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All about earth

2009-01-02T16:36:00.000-08:00

Earth (planet)

INTRODUCTION

Earth

An oxygen-rich and protective atmosphere, moderate temperatures, abundant water, and a varied chemical composition enable Earth to support life, the only planet known to harbor life. The planet is composed of rock and metal, which are present in molten form beneath its surface. The Apollo 17 spacecraft took this snapshot in 1972 of the Arabian Peninsula, the African continent, and Antarctica (most of the white area near the bottom).

NASA/Science Source/Photo Researchers, Inc.

Earth (planet), one of nine planets in the solar system, the only planet known to harbor life, and the “home” of human beings. From space Earth resembles a big blue marble with swirling white clouds floating above blue oceans. About 71 percent of Earth’s surface is covered by water, which is essential to life. The rest is land, mostly in the form of continents that rise above the oceans.

Earth’s surface is surrounded by a layer of gases known as the atmosphere, which extends upward from the surface, slowly thinning out into space. Below the surface is a hot interior of rocky material and two core layers composed of the metals nickel and iron in solid and liquid form.

Unlike the other planets, Earth has a unique set of characteristics ideally suited to supporting life as we know it. It is neither too hot, like Mercury, the closest planet to the Sun, nor too cold, like distant Mars and the even more distant outer planets—Jupiter, Saturn, Uranus, Neptune, and tiny Pluto. Earth’s atmosphere includes just the right amount of gases that trap heat from the Sun, resulting in a moderate climate suitable for water to exist in liquid form. The atmosphere also helps block radiation from the Sun that would be harmful to life. Earth’s atmosphere distinguishes it from the planet Venus, which is otherwise much like Earth. Venus is about the same size and mass as Earth and is also neither too near nor too far from the Sun. But because Venus has too much heat-trapping carbon dioxide in its atmosphere, its surface is extremely hot—462°C (864°F)—hot enough to melt lead and too hot for life to exist.

Although Earth is the only planet known to have life, scientists do not rule out the possibility that life may once have existed on other planets or their moons, or may exist today in primitive form. Mars, for example, has many features that resemble river channels, indicating that liquid water once flowed on its surface. If so, life may also have evolved there, and evidence for it may one day be found in fossil form. Water still exists on Mars, but it is frozen in polar ice caps, in permafrost, and possibly in rocks below the surface.

Earth from the Moon

In the late 1960s, people saw for the first time what Earth looked like from space. This famous photo of Earth was taken by astronauts on the Apollo 8 mission as they orbited the Moon in 1968.

NASA

For thousands of years, human beings could only wonder about Earth and the other observable planets in the solar system. Many early ideas—for example, that the Earth was a sphere and that it traveled around the Sun—were based on brilliant reasoning. However, it was only with the development of the scientific method and scientific instruments, especially in the 18th and 19th centuries, that humans began to gather data that could be used to verify theories about Earth and the rest of the solar system. By studying fossils found in rock layers, for example, scientists realized that the Earth was much older than previously believed. And with the use of telescopes, new planets such as Uranus, Neptune, and Pluto were discovered.

In the second half of the 20th century, more advances in the study of Earth and the solar system occurred due to the development of rockets that could send spacecraft beyond Earth. Human beings were able to study and observe Earth from space with satellites equipped with scientific instruments. Astronauts landed on the Moon and gathered ancient rocks that revealed much about the early solar system. During this remarkable advancement in human history, humans also sent unmanned spacecraft to the other planets and their moons. Spacecraft have now visited all of the planets except Pluto. The study of other planets and moons has provided new insights about Earth, just as the study of the Sun and other stars like it has helped shape new theories about how Earth and the rest of the solar system formed.

As a result of this recent space exploration, we now know that Earth is one of the most geologically active of all the planets and moons in the solar system. Earth is constantly changing. Over long periods of time land is built up and worn away, oceans are formed and re-formed, and continents move around, break up, and merge.

Life itself contributes to changes on Earth, especially in the way living things can alter Earth’s atmosphere. For example, Earth at one time had the same amount of carbon dioxide in its atmosphere as Venus now has, but early forms of life helped remove this carbon dioxide over millions of years. These life forms also added oxygen to Earth’s atmosphere and made it possible for animal life to evolve on land.

A variety of scientific fields have broadened our knowledge about Earth, including biogeography, climatology, geology, geophysics, hydrology, meteorology, oceanography, and zoogeography. Collectively, these fields are known as Earth science. By studying Earth’s atmosphere, its surface, and its interior and by studying the Sun and the rest of the solar system, scientists have learned much about how Earth came into existence, how it changed, and why it continues to change.

EARTH, THE SOLAR SYSTEM, AND THE GALAXY

Solar System

Earth is the third planet from the Sun, after Mercury and Venus. The average distance between Earth and the Sun is 150 million km (93 million mi). Earth and all the other planets in the solar system revolve, or orbit, around the Sun due to the force of gravitation. The Earth travels at a velocity of about 107,000 km/h (about 67,000 mph) as it orbits the Sun. All but one of the planets orbit the Sun in the same plane—that is, if an imaginary line were extended from the center of the Sun to the outer regions of the solar system, the orbital paths of the planets would intersect that line. The exception is Pluto, which has an eccentric (unusual) orbit.

Earth’s orbital path is not quite a perfect circle but instead is slightly elliptical (oval-shaped). For example, at maximum distance Earth is about 152 million km (about 95 million mi) from the Sun; at minimum distance Earth is about 147 million km (about 91 million mi) from the Sun. If Earth orbited the Sun in a perfect circle, it would always be the same distance from the Sun.

The solar system, in turn, is part of the Milky Way Galaxy, a collection of billions of stars bound together by gravity. The Milky Way has armlike discs of stars that spiral out from its center. The solar system is located in one of these spiral arms, known as the Orion arm, which is about two-thirds of the way from the center of the Galaxy. In most parts of the Northern Hemisphere, this disc of stars is visible on a summer night as a dense band of light known as the Milky Way.

Milky Way Galaxy

Our own solar system exists within one of the spiral arms of the disk-shaped galaxy called the Milky Way. This false-color image looks toward the center of the Milky Way, located 30,000 light-years away. Bright star clusters are visible along with darker areas of dust and gas.

Morton-Milon/Science Source/Photo Researchers, Inc.

Earth is the fifth largest planet in the solar system. Its diameter, measured around the equator, is 12,756 km (7,926 mi). Earth is not a perfect sphere but is slightly flattened at the poles. Its polar diameter, measured from the North Pole to the South Pole, is somewhat less than the equatorial diameter because of this flattening. Although Earth is the largest of the four planets—Mercury, Venus, Earth, and Mars—that make up the inner solar system (the planets closest to the Sun), it is small compared with the giant planets of the outer solar system—Jupiter, Saturn, Uranus, and Neptune. For example, the largest planet, Jupiter, has a diameter at its equator of 143,000 km (89,000 mi), 11 times greater than that of Earth. A famous atmospheric feature on Jupiter, the Great Red Spot, is so large that three Earths would fit inside it.

Earth has one natural satellite, the Moon. The Moon orbits the Earth, completing one revolution in an elliptical path in 27 days 7 hr 43 min 11.5 sec. The Moon orbits the Earth because of the force of Earth’s gravity. However, the Moon also exerts a gravitational force on the Earth. Evidence for the Moon’s gravitational influence can be seen in the ocean tides. A popular theory suggests that the Moon split off from Earth more than 4 billion years ago when a large meteorite or small planet struck the Earth.

As Earth revolves around the Sun, it rotates, or spins, on its axis, an imaginary line that runs between the North and South poles. The period of one complete rotation is defined as a day and takes 23 hr 56 min 4.1 sec. The period of one revolution around the Sun is defined as a year, or 365.2422 solar days, or 365 days 5 hr 48 min 46 sec. Earth also moves along with the Milky Way Galaxy as the Galaxy rotates and moves through space. It takes more than 200 million years for the stars in the Milky Way to complete one revolution around the Galaxy’s center.

Earth’s axis of rotation is inclined (tilted) 23.5° relative to its plane of revolution around the Sun. This inclination of the axis creates the seasons and causes the height of the Sun in the sky at noon to increase and decrease as the seasons change. The Northern Hemisphere receives the most energy from the Sun when it is tilted toward the Sun. This orientation corresponds to summer in the Northern Hemisphere and winter in the Southern Hemisphere. The Southern Hemisphere receives maximum energy when it is tilted toward the Sun, corresponding to summer in the Southern Hemisphere and winter in the Northern Hemisphere. Fall and spring occur in between these orientations.

III

EARTH’S ATMOSPHERE

The atmosphere is a layer of different gases that extends from Earth’s surface to the exosphere, the outer limit of the atmosphere, about 9,600 km (6,000 mi) above the surface. Near Earth’s surface, the atmosphere consists almost entirely of nitrogen (78 percent) and oxygen (21 percent). The remaining 1 percent of atmospheric gases consists of argon (0.9 percent); carbon dioxide (0.03 percent); varying amounts of water vapor; and trace amounts of hydrogen, nitrous oxide, ozone, methane, carbon monoxide, helium, neon, krypton, and xenon.

Layers of the Atmosphere

Divisions of the Atmosphere

Without our atmosphere, there would be no life on Earth. A relatively thin envelope, the atmosphere consists of layers of gases that support life and provide protection from harmful radiation.

The layers of the atmosphere are the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. The troposphere is the layer in which weather occurs and extends from the surface to about 16 km (about 10 mi) above sea level at the equator. Above the troposphere is the stratosphere, which has an upper boundary of about 50 km (about 30 mi) above sea level. The layer from 50 to 90 km (30 to 60 mi) is called the mesosphere. At an altitude of about 90 km, temperatures begin to rise. The layer that begins at this altitude is called the thermosphere because of the high temperatures that can be reached in this layer (about 1200°C, or about 2200°F). The region beyond the thermosphere is called the exosphere. The thermosphere and the exosphere overlap with another region of the atmosphere known as the ionosphere, a layer or layers of ionized air extending from almost 60 km (about 50 mi) above Earth’s surface to altitudes of 1,000 km (600 mi) and more.

Greenhouse Effect

Earth’s atmosphere and the way it interacts with the oceans and radiation from the Sun are responsible for the planet’s climate and weather. The atmosphere plays a key role in supporting life. Almost all life on Earth uses atmospheric oxygen for energy in a process known as cellular respiration, which is essential to life. The atmosphere also helps moderate Earth’s climate by trapping radiation from the Sun that is reflected from Earth’s surface. Water vapor, carbon dioxide, methane, and nitrous oxide in the atmosphere act as “greenhouse gases.” Like the glass in a greenhouse, they trap infrared, or heat, radiation from the Sun in the lower atmosphere and thereby help warm Earth’s surface. Without this greenhouse effect, heat radiation would escape into space, and Earth would be too cold to support most forms of life.

Other gases in the atmosphere are also essential to life. The trace amount of ozone found in Earth’s stratosphere blocks harmful ultraviolet radiation from the Sun. Without the ozone layer, life as we know it could not survive on land. Earth’s atmosphere is also an important part of a phenomenon known as the water cycle or the hydrologic cycle. See also Atmosphere.

The Atmosphere and the Water Cycle

Water Cycle

The water cycle simply means that Earth’s water is continually recycled between the oceans, the atmosphere, and the land. All of the water that exists on Earth today has been used and reused for billions of years. Very little water has been created or lost during this period of time. Water is constantly moving on Earth’s surface and changing back and forth between ice, liquid water, and water vapor.

The water cycle begins when the Sun heats the water in the oceans and causes it to evaporate and enter the atmosphere as water vapor. Some of this water vapor falls as precipitation directly back into the oceans, completing a short cycle. Some of the water vapor, however, reaches land, where it may fall as snow or rain. Melted snow or rain enters rivers or lakes on the land. Due to the force of gravity, the water in the rivers eventually empties back into the oceans. Melted snow or rain also may enter the ground. Groundwater may be stored for hundreds or thousands of years, but it will eventually reach the surface as springs or small pools known as seeps. Even snow that forms glacial ice or becomes part of the polar caps and is kept out of the cycle for thousands of years eventually melts or is warmed by the Sun and turned into water vapor, entering the atmosphere and falling again as precipitation. All water that falls on land eventually returns to the ocean, completing the water cycle.

EARTH’S SURFACE

Earth’s surface is the outermost layer of the planet. It includes the hydrosphere, the crust, and the biosphere.

Hydrosphere

The hydrosphere consists of the bodies of water that cover 71 percent of Earth’s surface. The largest of these are the oceans, which contain over 97 percent of all water on Earth. Glaciers and the polar ice caps contain just over 2 percent of Earth’s water in the form of solid ice. Only about 0.6 percent is under the surface as groundwater. Nevertheless, groundwater is 36 times more plentiful than water found in lakes, inland seas, rivers, and in the atmosphere as water vapor. Only 0.017 percent of all the water on Earth is found in lakes and rivers. And a mere 0.001 percent is found in the atmosphere as water vapor. Most of the water in glaciers, lakes, inland seas, rivers, and groundwater is fresh and can be used for drinking and agriculture. Dissolved salts compose about 3.5 percent of the water in the oceans, however, making it unsuitable for drinking or agriculture unless it is treated to remove the salts.

Crust

The crust consists of the continents, other land areas, and the basins, or floors, of the oceans. The dry land of Earth’s surface is called the continental crust. It is about 15 to 75 km (9 to 47 mi) thick. The oceanic crust is thinner than the continental crust. Its average thickness is 5 to 10 km (3 to 6 mi). The crust has a definite boundary called the Mohorovičić discontinuity, or simply the Moho. The boundary separates the crust from the underlying mantle, which is much thicker and is part of Earth’s interior.

Oceanic crust and continental crust differ in the type of rocks they contain. There are three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks form when molten rock, called magma, cools and solidifies. Sedimentary rocks are usually created by the breakdown of igneous rocks. They tend to form in layers as small particles of other rocks or as the mineralized remains of dead animals and plants that have fused together over time. The remains of dead animals and plants occasionally become mineralized in sedimentary rock and are recognizable as fossils. Metamorphic rocks form when sedimentary or igneous rocks are altered by heat and pressure deep underground.

Oceanic crust consists of dark, dense igneous rocks, such as basalt and gabbro. Continental crust consists of lighter-colored, less dense igneous rocks, such as granite and diorite. Continental crust also includes metamorphic rocks and sedimentary rocks.

Biosphere

The biosphere includes all the areas of Earth capable of supporting life. The biosphere ranges from about 10 km (about 6 mi) into the atmosphere to the deepest ocean floor. For a long time, scientists believed that all life depended on energy from the Sun and consequently could only exist where sunlight penetrated. In the 1970s, however, scientists discovered various forms of life around hydrothermal vents on the floor of the Pacific Ocean where no sunlight penetrated. They learned that primitive bacteria formed the basis of this living community and that the bacteria derived their energy from a process called chemosynthesis that did not depend on sunlight. Some scientists believe that the biosphere may extend relatively deep into Earth’s crust. They have recovered what they believe are primitive bacteria from deeply drilled holes below the surface.

Changes to Earth’s Surface

Earth’s surface has been constantly changing ever since the planet formed. Most of these changes have been gradual, taking place over millions of years. Nevertheless, these gradual changes have resulted in radical modifications, involving the formation, erosion, and re-formation of mountain ranges, the movement of continents, the creation of huge supercontinents, and the breakup of supercontinents into smaller continents.

The weathering and erosion that result from the water cycle are among the principal factors responsible for changes to Earth’s surface. Another principal factor is the movement of Earth’s continents and seafloors and the buildup of mountain ranges due to a phenomenon known as plate tectonics. Heat is the basis for all of these changes. Heat in Earth’s interior is believed to be responsible for continental movement, mountain building, and the creation of new seafloor in ocean basins. Heat from the Sun is responsible for the evaporation of ocean water and the resulting precipitation that causes weathering and erosion. In effect, heat in Earth’s interior helps build up Earth’s surface while heat from the Sun helps wear down the surface.

Weathering

Weathering is the breakdown of rock at and near the surface of Earth. Most rocks originally formed in a hot, high-pressure environment below the surface where there was little exposure to water. Once the rocks reached Earth’s surface, however, they were subjected to temperature changes and exposed to water. When rocks are subjected to these kinds of surface conditions, the minerals they contain tend to change. These changes constitute the process of weathering. There are two types of weathering: physical weathering and chemical weathering.

Physical weathering involves a decrease in the size of rock material. Freezing and thawing of water in rock cavities, for example, splits rock into small pieces because water expands when it freezes.

Chemical weathering involves a chemical change in the composition of rock. For example, feldspar, a common mineral in granite and other rocks, reacts with water to form clay minerals, resulting in a new substance with totally different properties than the parent feldspar. Chemical weathering is of significance to humans because it creates the clay minerals that are important components of soil, the basis of agriculture. Chemical weathering also causes the release of dissolved forms of sodium, calcium, potassium, magnesium, and other chemical elements into surface water and groundwater. These elements are carried by surface water and groundwater to the sea and are the sources of dissolved salts in the sea.

Erosion

Glacial Erosion

Glaciers erode the Earth’s surface through processes such as abrasion, crushing, and fracturing of the material in the glacier’s path. Glaciers move by growing or shrinking, depending on the climate. Moving glaciers erode and transport large quantities of rocks, sand, and other particles along their path. The icy path shown here is a moraine formed by a glacier in Switzerland.

Paolo Koch/Photo Researchers, Inc.

Erosion is the process that removes loose and weathered rock and carries it to a new site. Water, wind, and glacial ice combined with the force of gravity can cause erosion.

Erosion by running water is by far the most common process of erosion. It takes place over a longer period of time than other forms of erosion. When water from rain or melted snow moves downhill, it can carry loose rock or soil with it. Erosion by running water forms the familiar gullies and V-shaped valleys that cut into most landscapes. The force of the running water removes loose particles formed by weathering. In the process, gullies and valleys are lengthened, widened, and deepened. Often, water overflows the banks of the gullies or river channels, resulting in floods. Each new flood carries more material away to increase the size of the valley. Meanwhile, weathering loosens more and more material so the process continues.

Erosion by glacial ice is less common, but it can cause the greatest landscape changes in the shortest amount of time. Glacial ice forms in a region where snow fails to melt in the spring and summer and instead builds up as ice. For major glaciers to form, this lack of snowmelt has to occur for a number of years in areas with high precipitation. As ice accumulates and thickens, it flows as a solid mass. As it flows, it has a tremendous capacity to erode soil and even solid rock. Ice is a major factor in shaping some landscapes, especially mountainous regions. Glacial ice provides much of the spectacular scenery in these regions. Features such as horns (sharp mountain peaks), arêtes (sharp ridges), glacially formed lakes, and U-shaped valleys are all the result of glacial erosion.

Wind is an important cause of erosion only in arid (dry) regions. Wind carries sand and dust, which can scour even solid rock.

Many factors determine the rate and kind of erosion that occurs in a given area. The climate of an area determines the distribution, amount, and kind of precipitation that the area receives and thus the type and rate of weathering. An area with an arid climate erodes differently than an area with a humid climate. The elevation of an area also plays a role by determining the potential energy of running water. The higher the elevation the more energetically water will flow due to the force of gravity. The type of bedrock in an area (sandstone, granite, or shale) can determine the shapes of valleys and slopes, and the depth of streams.

A landscape’s geologic age—that is, how long current conditions of weathering and erosion have affected the area—determines its overall appearance. Relatively young landscapes tend to be more rugged and angular in appearance. Older landscapes tend to have more rounded slopes and hills. The oldest landscapes tend to be low-lying with broad, open river valleys and low, rounded hills. The overall effect of the wearing down of an area is to level the land; the tendency is toward the reduction of all land surfaces to sea level.

Plate Tectonics

Opposing this tendency toward leveling is a force responsible for raising mountains and plateaus and for creating new landmasses. These changes to Earth’s surface occur in the outermost solid portion of Earth, known as the lithosphere. The lithosphere consists of the crust and another region known as the upper mantle and is approximately 65 to 100 km (40 to 60 mi) thick. Compared with the interior of the Earth, however, this region is relatively thin. The lithosphere is thinner in proportion to the whole Earth than the skin of an apple is to the whole apple.

Scientists believe that the lithosphere is broken into a series of plates, or segments. According to the theory of plate tectonics, these plates move around on Earth’s surface over long periods of time. Tectonics comes from the Greek word, tektonikos, which means “builder.”

According to the theory, the lithosphere is divided into large and small plates. The largest plates include the Pacific plate, the North American plate, the Eurasian plate, the Antarctic plate, the Indo-Australian plate, and the African plate. Smaller plates include the Cocos plate, the Nazca plate, the Philippine plate, and the Caribbean plate. Plate sizes vary a great deal. The Cocos plate is 2,000 km (1,000 mi) wide, while the Pacific plate is nearly 14,000 km (nearly 9,000 mi) wide.

These plates move in three different ways in relation to each other. They pull apart or move away from each other, they collide or move against each other, or they slide past each other as they move sideways. The movement of these plates helps explain many geological events, such as earthquakes and volcanic eruptions as well as mountain building and the formation of the oceans and continents.

D3a

When Plates Pull Apart

Magma Upwelling

Mid-ocean ridges occur along boundaries between plates of Earth’s outer shell where new seafloor is created as the plates spread apart. As plates move apart under the ocean, molten rock, or magma, wells up from deep below the surface of the seafloor. Some of the magma that ascends to the seafloor produces enormous volcanic eruptions. The rest solidifies on the edges of the plates as they spread apart, creating new rocky seafloor material.

Archive Photos

When the plates pull apart, two types of phenomena occur depending on whether the movement takes place in the oceans or on land. When plates pull apart on land, deep valleys known as rift valleys form. An example of a rift valley is the Great Rift Valley that extends from Syria in the Middle East to Mozambique in Africa. When plates pull apart in the oceans, long, sinuous chains of volcanic mountains called mid-ocean ridges form, and new seafloor is created at the site of these ridges. Rift valleys are also present along the crests of the mid-ocean ridges.

Most scientists believe that gravity and heat from the interior of the Earth cause the plates to move apart and to create new seafloor. According to this explanation, molten rock known as magma rises from Earth’s interior to form hot spots beneath the ocean floor. As two oceanic plates pull apart from each other in the middle of the oceans, a crack, or rupture, appears and forms the mid-ocean ridges. These ridges exist in all the world’s ocean basins and resemble the seams of a baseball. The molten rock rises through these cracks and creates new seafloor.

D3b

When Plates Collide

Converging Plates

The outer layer of the Earth, the lithosphere, is broken into about 20 pieces, called tectonic plates. These plates slowly slide around on the asthenosphere below, periodically colliding with each other.

When plates collide or push against each other, regions called convergent plate margins form. Along these margins, one plate is usually forced to dive below the other. As that plate dives, it triggers the melting of the surrounding lithosphere and a region just below it known as the asthenosphere. These pockets of molten crust rise behind the margin through the overlying plate, creating curved chains of volcanoes known as arcs. This process is called subduction.

If one plate consists of oceanic crust and the other consists of continental crust, the denser oceanic crust will dive below the continental crust. If both plates are oceanic crust, then either may be subducted. If both are continental crust, subduction can continue for a while but will eventually end because continental crust is not dense enough to be forced very far into the upper mantle.

Mount Everest

Mount Everest, the world’s highest mountain at 8,850 m (29,035 ft), is located in the Himalayas. The Himalayas form the highest mountain system in the world, with more than 30 peaks towering 7,600 m (25,000 ft) or more.

Keren Su/Tony Stone Images

The results of this subduction process are readily visible on a map showing that 80 percent of the world’s volcanoes rim the Pacific Ocean where plates are colliding against each other. The subduction zone created by the collision of two oceanic plates—the Pacific plate and the Philippine plate—can also create a trench. Such a trench resulted in the formation of the deepest point on Earth, the Mariana Trench, which is estimated to be 11,033 m (36,198 ft) below sea level.

On the other hand, when two continental plates collide, mountain building occurs. The collision of the Indo-Australian plate with the Eurasian plate has produced the Himalayan Mountains. This collision resulted in the highest point of Earth, Mount Everest, which is 8,850 m (29,035 ft) above sea level.

D3c

When Plates Slide Past Each Other

San Andreas Fault, California

The San Andreas Fault, unlike most faults that stay below the ocean, emerges from the Pacific Ocean and traverses hundreds of miles of land. It runs through California for about 1,000 km (about 600 mi) from Point Arena to the Imperial Valley. The fault marks the boundary between the North American and Pacific tectonic plates; earthquakes are caused by these plates sliding together.

Francois Gohier/Photo Researchers, Inc.

Finally, some of Earth’s plates neither collide nor pull apart but instead slide past each other. These regions are called transform margins. Few volcanoes occur in these areas because neither plate is forced down into Earth’s interior and little melting occurs. Earthquakes, however, are abundant as the two rigid plates slide past each other. The San Andreas Fault in California is a well-known example of a transform margin.

The movement of plates occurs at a slow pace, at an average rate of only 2.5 cm (1 in) per year. But over millions of years this gradual movement results in radical changes. Current plate movement is making the Pacific Ocean and Mediterranean Sea smaller, the Atlantic Ocean larger, and the Himalayan Mountains higher.

EARTH’S INTERIOR

Internal Structure of the Earth

Earth is made up of a series of layers that formed early in the planet’s history, as heavier material gravitated toward the center and lighter material floated to the surface. The dense, solid, inner core of iron is surrounded by a liquid, iron, outer core. The lower mantle consists of molten rock, which is surrounded by partially molten rock in the asthenosphere and solid rock in the upper mantle and crust. Between some of the layers, there are chemical or structural changes that form discontinuities. Lighter elements, such as silicon, aluminum, calcium, potassium, sodium, and oxygen, compose the outer crust.

The interior of Earth plays an important role in plate tectonics. Scientists believe it is also responsible for Earth’s magnetic field. This field is vital to life because it shields the planet’s surface from harmful cosmic rays and from a steady stream of energetic particles from the Sun known as the solar wind.

Composition of the Interior

Earth’s interior consists of the mantle and the core. The mantle and core make up by far the largest part of Earth’s mass. The distance from the base of the crust to the center of the core is about 6,400 km (about 4,000 mi).

Scientists have learned about Earth’s interior by studying rocks that formed in the interior and rose to the surface. The study of meteorites, which are believed to be made of the same material that formed the Earth and its interior, has also offered clues about Earth’s interior. Finally, seismic waves generated by earthquakes provide geophysicists with information about the composition of the interior. The sudden movement of rocks during an earthquake causes vibrations that transmit energy through the Earth in the form of waves. The way these waves travel through the interior of Earth reveals the nature of materials inside the planet.

The mantle consists of three parts: the lower part of the lithosphere, the region below it known as the asthenosphere, and the region below the asthenosphere called the lower mantle. The entire mantle extends from the base of the crust to a depth of about 2,900 km (about 1,800 mi). Scientists believe the asthenosphere is made up of mushy plastic-like rock with pockets of molten rock. The term asthenosphere is derived from Greek and means “weak layer.” The asthenosphere’s soft, plastic quality allows plates in the lithosphere above it to shift and slide on top of the asthenosphere. This shifting of the lithosphere’s plates is the source of most tectonic activity. The asthenosphere is also the source of the basaltic magma that makes up much of the oceanic crust and rises through volcanic vents on the ocean floor.

The mantle consists of mostly solid iron-magnesium silicate rock mixed with many other minor components including radioactive elements. However, even this solid rock can flow like a “sticky” liquid when it is subjected to enough heat and pressure.

The core is divided into two parts, the outer core and the inner core. The outer core is about 2,260 km (about 1,404 mi) thick. The outer core is a liquid region composed mostly of iron, with smaller amounts of nickel and sulfur in liquid form. The inner core is about 1,220 km (about 758 mi) thick. The inner core is solid and is composed of iron, nickel, and sulfur in solid form. Because the inner core is surrounded by a liquid region, it can rotate independently. Recent scientific studies indicate that the inner core may actually rotate faster than the rest of the planet, making one full extra spin over a period of 700 to 1,200 years. The inner core and the outer core also contain a small percentage of radioactive material. The existence of radioactive material is one of the sources of heat in Earth’s interior because as radioactive material decays, it gives off heat. Temperatures in the inner core may be as high as 6650°C (12,000°F).

The Core and Earth’s Magnetism

Earth’s Magnetic Field

Scientists believe that Earth’s liquid iron core is instrumental in creating a magnetic field that surrounds Earth and shields the planet from harmful cosmic rays and the Sun’s solar wind. The idea that Earth is like a giant magnet was first proposed in 1600 by English physician and natural philosopher William Gilbert. Gilbert proposed the idea to explain why the magnetized needle in a compass points north. According to Gilbert, Earth’s magnetic field creates a magnetic north pole and a magnetic south pole. The magnetic poles do not correspond to the geographic North and South poles, however. Moreover, the magnetic poles wander and are not always in the same place. The north magnetic pole is currently close to Ellef Ringnes Island in the Queen Elizabeth Islands near the boundary of Canada’s Northwest Territories with Nunavut. The south magnetic pole lies just off the coast of Wilkes Land, Antarctica.

Not only do the magnetic poles wander, but they also reverse their polarity—that is, the north magnetic pole becomes the south magnetic pole and vice versa. Magnetic reversals have occurred at least 170 times over the past 100 million years. The reversals occur on average about every 200,000 years and take place gradually over a period of several thousand years. Scientists still do not understand why these magnetic reversals occur but think they may be related to Earth’s rotation and changes in the flow of liquid iron in the outer core.

Aurora Borealis

The aurora borealis, commonly known as the northern lights, creates a spectacular light show near Fairbanks, Alaska. Auroras, most frequently seen in the far northern and far southern regions of the globe, are common sights in the Alaskan sky. Luminous displays visible to the naked eye only at night, auroras occur when charged particles from the Sun interact with gases in Earth’s atmosphere.

Jack Finch/Science Source/Photo Researchers, Inc.

Some scientists theorize that the flow of liquid iron in the outer core sets up electrical currents that produce Earth’s magnetic field. Known as the dynamo theory, this theory appears to be the best explanation yet for the origin of the magnetic field. Earth’s magnetic field operates in a region above Earth’s surface known as the magnetosphere. The magnetosphere is shaped somewhat like a teardrop with a long tail that trails away from the Earth due to the force of the solar wind.

Inside the magnetosphere are the Van Allen radiation belts, named for the American physicist James A. Van Allen who discovered them in 1958. The Van Allen belts are regions where charged particles from the Sun and from cosmic rays are trapped and sent into spiral paths along the lines of Earth’s magnetic field. The radiation belts thereby shield Earth’s surface from these highly energetic particles. Occasionally, however, due to extremely strong magnetic fields on the Sun’s surface, which are visible as sunspots, a brief burst of highly energetic particles streams along with the solar wind. Because Earth’s magnetic field lines converge and are closest to the surface at the poles, some of these energetic particles sneak through and interact with Earth’s atmosphere, creating the phenomenon known as an aurora.

EARTH’S PAST

Origin of Earth

Most scientists believe that the Earth, Sun, and all of the other planets and moons in the solar system formed about 4.6 billion years ago from a giant cloud of gas and dust known as the solar nebula. The gas and dust in this solar nebula originated in a star that ended its life in a violent explosion known as a supernova. The solar nebula consisted principally of hydrogen, the lightest element, but the nebula was also seeded with a smaller percentage of heavier elements, such as carbon and oxygen. All of the chemical elements we know were originally made in the star that became a supernova. Our bodies are made of these same chemical elements. Therefore, all of the elements in our solar system, including all of the elements in our bodies, originally came from this star-seeded solar nebula.

Due to the force of gravity tiny clumps of gas and dust began to form in the early solar nebula. As these clumps came together and grew larger, they caused the solar nebula to contract in on itself. The contraction caused the cloud of gas and dust to flatten in the shape of a disc. As the clumps continued to contract, they became very dense and hot. Eventually the atoms of hydrogen became so dense that they began to fuse in the innermost part of the cloud, and these nuclear reactions gave birth to the Sun. The fusion of hydrogen atoms in the Sun is the source of its energy.

Many scientists favor the planetesimal theory for how the Earth and other planets formed out of this solar nebula. This theory helps explain why the inner planets became rocky while the outer planets, except for Pluto, are made up mostly of gases. The theory also explains why all of the planets orbit the Sun in the same plane.

According to this theory, temperatures decreased with increasing distance from the center of the solar nebula. In the inner region, where Mercury, Venus, Earth, and Mars formed, temperatures were low enough that certain heavier elements, such as iron and the other heavy compounds that make up rock, could condense out—that is, could change from a gas to a solid or liquid. Due to the force of gravity, small clumps of this rocky material eventually came together with the dust in the original solar nebula to form protoplanets or planetesimals (small rocky bodies). These planetesimals collided, broke apart, and re-formed until they became the four inner rocky planets. The inner region, however, was still too hot for other light elements, such as hydrogen and helium, to be retained. These elements could only exist in the outermost part of the disc, where temperatures were lower. As a result two of the outer planets—Jupiter and Saturn—are mostly made of hydrogen and helium, which are also the dominant elements in the atmospheres of Uranus and Neptune.

The Early Earth

The Early Earth

Life originated on Earth about four billion years ago, when oceans dotted with volcanic islands covered most of Earth’s surface and continents were very small. The air was hot and contained almost no breathable oxygen. The Moon was much closer to Earth, and a day was less than 15 hours long. Meteorites fell more frequently, and there was more volcanic activity than there is today.

Within the planetesimal Earth, heavier matter sank to the center and lighter matter rose toward the surface. Most scientists believe that Earth was never truly molten and that this transfer of matter took place in the solid state. Much of the matter that went toward the center contained radioactive material, an important source of Earth’s internal heat. As heavier material moved inward, lighter material moved outward, the planet became layered, and the layers of the core and mantle were formed. This process is called differentiation.

Not long after they formed, more than 4 billion years ago, the Earth and the Moon underwent a period when they were bombarded by meteorites, the rocky debris left over from the formation of the solar system. The impact craters created during this period of heavy bombardment are still visible on the Moon’s surface, which is unchanged. Earth’s craters, however, were long ago erased by weathering, erosion, and mountain building. Because the Moon has no atmosphere, its surface has not been subjected to weathering or erosion. Thus, the evidence of meteorite bombardment remains.

Energy released from the meteorite impacts created extremely high temperatures on Earth that melted the outer part of the planet and created the crust. By 4 billion years ago, both the oceanic and continental crust had formed, and the oldest rocks were created. These rocks are known as the Acasta Gneiss and are found in Canada’s Northwest Territories. Due to the meteorite bombardment, the early Earth was too hot for liquid water to exist and so it was impossible for life to exist.

Geologic Time

Fossil-bearing Rocks

Sedimentary rocks, such as this fossil-bearing limestone, can help geologists determine geologic time. Because the bottom layers were deposited first, the oldest fossils are found in the bottom layers of sedimentary rocks. The accumulation of shells or shell fragments and other fossils in limestone provides geologists with a record of the evolution of the animals that used to live in the ancient oceans.

Carolina Biological Supply/Phototake NYC

Geologists divide the history of the Earth into three eons: the Archean Eon, which lasted from around 4 billion to 2.5 billion years ago; the Proterozoic Eon, which lasted from 2.5 billion to 543 million years ago; and the Phanerozoic Eon, which lasted from 543 million years ago to the present. Each eon is subdivided into different eras. For example, the Phanerozoic Eon includes the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. In turn, eras are further divided into periods. For example, the Paleozoic Era includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods.

Geologic Time Scale

The Archean Eon is subdivided into four eras, the Eoarchean, the Paleoarchean, the Mesoarchean, and the Neoarchean. The beginning of the Archean is generally dated as the age of the oldest terrestrial rocks, which are about 4 billion years old. The Archean Eon ended 2.5 billion years ago when the Proterozoic Eon began. The Proterozoic Eon is subdivided into three eras: the Paleoproterozoic Era, the Mesoproterozoic Era, and the Neoproterozoic Era. The Proterozoic Eon lasted from 2.5 billion years ago to 543 million years ago when the Phanerozoic Eon began. The Phanerozoic Eon is subdivided into three eras: the Paleozoic Era from 543 million to 248 million years ago, the Mesozoic Era from 248 million to 65 million years ago, and the Cenozoic Era from 65 million years ago to the present.

Stratigraphic Column

Fossils preserved in rock strata provide scientists with clues to evolutionary history. This stratigraphic column is based on paleontological evidence and shows the order in which organisms appeared in the fossil-rich Paleozoic era. Each layer represents a particular time frame and shows a representative organism that flourished during that time. Although fossils are rarely found in the idealized and localized fashion shown here, they are often in more or less chronological order. Generally, the oldest fossils appear in lower layers, and the most recent fossils at the top, so that placement may be used as an aid in dating the specimens.

Geologists base these divisions on the study and dating of rock layers or strata, including the fossilized remains of plants and animals found in those layers. Until the late 1800s scientists could only determine the relative ages of rock strata. They knew that in general the top layers of rock were the youngest and formed most recently, while deeper layers of rock were older. The field of stratigraphy shed much light on the relative ages of rock layers.

The study of fossils also enabled geologists to determine the relative ages of different rock layers. The fossil record helped scientists determine how organisms evolved or when they became extinct. By studying rock layers around the world, geologists and paleontologists saw that the remains of certain animal and plant species occurred in the same layers, but were absent or altered in other layers. They soon developed a fossil index that also helped determine the relative ages of rock layers.

Beginning in the 1890s, scientists learned that radioactive elements in rock decay at a known rate. By studying this radioactive decay, they could determine an absolute age for rock layers. This type of dating, known as radiometric dating, confirmed the relative ages determined through stratigraphy and the fossil index and assigned absolute ages to the various strata. As a result scientists were able to assemble Earth’s geologic time scale from the Archean Eon to the present. See also Geologic Time.

Precambrian

Cyanobacteria

Cyanobacteria (formerly blue-green algae) are among the most ancient organisms on earth. These photosynthetic organisms can be single-celled, connected in afilamentous form as shown here, or arranged in simple colonies. Cyanobacteria are capable of enduring a wide variety of environmental conditions ranging from freshwater and marine habitats to snowfields and glaciers. They are capable of surviving and flourishing even at extremely high temperatures.

Peter Parks/Oxford Scientific Films

The Precambrian is a time span that includes the Archean and Proterozoic eons and began about 4 billion years ago. The Precambrian marks the first formation of continents, the oceans, the atmosphere, and life. The Precambrian represents the oldest chapter in Earth’s history that can still be studied. Very little remains of Earth from the period of 4.6 billion to about 4 billion years ago due to the melting of rock caused by the early period of meteorite bombardment. Rocks dating from the Precambrian, however, have been found in Africa, Antarctica, Australia, Brazil, Canada, and Scandinavia. Some zircon mineral grains deposited in Australian rock layers have been dated to 4.2 billion years.

The Precambrian is also the longest chapter in Earth’s history, spanning a period of about 3.5 billion years. During this timeframe, the atmosphere and the oceans formed from gases that escaped from the hot interior of the planet as a result of widespread volcanic eruptions. The early atmosphere consisted primarily of nitrogen, carbon dioxide, and water vapor. As Earth continued to cool, the water vapor condensed out and fell as precipitation to form the oceans. Some scientists believe that much of Earth’s water vapor originally came from comets containing frozen water that struck Earth during the period of meteorite bombardment.

By studying 2-billion-year-old rocks found in northwestern Canada, as well as 2.5-billion-year-old rocks in China, scientists have found evidence that plate tectonics began shaping Earth’s surface as early as the middle Precambrian. About a billion years ago, the Earth’s plates were centered around the South Pole and formed a supercontinent called Rodinia. Slowly, pieces of this supercontinent broke away from the central continent and traveled north, forming smaller continents.

Life originated during the Precambrian. The earliest fossil evidence of life consists of prokaryotes, one-celled organisms that lacked a nucleus and reproduced by dividing, a process known as asexual reproduction. Asexual division meant that a prokaryote’s hereditary material was copied unchanged. The first prokaryotes were bacteria known as archaebacteria. Scientists believe they came into existence perhaps as early as 3.8 billion years ago, but certainly by about 3.5 billion years ago, and were anaerobic—that is, they did not require oxygen to produce energy. Free oxygen barely existed in the atmosphere of the early Earth.

Archaebacteria were followed about 3.46 billion years ago by another type of prokaryote known as cyanobacteria or blue-green algae. These cyanobacteria gradually introduced oxygen in the atmosphere as a result of photosynthesis. In shallow tropical waters, cyanobacteria formed mats that grew into humps called stromatolites. Fossilized stromatolites have been found in rocks in the Pilbara region of western Australia that are more than 3.4 billion years old and in rocks of the Gunflint Chert region of northwest Lake Superior that are about 2.1 billion years old.

For billions of years, life existed only in the simple form of prokaryotes. Prokaryotes were followed by the relatively more advanced eukaryotes, organisms that have a nucleus in their cells and that reproduce by combining or sharing their heredity makeup rather than by simply dividing. Sexual reproduction marked a milestone in life on Earth because it created the possibility of hereditary variation and enabled organisms to adapt more easily to a changing environment. The very latest part of Precambrian time some 560 million to 545 million years ago saw the appearance of an intriguing group of fossil organisms known as the Ediacaran fauna. First discovered in the northern Flinders Range region of Australia in the mid-1940s and subsequently found in many locations throughout the world, these strange fossils appear to be the precursors of many of the fossil groups that were to explode in Earth's oceans in the Paleozoic Era. See also Evolution; Natural Selection.

Paleozoic Era

The Earliest Animals

The earliest known animals on Earth were a bizarre collection of life forms that emerged just prior to and during the Cambrian Period, some of which were exquisitely preserved in fossil beds in various parts of the world. Some of the more extraordinary creatures (depicted in this artist's conception) were the formidable predator Anomalocaris (foreground upper right) about to make a meal of Waptia, which it holds in its extended claws. Just below Anomalocaris and slightly to its left is Opabinia using its long, trunklike snout to grasp Burgessochaeta, a bristle worm. The fernlike objects (left and center) are actually animals, as are the primitive sponges (center foreground) that resemble a saguaro cactus. The depictions of these fernlike animals are based on a group of fossils known as the Ediacaran fossils and date from about 550 million years ago.

D.W. Miller

At the start of the Paleozoic Era about 543 million years ago, an enormous expansion in the diversity and complexity of life occurred. This event took place in the Cambrian Period and is called the Cambrian explosion. Nothing like it has happened since. Almost all of the major groups of animals we know today made their first appearance during the Cambrian explosion. Almost all of the different “body plans” found in animals today—that is, the way an animal’s body is designed, with heads, legs, rear ends, claws, tentacles, or antennae—also originated during this period.

Fishes first appeared during the Paleozoic Era, and multicellular plants began growing on the land. Other land animals, such as scorpions, insects, and amphibians, also originated during this time. Just as new forms of life were being created, however, other forms of life were going out of existence. Natural selection meant that some species were able to flourish, while others failed. In fact, mass extinctions of animal and plant species were commonplace.

Most of the early complex life forms of the Cambrian explosion lived in the sea. The creation of warm, shallow seas, along with the buildup of oxygen in the atmosphere, may have aided this explosion of life forms. The shallow seas were created by the breakup of the supercontinent Rodinia. During the Ordovician, Silurian, and Devonian periods, which followed the Cambrian Period and lasted from 490 million to 354 million years ago, some of the continental pieces that had broken off Rodinia collided. These collisions resulted in larger continental masses in equatorial regions and in the Northern Hemisphere. The collisions built a number of mountain ranges, including parts of the Appalachian Mountains in North America and the Caledonian Mountains of northern Europe.

Toward the close of the Paleozoic Era, two large continental masses, Gondwanaland to the south and Laurasia to the north, faced each other across the equator. Their slow but eventful collision during the Permian Period of the Paleozoic Era, which lasted from 290 million to 248 million years ago, assembled the supercontinent Pangaea and resulted in some of the grandest mountains in the history of Earth. These mountains included other parts of the Appalachians and the Ural Mountains of Asia. At the close of the Paleozoic Era, Pangaea represented over 90 percent of all the continental landmasses. Pangaea straddled the equator with a huge mouthlike opening that faced east. This opening was the Tethys Ocean, which closed as India moved northward creating the Himalayas. The last remnants of the Tethys Ocean can be seen in today’s Mediterranean Sea.

The Paleozoic came to an end with a major extinction event, when perhaps as many as 90 percent of all plant and animal species died out. The reason is not known for sure, but many scientists believe that huge volcanic outpourings of lavas in central Siberia, coupled with an asteroid impact, were joint contributing factors.

Mesozoic Era

Extent of Pleistocene Epoch Glaciation

During the Pleistocene epoch of the Quaternary Ice Age, glaciers (represented on map in white) covered much of the Earth’s northern hemisphere. Ice Ages consist of glacial periods and warmer interglacial periods. Although the Pleistocene, the Earth’s most recent glacial event, ended 10,000 years ago, many scientists believe that the Earth remains in an interglacial state of the Quaternary Ice Age.

The Cenozoic Era, beginning about 65 million years ago, is the period when mammals became the dominant form of life on land. Human beings first appeared in the later stages of the Cenozoic Era. In short, the modern world as we know it, with its characteristic geographical features and its animals and plants, came into being. All of the continents that we know today took shape during this era.

A single catastrophic event may have been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico’s Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.

The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continues to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.

Glaciers

Discovery Enterprises, LLC

Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller microcontinents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today’s Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today indicates that seafloor spreading is still causing the country to grow.

Late in the Tertiary Period, about 6 million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between 2 million and 1.7 million years ago.

The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anatomically modern human beings, known as Homo sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that anatomically modern humans originated only recently in Earth’s 4.6-billion-year history, within the past 200,000 years. See also Human Evolution.

VII

EARTH’S FUTURE

With the rise of human civilization about 8,000 years ago and especially since the Industrial Revolution in the mid-1700s, human beings began to alter the surface, water, and atmosphere of Earth. In doing so, they have become active geological agents, not unlike other forces of change that influence the planet. As a result, Earth’s immediate future depends to a great extent on the behavior of humans. For example, the widespread use of fossil fuels is releasing carbon dioxide and other greenhouse gases into the atmosphere and threatens to warm the planet’s surface. This global warming could melt glaciers and the polar ice caps, which could flood coastlines around the world and many island nations. In effect, the carbon dioxide that was removed from Earth’s early atmosphere by the oceans and by primitive plant and animal life, and subsequently buried as fossilized remains in sedimentary rock, is being released back into the atmosphere and is threatening the existence of living things. See also Global Warming.

Even without human intervention, Earth will continue to change because it is geologically active. Many scientists believe that some of these changes can be predicted. For example, based on studies of the rate that the seafloor is spreading in the Red Sea, some geologists predict that in 200 million years the Red Sea will be the same size as the Atlantic Ocean is today. Other scientists predict that the continent of Asia will break apart millions of years from now, and as it does, Lake Baikal in Siberia will become a vast ocean, separating two landmasses that once made up the Asian continent.

In the far, far distant future, however, scientists believe that Earth will become an uninhabitable planet, scorched by the Sun. Knowing the rate at which nuclear fusion occurs in the Sun and knowing the Sun’s mass, astrophysicists (scientists who study stars) have calculated that the Sun will become brighter and hotter about 3 billion years from now, when it will be hot enough to boil Earth’s oceans away. Based on studies of how other Sun-like stars have evolved, scientists predict that the Sun will become a red giant, a star with a very large, hot atmosphere, about 7 billion years from now. As a red giant the Sun’s outer atmosphere will expand until it engulfs the planet Mercury. The Sun will then be 2,000 times brighter than it is now and so hot it will melt Earth’s rocks. Earth will end its existence as a burnt cinder. See also Sun.

Three billion years is the life span of millions of human generations, however. Perhaps by then, humans will have learned how to journey beyond the solar system to colonize other planets in the Milky Way Galaxy and find another place to call “home.”

Reviewed By:
Alan V. Morgan

All about animals

2009-01-02T16:34:00.000-08:00

Animal

INTRODUCTION

Swallowing Snake

BBC Worldwide Americas, Inc.

Animal, multicellular organism that obtains energy by eating food. With over 2 million known species, and many more awaiting identification, animals are the most diverse forms of life on earth. They range in size from 30-m (100-ft) long whales to microscopic organisms only 0.05 mm (0.002 in) long. They live in a vast range of habitats, from deserts and Arctic tundra to the deep-sea floor. Animals are the only living things that have evolved nervous systems and sense organs that monitor their surroundings. They are also the only forms of life that show flexible patterns of behavior that can be shaped by past experience. The study of animals is known as zoology.

Animals are multicellular organisms, a characteristic they share with plants and many fungi. But they differ from plants and fungi in several important ways. Foremost among these is the way they obtain energy. Plants obtain energy directly from sunlight through the process of photosynthesis, and they use this energy to build up organic matter from simple raw materials. Animals, on the other hand, eat other living things or their dead remains. They then digest this food to release the energy that it contains. Fungi also take in food, but instead of digesting it internally as animals do, they digest it before they absorb it.

Fierce Hunter

A flying eagle snatches a fish from the water with its long, curved talons. It will carry the fish to a feeding place on land before devouring it. Eagles hunt only during the day; by night, they perch safely in their nests or in some other high spot.

Oxford Scientific Films

Most animals start life as a single fertilized cell, which divides many times to produce the thousands or millions of cells needed to form a functioning body. During this process, groups of cells develop different characteristics and arrange themselves in tissues that carry out specialized functions. Epithelial tissue covers the body’s inner and outer surfaces, while connective tissue binds it together and provides support. Nervous tissue conducts the signals that coordinate the body (see Nervous System), and muscle tissue–which makes up over two-thirds of the body mass of some animals–contracts to make the body move. This mobility, coupled with rapid responses to opportunities and hazards, is one feature that distinguishes animals from other forms of life.

Some kinds of animal movement, such as the slow progress of a limpet as it creeps across rocks, are so slow that they are almost imperceptible. Others, such as the attacking dive of a peregrine falcon or the leap of a flea, are so fast that they are difficult or even impossible to follow. Many single-celled organisms can move, but in absolute terms, animals are by far the fastest-moving living things on earth.

Animal life spans vary from less than 3 weeks in some insects to over a century in giant tortoises. Some animals, such as sponges, mollusks, fish, and snakes, show indeterminate growth, which means that they continue to grow throughout life. Most, however, reach a pre-defined size at maturity, at which point their physical growth stops.

TYPES OF ANIMALS

Animal Kingdom

Kingdom Animalia includes more than one million living species, grouped into more than 30 phyla. Vertebrates, members of the phylum Chordata, comprise only one percent of these organisms. Phylum Arthropoda is more successful in sheer numbers, total mass, and distribution than all other groups of animals combined. The remaining animal phyla are composed of mostly marine-dwelling organisms. Illustrated here is the evolutionary relationship between all of these groups.

Like all living things, animals show similarities and differences that enable them to be classified into groups. Birds, for example, are the only animals that have feathers, while mammals are the only ones that have fur. The scientific classification of animals began in the late 18th century. At this time, animals were classified almost entirely by external features, mainly because these are easy to observe. But external features can sometimes be misleading. For example, in the past, comparison of physical features led to whales being classified as fish and some snakes being classified as worms.

Body Plans of Animals

The basic body plan of an animal, shown here in cross section, is one of the characteristics used to classify animals into separate phyla. Cnidarians such as jellyfishes and sea anemones have two layers of tissue, endoderm and ectoderm, surrounding a digestive cavity. In some animals a third layer, mesoderm, develops between the endoderm and ectoderm. Among these, flatworms and ribbon worms are called acoelomates, because they lack a separate body cavity, or coelom. Nematodes have an extra, epithelial-lined cavity called a pseudocoelom, but only animals such as annelids and chordates have a true coelom, a fluid-filled chamber situated actually within the mesoderm.

Presently, animals are classified according to a broader range of characteristics, including their internal anatomy, patterns of development, and genetic makeup. These features provide a much more reliable guide to an animal’s place in the living world. They also help to show how different species are linked through evolution. Scientists divide the animal kingdom into approximately 30 groups, each called a phylum (plural phyla).

Vertebrates and Invertebrates

Vertebrate Embryos

Vertebrates that evolved from fish pass through similar embryonic stages. As a flexible notochord develops in the back, blocks of tissue called somites form along each side of it. These somites will become major structures, such as muscle, vertebrae, connective tissue, and, later, the larger glands of the body. Just above the notochord lies a hollow nerve cord. Such similarities formed the basis for German biologist Ernst Haeckel’s biogenetic law, which states that an animal’s embryonic development recapitulates its evolution. Although scientists now know that this law does not hold absolutely, Haeckel’s idea has remained influential.

One phylum of animals, the chordates, has been more intensively studied than has any other, because it comprises nearly all the world’s largest and most familiar animals as well as humans. This phylum includes mammals, birds, reptiles, amphibians, and fish together with a collection of lesser-known organisms, such as sea squirts and their relatives (see Tunicates). The feature uniting these animals is that at some stage in their lives, all have a flexible supporting rod, called a notochord, running the length of their bodies. In the great majority of chordates, the notochord is replaced by a series of interlocking bones called vertebrae during early development. These bones form the backbone, and they give these animals their name—the vertebrates.

Vertebrates total about 40,000 species. Thanks to their highly developed nervous systems and internal skeletons, they have become very successful on land, sea, and air. Yet vertebrates account for only about 2 percent of animal species. The remaining 98 percent, collectively called invertebrates, are far more numerous and diverse and include an immense variety of animals from sponges, worms, and jellyfish to mollusks and insects. The only feature these diverse creatures share in common is the lack of a backbone.

Some invertebrate phyla contain relatively few species. An extreme example is the phylum Placozoa, which contains just one species. Measuring less than 0.5 mm (0.02 in) across, this unique animal was first discovered in 1883 in a saltwater aquarium in Austria. Its flat body consists of just two layers of cells, making it the simplest known member of the animal kingdom, although not the smallest. Another minor phylum, the loriciferans, was classified in 1983 with the chance discovery of a tiny organism dredged up in marine gravel. Several other species of loriciferans have since been identified, but little is known about how they live.

At the other end of the spectrum, some invertebrate phyla contain immense numbers of species. These major phyla include the annelids (segmented worms), with 12,000 known species; the nematodes (roundworms), also with 12,000 known species; and the mollusks, including bivalves, snails, and octopuses, with at least 100,000 species. The arthropods, with about 1 million known species, include the insects, spiders, and crustaceans. These figures include only species that have been described and named, which are only a portion of those that actually exist. Some biologists estimate that the total number of nematode species may be as high as a quarter of a million, while the total number of arthropods could exceed 10 million.

Compared to vertebrates, most invertebrates are animals of modest dimensions. Giant squids, which are the largest invertebrates, can exceed 18 m (60 ft) in length, but the great majority of invertebrate animals are less than 2.5 cm (1 in) long. Their small size enables them to exploit food sources and infiltrate habitats that larger animals cannot use, but it also leaves them exposed to changing environmental conditions. This is not often a problem in the sea, but it can create difficulties on land. Land-dwelling invertebrates have to cope with the constant threat of drying out, and most of them quickly become inactive in low temperatures.

Cold-blooded and Warm-blooded Animals

Penguin Keeping Its Young Warm

Penguins always return to their ancestral nesting sites to lay their eggs and rear their young. The emperor penguin, the largest of the penguins, lays its single egg during the coldest time of the Antarctic year, when temperatures drop as low as -62 degrees C (-80 degrees F). The egg is incubated on top of the parent’s feet, protected by abdominal folds of skin. Young chicks remain under these abdominal folds until they are able to regulate their own body temperature.

Doug Allan/Oxford Scientific Films

On land, some invertebrates manage to overcome the problem of cold by using muscles to warm themselves. For example, many large moths and bumblebees use a special form of shivering to raise their body temperature to 35°C (95°F) before they take off, which allows them to fly in cool weather. Bees also maintain warm conditions in their nests, which speeds up the development of their young. But in invertebrates as a whole, temperature regulation is very unusual. In vertebrates, on the other hand, it has developed to a high degree.

Vertebrates are customarily divided into cold-blooded and warm-blooded animals, but these labels are not very precise. Biologists normally use the terms ectoderm and endoderm to describe temperature regulation more accurately. An ectoderm is an animal whose temperature is dictated by its surroundings, while an endoderm is one that keeps its body at a constant warm temperature by generating internal heat.

Reptiles, amphibians, and fish are ectoderms. Although they do not maintain a constant warm temperature, some of these animals do manage to raise their body temperature far above that of their surroundings. They do this by behavioral means, such as basking in direct sunshine when the surrounding air is cool. Mammals and birds are endoderms. These animals generate heat through their metabolic processes, and they retain it by having insulating layers of fat, fur, or feathers. Because their bodies are always warm, they can remain active in some of the coldest conditions on earth.

III

ANIMAL HABITATS

Few parts of Earth’s surface are entirely devoid of animal life. Animals cannot survive in places where water is unavailable or permanently frozen, or where temperatures regularly exceed 55° C (130° F). However, in all habitats that lie between these extremes, animal life abounds.

Aquatic Habitats

Tidal Pool

The fluctuation of the tide allows for a unique environment along shorelines. The current continually circulates and replenishes a rich supply of nutrients along beaches, but organisms living there must be adapted to both buffeting waves and frequent shifts from open air to complete submersion. Marine organisms adapt to the constantly changing surroundings in a variety of ways. Starfish use suction-cup feet, barnacles fix permanently to large objects like rocks and boats, and seaweed anchors firmly to the ocean floor. When the tide goes out, pockets of water remain trapped in rocks, depressions in the sand, and natural basins called tidal pools, like the one shown here during low tide.

Pat O'Hara Photography

Animal life first arose in water. Millions of years later, marine and freshwater habitats continue to support a large proportion of the animal life on earth. Aquatic habitats—particularly in the seas and oceans–rarely experience abrupt changes in conditions, which is a major advantage for living things.

In the seas and oceans, the greatest diversity of animal life is found in habitats close to shores. The richest of all these habitats are coral reefs, underwater ridges that form in clear water where the minimum temperature is 20° C (68° F) or above. Coral reefs are composed of an accumulation of the remains of coral—invertebrates with stony skeletons—calcareous red algae, and mollusks. One of the reasons for the great diversity of animal life in reefs is that living coral creates a complex three-dimensional landscape, with many different microhabitats. The smallest crevices provide hiding places for scavengers such as crabs and shrimps, while larger ones conceal predators such as octopuses and moray eels. Over half the world’s fish species live in coral reefs, many hiding away by day and emerging after dark to feed.

Ratfish

The ratfish is a member of a species of deep-water fish related to sharks. The deep-water habitat of the ratfish is dark, cold, and vast. Like many deep-water predators, the ratfish uses several senses to track prey; its eyes are used to located bioluminescent prey The poisonous spine in front of the dorsal fin is used defensively.

Dave Fleetham/Tom Stack and Associates

On reefs and rocky shores, many animals are sessile, meaning that they spend their entire adult lives fixed in one place. These species, which include sponges, barnacles, and mollusks, as well as reef-building corals themselves, typically spend the early part of their lives as drifting larvae, before settling on a solid surface and changing shape. Sessile animals are common in aquatic habitats because it is relatively easy for them to collect food, which typically is pushed in the animal’s direction by water currents. By contrast, very few sessile animals have evolved on land.

In open water, depth has a marked influence on animal lifestyles. The surface layers of the open sea teem with small and submicroscopic animals, which feed either on algae and other plantlike organisms or on each other. These animals form part of the plankton, a complex community of living things that drifts passively with the currents. Many planktonic animals can adjust the depth at which they float, but larger animals such as fish, squid, and marine mammals, are strong enough to commute between the surface and the depths far below.

Even in the clearest water, light quickly fades with increasing depth. Deeper than about 150 m (500 ft), not enough light penetrates for photosynthesis to occur, so algae are unable to survive. With increasing depth, water pressure rises and temperature falls, ultimately coming close to the freezing point on the ocean floor. Despite these extreme conditions, animal life is found in the ocean’s greatest depths, fueled by the constant rain of organic debris that drifts down from far above. In a habitat where prey is widely scattered, many deep-sea fish can swallow animals larger than themselves, an adaptation that allows them to go weeks or months between meals.

Land Habitats

Zoogeographic Regions

The world’s land area is divided into six zoogeographic regions, each with different fauna. Within these regions, animals are grouped by the particular habitat they occupy. Land animals will tend toward habitats based on many factors, including indigenous food and availability of natural protection from predators.

On land, animal habitats are strongly influenced by climate, the combination of precipitation and temperature conditions experienced in a region. At or near the equator, year-round moisture and warmth generates a constant supply of food. Further north or south, seasonal changes become much more pronounced, shaping the type of animals that live in different habitats and their strategies for survival (see Animal Distribution).

Tropical and subtropical forests are home to by far the largest number of animal species on land. These animals include the majority of the world’s insects, most of its primates, and a large proportion of its birds. Tropical forests have existed longer than any other forests on earth and their plants and animals have evolved an elaborate web of interrelationships.

Much of the animal life of tropical forests is still poorly known, and new species are constantly being discovered. The majority of these newly identified animals are invertebrates, but larger animals have also come to light during the 20th century. Major discoveries have included three large but secretive plant-eating mammals: the okapi, discovered in Central Africa in 1900; the kouprey, discovered in the forests of Cambodia in 1937; and the sao la, which was identified in forests bordering Laos and Vietnam in 1993.

Unlike tropical forests, temperate forests provide animals with an abundance of food during spring and summer, but a dearth during the winter. In this habitat, animals have evolved several different strategies for avoiding starvation during the winter months. Food hoarders, such as squirrels and jay birds, bury surplus food during the fall, and dig it up again when other food supplies run out. Other forest animals, such as the common dormouse, avoid food shortages by hibernation, a period of inactivity when body temperature is lowered. A third group of animals—composed chiefly of birds, but also including some bats and insects–migrates to warmer regions before the winter begins and returns again in spring. In boreal forests, which are found in the far north, the seasonal swings are more extreme. Here only a few species stay and remain active during the winter months.

For land animals, the most testing habitats are ones that experience intense drought or extreme cold. Desert animals cope with heat and water shortage by behavioral adaptations, such as remaining below ground by day, and also by physiological adaptations. North American kangaroo rats, for example, can live entirely on dry seeds without ever drinking liquid water. They do this by losing very little moisture from their bodies and using all the “metabolic water” that is formed when food is broken down to release energy.

In tundra and on polar ice, winter air temperatures can fall to below -40° C (-40° F), which is far colder than the temperature of the surrounding seas. The smallest inhabitants of tundra, which include vast numbers of mosquitoes and other biting flies, spend winter in a state of suspended animation and are kept alive by chemical antifreeze within their tissues. The few animals that do remain active on land or ice during winter, such as seals and male emperor penguins, rely on a thick layer of insulating fat to prevent their body heat leaking away. Without this fat, they would die within a matter of minutes.

FEEDING

Animals all feed on organic matter, but their diets and way of obtaining food vary enormously. Some animals are omnivores, meaning that they are capable of surviving on a very wide range of foods. Many other animals, from giant pandas to fleas, have extremely precise requirements and cannot deviate from their highly specialized diet.

Herbivores and Carnivores

Meat Eaters and Plant Eaters

In carnivores (right), the front of the skull has a pair of enlarged canine teeth and the lower jaw moves only in an up and down direction, which assists with the capture and holding of prey. In herbivores (left), the canine teeth are absent and the premolars and molars are well developed. The jaw construction also allows for the lateral movement of the lower jaw in relation to the upper jaw, which helps to provide a grinding motion necessary for rendering plant materials into a state suitable for swallowing and digestion.

Dorling Kindersley

In general, animals eat plants, other animals, or the remains of living things. Plant-eaters, or herbivores, often do not have to search far to find things to eat, and in some cases—for example wood-boring insects—they are entirely surrounded by their food. The disadvantage of a plant-based diet is that it can be difficult to digest and is often low in nutrients.

To overcome the first of these problems, most herbivores have tough mouthparts for chewing and grinding their food. Many plant-eating animals, from termites to cattle, have complex digestive systems containing microorganisms that break down cellulose and other indigestible plant substances, turning them into nutrients that the animals can absorb. The second problem—lack of nutrients–is harder to sidestep, particularly in a diet made up largely of leaves. As a result, leaf-eaters often have to feed for many hours each day to obtain the nutrients that they need.

Alligator Snapping Turtle

The alligator snapping turtle, the largest of the freshwater turtles, has a ridged, camouflaged shell and powerful jaws. When a fish, mistaking a small, wriggling projection on the turtle’s tongue for a worm, swims within reach, the turtle captures it by quickly snapping its jaws shut.

Dorling Kindersley

Carnivores live on flesh from other animals that is often nutrient-rich and easy to digest but difficult to obtain. Finding and capturing this kind of food calls for keen senses. But even though a hunter has acute vision or a highly developed sense of smell, a large proportion of a hunter’s victims manage to escape. If this happens too often, a predator quickly starves.

Some mammalian predators, such as the lion and wolf, increase their chances of success by hunting in groups. While this strategy enables them to tackle larger prey, a successful kill has to be shared among members of the group. But in the animal world as a whole, many other predators adopt a less energy-intensive approach to catching their food. Instead of actively searching out their prey, they position themselves in a suitable location and wait for their prey to come within striking distance.

Anglerfish

Anglerfish have appendages that serve as fishing rods or lures to attract prey, mainly other fish. They are found in oceans all over the world and generally inhabit deep waters. Certain species can grow to lengths of about 1.5 m (5 ft), and have huge mouths capable of swallowing fish of equal size.

Zig Leszczynski/Animals Animals

In this method of hunting, camouflage and other forms of deception play a prominent role. Most animals that use a lie-and-wait strategy blend in with their surroundings, but a few use lures to entice their prey within range. A typical example is the alligator snapping turtle of North America, which waves a ribbon of pink flesh on its tongue that resembles a worm. Any fish venturing toward it is swallowed whole.

In predatory animals, teeth or other mouthparts often play a part in catching and subduing food as well as in preparing it for digestion. These mouthparts include canine teeth in carnivorous mammals, venomous fangs in snakes, and poisonous “harpoons” in some marine mollusks. These harpoons can impale and kill small fish. Each harpoon is used just once, and afterwards it is expelled and another is formed in its place.

Other Feeding Strategies

Giant Anteater

Anteaters are native to Central and South America, inhabiting both forest and open-plain regions. The giant anteater, shown here, is the largest of the species, weighing up to 23 kg (50 lb). The animal is well-adapted to hunt for insects, its sole source of food, because of its long front claws and sticky tongue, which can extend to 60 cm (24 in).

Most predators hunt the largest animals that they can catch without putting themselves unduly at risk. However, some animals concentrate on food items that are much too small to be worth collecting one by one. Instead of catching food individually, they have special feeding adaptations for sweeping it up in bulk.

Whale Shark

Strictly a filter feeder, the whale shark strains plankton and small fish from the upper waters of tropical and subtropical seas by lying motionless beneath the water’s surface. Considered the largest living species of fish, a whale shark may measure more than 15 m (50 ft) in length and weigh more than 18 metric tons. The whale shark poses little risk to humans; however, whale sharks have been known to ram boats that they have mistaken for rival sharks.

James D. Watt/Animals Animals

On land, these animals include insect-eating mammals, such as anteaters and pangolins. Using their long and sticky tongues, they lick up ants and termites and can consume over 20,000 insects a day. In water, this kind of feeding strategy is mirrored by animals called filter feeders, which sieve small animals or food particles from their surroundings. Many of these filter feeders are sessile animals that sieve food from the water immediately around them. Others, such as some whales, scoop up their food while on the move and filter it out in their mouths, using specialized gills or plates of a fibrous material called baleen. This feeding technique is extremely efficient, allowing whales to grow to an immense size.

Female Mosquito Sucking Blood

There are approximately 2,000 species of mosquitoes ranging from the tropics to the Arctic Circle and from sea level to mountaintops. All mosquitoes belong to the insect order Diptera, which includes all of the flies, or two-winged insects. All species of Dipterans have a single pair of wings for flying and a second vestigial pair called halteres, which act as organs of balance. Female mosquitoes have hypodermic mouthparts which enable them to pierce the skin and suck the blood of mammals, birds, reptiles, and other arthropods. The males have reduced mouthparts and feed instead on nectar and water.

Tim Shepherd/Oxford Scientific Films

In another feeding technique, predators seek out sources of food that are much larger than themselves but only eat part of their prey—usually its blood. This way of life is has been pursued with great success by several groups of flying insects, such as mosquitoes and horseflies. But in the animal world as a whole, fluid diets are much more common in animals that feed on plants. Aphids, cicadas, and other true bugs use piercing mouthparts to suck sap from plant stems. Many different animals, including moths, butterflies, hummingbirds, and bats, use probing beaks and tongues to reach nectar in flowers.

Life Cycle of Human Blood Flukes

Flukes of the genus Schistosoma parasitize two hosts. The young hatch from their eggs in rivers and lakes and enter a specific kind of aquatic snail, where they develop into tadpole-like larvae called cercariae. When the cercariae leave the snail, they burrow through the skin of a human host swimming or wading in infested water. Adult flukes mature in the host’s bloodstream and settle in the veins of the gut. Their eggs, deposited in the lining of the human intestine and bladder, pass back into water via the sewage system, and the cycle begins again. More than 200 million people worldwide suffer from schistosomiasis, a disease characterized by the abscesses and bleeding caused by the flukes’ infestation.

To avoid the need to track down food, some animals use a highly specialized feeding strategy, called parasitism (see Parasite). A parasite lives on or inside other animals and simply siphons off some of its host’s food or, more commonly, feeds on the host itself. External parasites, such as fleas, have well-developed senses and adaptations that enable them to cling to their hosts. Internal parasites, such as tapeworms and liver flukes, are highly modified for a life inside their hosts. The sense organs of internal parasites are rudimentary or absent because they do not need to find food or avoid enemies. Instead, they devote their time entirely to the twin tasks of feeding and reproduction.

BREATHING

Evolution of Air-Breathing Organisms

Both the lung structure of air-breathing organisms and the swim bladders of most modern fishes evolved from paired air sacs of primitive bony fishes. In the primitive fish, as in the modern bony fishes, these sacs served as a buoyancy device that inflated and deflated to alter the fish’s depth in the water. In other fish, they became primitive lung structures, repeatedly folding inward to maximize oxygen uptake in an oxygen-deprived environment. Both kinds of fishes improved upon a preexisting adaptation but in so doing evolved into very different groups of organisms.

Wherever they live, animals need oxygen in order to survive. By breathing, or respiring, they extract oxygen from their surroundings and dispose of carbon dioxide waste (see Respiration).

How Fish Breathe

A fish breathes by absorbing oxygen from the water it drinks. Water flows into the mouth, through the gills, and out of the body through gill slits. As water flows through the gills, the oxygen it contains passes into blood circulating through gill structures called filaments and lamellae. At the same time, carbon dioxide in the fish’s bloodstream passes into the water and is carried out of the body.

Very small animals do not need any special adaptations for obtaining oxygen. Oxygen simply diffuses in through their body surface, with carbon dioxide traveling out the same way. Larger animals cannot rely on this system because they have a much bigger volume relative to their surface area. To obtain sufficient oxygen, large animals have to boost their oxygen intake by using special respiratory organs. In water, many animals breathe by using gills. A typical gill consists of a stack of thin flaps connected to the animal’s blood supply. Water moves past the flaps in a one-way flow, either when the animal moves, or when it pumps water through its body. The flaps extract oxygen from the water and pass it into the blood, which transfers it to needed tissues. The blood releases carbon dioxide in exchange.

Axolotl Showing External Gills

The axolotl is actually the aquatic larval stage of the brown salamander. Axolotls are of interest to scientists because not all axolotls metamorphose, or change, into adult salamanders. More interestingly, those axolotls that do not transform may become sexually mature while in the larval stage. In captivity, axolotls can be induced to change into adult salamanders by the addition of thyroid extract to the surrounding water.

G.I. Bernard/Oxford Scientific Films

Gills do not work on land because their flaps collapse and stick together. Instead, land animals have evolved two different kinds of respiratory organs: tracheal systems and lungs. Tracheal systems are found in insects and many other arthropods. They consist of slender hollow tubes, called tracheae, that reach deep into the body, delivering oxygen from outside. Lungs are hollow cavities that have a large surface area. They are found in vertebrates and also in some invertebrates, such as terrestrial mollusks.

Dolphin Surfacing for Air

A Pacific whitesided dolphin breaks the surface of the water while swimming to breathe through a blowhole on the top of its head. Underwater, a dolphin communicates with whistles emitted in single-toned squeals to convey alarm, sexual excitement, and perhaps other emotional states. Dolphins inhabit all the world’s oceans, using their streamlined bodies to reach underwater speeds of 40 km/h (25 mph). This swimming ability coupled with sharp teeth enables dolphins to capture fish and squid, their principal prey.

In tracheae and most lungs, gases move in a two-way flow. Most vertebrates actively pump air in and out of their lungs to step up the rate of gas exchange. By stretching and squeezing their bodies, some arthropods behave in a similar way.

MOVEMENT

All animals can move parts of their bodies. The majority are also capable of locomotion—movement of the whole body from place to place. Many simple animals, such as rotifers and flatworms, move with the help of microscopic hairlike structures called cilia. These beat in a coordinated way, propelling the animal through water or making it glide over solid surfaces at the rate of a few inches an hour. Another form of creeping movement, seen in earthworms, involves changes in body shape. The worm’s segments extend and contract in a set sequence, allowing it to force its way through the surrounding soil.

Some of the earthworm’s relatives have flaps called parapodia that help them to move, but even with these, their speed is fairly modest. With a few notable exceptions—such as squid and octopuses, which can move by a form of jet propulsion—the fastest animals by far are ones that have skeletons and jointed limbs.

Jointed Limbs

Cheetah Running

The cheetah is believed to be the fastest animal on Earth, reaching speeds of more than 97 km/h (60 mph) while chasing prey. Wildebeests, gazelles, impalas, and other hoofed mammals make up much of the cheetah’s diet. Cheetahs generally stalk their prey to within 10 m (33 ft) and then burst into a sprint to close the gap. Studies indicate that approximately half of the chases initiated by the cheetah are successful.

National Geographic Society/Worldwide Television News

Jointed limbs are found in only two groups of animals: the arthropods and vertebrates. An arthropod’s limbs are made of a number of hard tubular segments, which form part of its external skeleton, or exoskeleton. The muscles that operate them are hidden away inside this strong outer framework. In vertebrates, the plan is reversed. The bony skeleton forms an internal framework, with muscles attached around it.

Pigeon in Flight

A bird moves its wings in two ways during flapping flight. The part of the wing closest to the bird’s body moves up and down. Simultaneously, the tip of the wing moves in a circular motion, propelling the bird forward. The way a bird flies depends on the shape of its wings. Most small birds flap their wings the entire time they are airborne, while gulls and other large birds with long, pointed wings soar or glide. The fastest fliers have sharply tapered wings.

Oxford Scientific Films

During the course of evolution, both these kinds of limbs have become modified in many different ways. Aquatic animals often have paddlelike limbs that push against the water, enabling them to speed away from predators or after food, or to maneuver their way around confined spaces. On land, the fastest animals, such as the horse and cheetah, have long legs and a flexible backbone, which helps to increase the length of their stride. Land animals that move by jumping often have highly developed hind legs, with extra-large muscles. In fleas, the muscles squeeze an elastic material called resilin, which flicks the legs back when released. This extremely rapid flick is faster than a jump triggered by muscles alone, and it throws a flea up to 30 cm (12 in) into the air.

Many animals can glide, but only insects, birds, and bats are capable of powered flight. The fastest flying insects are dragonflies, which can reach speeds of about 29 km/h (about 18 mph) in short bursts. However, in terms of speed and endurance, birds are by far the most successful animal aviators. Swans and geese can cruise at 64 km/h (40 mph) for many hours at a time, while peregrine falcons can briefly reach 145 km/h (90 mph) when they swoop down on their prey.

Patterns of Movement

Migrating Wildebeest

The blue wildebeest, or brindled gnu, migrates annually from Kenya to northern South Africa. Along their migratory route the wildebeests stop at watering holes on the Grameti River, where they become the chief source of food for Nile crocodiles. Scientists speculate that the crocodiles of the Grameti River may feed only once a year, when blue wildebeests arrive during their annual migration.

John Downer/Oxford Scientific Films

Being able to move gives animals many advantages, but it also generates its own demands. For any animal, random movement can be unhelpful or even dangerous. To be useful, movement has to be carefully guided.

Animals are guided by their senses, which provide feedback about their changing surroundings. In animals that have radial symmetry (symmetry around a central point), such as jellyfishes, sensory nerves are arranged more or less evenly around the body. This arrangement makes the animal equally sensitive to stimuli from any direction. In bilaterally symmetrical animals (animals made of equal halves), sensory nerves are concentrated in the head. They convey signals to the brain from organs such as ears and eyes, telling an animal about the surroundings that it is about to encounter.

Monarch Migration

The monarch butterfly, Danaus plexippus, is known for its extraordinarily long migrations. During the summer months, monarchs can be found throughout the continental United States and parts of Canada, and they migrate to the California coast and central Mexico for the winter. The longest recorded flight for a tagged adult is 2,900 km (1,800 mi). A large number of monarchs spend their winters in the mountains west of Mexico City. Scientists speculate that the mountainous climate provides a favorable mix of moist air and cool, but not freezing, temperatures. These conditions keep the butterfly from drying out and keep its metabolism low enough to conserve fat stores but high enough to maintain life.

G. G. Dimijian/Photo Researchers, Inc.

These sensory systems help animals to move toward food and away from possible danger. On a longer time span, they also guide them through much more complex patterns of movement that are essential for their survival. These movements include special kinds of behavior needed to locate a partner, and also seasonal movements or migrations.

Some of the shortest migrations are carried out by microscopic flatworms that live on sandy shores. These worms migrate up to the surface of the sand at low tide and back into it at high tide—a total distance of about 20 cm (about 8 in) roughly twice a day. In the open ocean, many planktonic animals carry out larger daily migrations, rising to the surface at dusk and then sinking at sunrise. By doing this, they reduce the chances of being eaten.

The longest migrations are annual ones, undertaken by animals in response to the changing seasons. By carrying out these journeys, animals can breed in places where food is abundant for just a few months each year. Long-distance annual migration is seen in some plant-eating mammals, such as wildebeest and caribou, and also in whales, but it is most common in animals that fly. Some birds, such as terns and shearwaters, migrate over 32,000 km (20,000 mi) each year. Research has shown that during these epic journeys, they use a variety of cues to help them navigate. These include familiar landmarks, the position of the sun and stars, and the also the orientation of Earth’s magnetic field (see Animal Migration).

VII

REPRODUCTION

Like all living things, animals have limited life spans. Although individual animals eventually die, reproduction ensures that they hand on their characteristics to future generations. Animals reproduce at markedly different rates, but all have the potential to increase their numbers if resources allow it. In practice, sharp increases are rare, kept in check by predators and food shortages.

Forms of Reproduction

Animal reproduction takes two overall forms. In the first form, called asexual reproduction, animals produce offspring without needing a partner. Asexual reproduction is most common in simple animals such as flatworms and cnidarians. In flatworms, the parent often develops a constriction in its body, and the rear part eventually tears itself free. The rear part grows a new head, while the front part grows a new tail. Some cnidarians can also divide in two, but many reproduce by a different process, called budding. During budding, a small outgrowth of the body slowly develops into a complete new animal, which eventually takes up life on its own.

Asexual reproduction also occurs in insects such as aphids and in a few unusual vertebrates, such as whiptail lizards. However, in general, it is rarely used as an animal’s sole method of reproduction. This is because asexual reproduction produces offspring that are genetically identical to their parent. They inherit all their parent’s weak points and are equally vulnerable if a disease or other changes in the environment threaten the group’s survival.

A second and much more common form of reproduction, sexual reproduction, involves two parents. The parents produce sperm and egg cells (gametes), which are brought together to form a fertilized cell (zygote) with a new and unique combination of genes. In this genetic lottery, offspring inherit unique combinations of characteristics that increase the likelihood that at least some individuals in the population can survive changes in the environment.

Sexual reproduction is used by the vast majority of the world’s animals. However, a significant number of species, particularly in the world of insects, use both forms of reproduction at different stages of their life cycles. They reproduce asexually when food is abundant, but turn to sexual reproduction when conditions become more severe.

Reproductive Strategies

Modes of Frog Reproduction

BBC Worldwide Americas, Inc.

Asexual reproduction is relatively easy to achieve because it involves only a single animal. Sexual reproduction is much more complex because the partners often have to find each other and precisely coordinate their reproductive behavior. In most cases, each partner is either male or female, but in some animals—such as earthworms, slugs, and snails–each one is a hermaphrodite, an animal that has both male and female organs. Hermaphrodites usually fertilize each other, with both partners producing young (see Hermaphroditism).

Western Grebe Courtship

During the spring, western grebes perform spectacular courtship dances. In the “rushing” display, the mating pair swim side-by-side with their wings held back, their long necks arched, and their yellow beaks angled upward. They swim so quickly that their bodies are pushed up out of the water and they appear to run across the surface. After courtship the male and female build a floating nest out of plant material.

BBC Worldwide Americas, Inc.

Most aquatic animals shed their eggs and sperm into the water, where external fertilization takes place. In corals and many other sessile species, the moment of spawning is often triggered by the tides, maximizing the chances that the egg and sperm will meet. In a minority of marine animals, fertilization is internal, meaning that the male mates with the female, inserting his sperm into her body. For this to work, the male needs special adaptations to make the transfer. Male sharks and rays use special claspers that are attached to their pelvic fins, while barnacles, which are often hermaphrodites, use a threadlike penis that can be almost as long as their bodies.

Internal Fertilization

Terrestrial vertebrates clasp each other tightly during copulation, the act by which the male deposits his sperm into the female’s reproductive tract. In the giant Galápagos tortoises pictured here, mating may take hours.

Tui De Roy/Oxford Scientific Films

On land, external fertilization is rare because egg and sperm cells cannot survive for long in the open. As a result, almost all land animals must mate to trigger internal fertilization in order to reproduce. Different groups of animals have evolved a wide variety of mechanisms to make sure that males and females manage to locate suitable partners. Some female insects emit chemicals called pheromones, which guide males towards them, while others use sound signals or biochemically produced light (see Bioluminescence). In birds, elaborate plumage and courtship displays help to attract females towards the males (see Animal Courtship and Mating).

Honey Bee From Egg to Adult

The queen honey bee may lay 1500 eggs in a single day. Worker bees feed the wormlike larva constantly—as many as 1300 times a day—after it hatches, sealing the cell when the grub has grown to fill it. The larva pupates in about 12 days, and the adult bee chews through the wax cap of its cell approximately three weeks after the eggs were first laid. Newly emerged adults perform various maintenance tasks until they are ready to begin foraging outside the hive.

The males of many insects and virtually all mammals use a penis to transfer sperm to the female, who harbors the eggs, in a process known as copulation. The penis ensures that sperm is transferred successfully without being carried away by wind, water, or other environmental elements. Most birds and reptiles mate using a cloaca, a single opening located on the lower abdomen. During mating, these animals align their cloacas for transfer of sperm. Some birds, such as bald eagles, can perform this feat in mid-air.

Once a female has mated, egg development can proceed in two different ways. In oviparous species, which include the majority of vertebrates except mammals, and also most insects, the fertilized eggs are laid and develop outside the mother’s body. In viviparous animals, which include nearly all mammals together with some reptiles and sharks, the young develop inside the mother and are born live.

Most animals that are born live look similar to their parents, although they are not fully developed. By contrast, many egg-laying invertebrates look completely different from their parents when they hatch and often live in a completely different way. Known as larvae, these young change shapes as they grow up, during a process called metamorphosis. Larvae are also found in some fish and most amphibians.

Mating Systems

Animals that reproduce sexually have evolved a wide variety of different systems for maximizing the number of young that can be raised. In the simplest system, each female is partnered by a male, and the partnership lasts for life. In more complex systems, the fittest adults have many partners while others have none at all.

In polygynous breeding systems, successful males mate with more than one female. Polygyny is common in birds, particularly in species where the males establish breeding territories that provide access to food. A male with a good territory may attract several mates, while one with an inferior territory may attract few or none. Polygyny can also be seen in some mammals and is taken to extremes in species such as elephant seals. The largest and most powerful male elephant seals, weighing up to four times as much as the females, clash viciously for dominance on a breeding beach. A successful male can assemble a harem of over twenty females, but weaker males are excluded from breeding altogether.

In polyandrous breeding systems, one female mates with several males. This kind of breeding system is rare and usually occurs in species where the males take on the work of raising the young. An example of a polyandrous bird is the North American spotted sandpiper. In this species, females compete for males. A single female can lay up to five sets, or clutches, of eggs, and each clutch is incubated by a different partner.

The most specialized mating systems of all occur in animals that form permanent family groups. In social insects, which include many bees and wasps and all ants and termites, each group or colony is founded by a single female or queen. The queen is the only individual in the colony to reproduce. Her offspring, which can number more than a million, forage for food, maintain the nest, and care for the young.

Parental Care

Killer Whale Family

Showing the characteristic contrasting white patches above the eyes and under the jaws, a male and female killer whale, Orcinus orca, swim protectively on either side of their baby. Killer whales maintain close ties to the social structure of their natal pods, or groups, for life. To prevent inbreeding, however, the whales typically seek mates outside of their original pod.

David E Myers/Tony Stone Images

With the exception of birds, the majority of egg-laying animals play no part in helping their young to survive. A large proportion of their young die, and to offset this, they often produce a huge number of eggs. A housefly, for example, can lay over a thousand eggs in the course of its life, while a female cod can lay 3 million.

Most amphibians and reptiles lay smaller clutches of eggs, and some of them remain with their eggs and guard them until they hatch. Birds lay smaller clutches still, and the parents incubate the eggs, or keep them warm until they hatch, and continue to care for their young once they have hatched. Most ground-nesting species protect their young and lead them to food, but typical tree-nesting birds provide their young with both food and shelter until they are able to fend for themselves. Without this parental care, the young birds would have no hope of survival.

American Redstart Feeding Young

A female American redstart turns toward her offspring, which begs for food with its gaping mouth.

Ralph A. Reinhold/Animals Animals

Parental care is equally important in mammals, which provide food for their young in the form of milk. Raising a family in this way creates a close link between the mother and her young. This method also allows the young to learn important patterns of behavior by watching their mother at work. In small rodents, this learning period lasts for just a few days, but in larger mammals, it can last for more than a year.

VIII

STRATEGIES FOR SURVIVAL

Animals in Extremes

Discovery Enterprises, LLC

In the living world, resources such as food and space are limited. As a result, survival is a constant struggle. Through evolution, animals have developed a range of adaptations that give them the best chances of success.

The most obvious of these adaptations are physical ones that affect the shape or structure of an animal’s body. Equally important, although often less conspicuous, are adaptations that affect behavior and body processes. Together, these different adaptations allow each species to pursue a distinctive way of life.

Physical Adaptations

Tulip-Tree Beauty

The tulip-tree beauty is a large moth that feeds on the foliage of the tulip tree as a caterpillar. Found from southern Canada to Florida, the moths often have banded coloration that camouflages them against tree bark.

John R. MacGregor/Peter Arnold, Inc.

The need to eat exposes animals to the danger of being attacked and eaten themselves. To avoid this fate, all animals have physical adaptations that enable them to escape being attacked or to survive an attack once it is underway.

Turtle Skeleton

The turtle or tortoise body is encased in a shell made up of a series of bony plates covered with a horny shield. The vertebrae and ribs are fused to the inside of this shell, which gives it additional support and strength. It is impossible for turtles or tortoises to crawl out of their shells. Turtles have a relatively flattened shell and are aquatic, while tortoises have a dome-shaped shell and are terrestrial.

Dorling Kindersley

The simplest form of defense is a rapid escape, which calls for keen senses and well-developed systems for movement. Many plant-eating mammals depend on this strategy for survival and must maintain a constant lookout for danger. A less-demanding survival strategy, found in many small animals such as insects, involves deception. These animals use camouflage to blend in with their backgrounds, or they mimic inedible objects such as twigs or bird droppings. If a predator does come too close, they still have the option of making a dash for safety.

Regal Horned Lizard

The well-camouflaged regal horned lizard, Phrynosoma solare, requires so many ants a day to sustain it that it almost always is found near an anthill. Regal horned lizards usually fare poorly in captivity, where quantities of ants are often insufficient. It remains motionless if approached, but if picked up, it may attempt to disconcert its attacker by puffing up its body and squirting blood, sometimes as far as a few feet, from a reserve behind its eyes. The regal horned lizard is the largest of the American species of horned lizards and can be recognized by the four large horns on the back of its head.

M.P.L. Fogden/Oxford Scientific Films

A more sophisticated form of mimicry occurs in animals that resemble species that are poisonous. This is common in insects, and it also occurs in some snakes. Poisonous insects, such as bees and wasps, are often brightly colored to warn other animals that they are best left alone. By adopting these colors and developing similar body shapes, non-poisonous insects benefit from the same protection. The physical adaptations involved can be elaborate. The hornet clearwing moth, for example, is yellow and brown like a stinging hornet. On its first flight, it loses most of its wing scales, resulting in transparent wings that make the resemblance even more convincing.

Snowshoe Hare in Summer and Winter

The snowshoe hare uses camouflage to hide from predators. The summer coat of the snowshoe hare provides excellent camouflage among the grasses and shrubs of its summer habitat, while the white winter coat blends in perfectly with the snowy forest.

Judd Cooney/Oxford Scientific Films

An alternative defense, seen in a wide range of animals, uses armor or spines to fend off an attack. Animal armor includes hard shells, overlapping scales, and in the case of armadillos, bands of hardened plates connected by areas of softer skin. If they are threatened, many of these animals can shut their bodies away inside their armor, making them difficult to attack. The disadvantage of this defense is that the animal cannot escape. If its armor is broken open, death is almost certain.

Behavioral Adaptations

Egyptian Vulture

This Egyptian vulture holds a stone in its beak in preparation for smashing an ostrich egg. Egyptian vultures are unusual among birds because they use stones as tools for obtaining food.

Roy Toft/Tom Stack and Associates

In simple animals, behavior is governed almost entirely by instinct, meaning that it is pre-programmed by an animal’s genes. In more complex animals, instinctive behavior is often modified by learning, producing more-flexible responses to the outside world.

Many forms of behavior help animals to survive severe environmental conditions. Two examples are hibernation, which enables animals to survive cold and food shortages in winter; and estivation, which allows animals to survive drought and heat in summer. True hibernators, such as bats and some rodents, become completely inactive during winter, and their body temperature falls close to freezing. While in this state, they survive entirely on food reserves stored in their bodies. Estivating animals, which include land snails and some amphibians, seal themselves up when conditions become dry and only become active again when it rains. Between these two extremes, many other animals show less drastic patterns of behavior that are triggered by cold or heat. Winter wrens, for example, often crowd together for sleep when temperatures fall below freezing. On warmer nights, they sleep on their own.

Special forms of behavior also help animals to find food, to avoid being eaten, and to protect their young. One of the most advanced forms of this behavior is the use of tools. Several kinds of animals, particularly primates and birds, pick up implements such as twigs and stones and use them to get at food. More rarely, some tool-using animals seek out a particular object and then shape it so that it can be used. Woodpecker finches probe for insect grubs by making tools from cactus spines, and chimpanzees sometimes dig for termites using specially prepared twigs.

Defensive behavior is exhibited by individual animals and also by animal groups. Group defense is common in herding mammals, particularly in species such as the musk-ox, which form a protective ring around their calves when threatened by wolves. It can also be seen in swallows, starlings, and other songbirds, which instinctively mob hawks and other birds of prey. By grouping together to harass their enemies, they reduce the chances that they or their young will be singled out and attacked.

Individual defensive behavior is often based on threatening gestures that make an animal look larger or more dangerous than it actually is. Sometimes it involves some highly specialized forms of deception. One of the most remarkable is playing dead. Seen in animals such as the Virginia opossum and some snakes, this last-ditch defense is effective against predators that habitually hunt moving prey but leave dead animals alone. After the predator has inspected the “dead” animal and moved on, the prey comes back to life and makes its escape.

ORIGINS OF ANIMALS

Purple and Yellow Tube Sponge

The purple and yellow tube sponge displays one of the many different body forms typical of sponges. Sponges, considered to be the most primitive of the multicellular animals, are represented in the fossil record back to the Cambrian Period, at least 600 million years ago. The interior body cavities of sponges provide shelter for a variety of small crabs, sea stars, and other marine invertebrates.

Joe Dorsey/Oxford Scientific Films

Most biologists agree that animals evolved from simpler single-celled organisms. Exactly how this happened is unclear, because few fossils have been left to record the sequence of events. Faced with this lack of fossil evidence, researchers have attempted to piece together animal origins by examining the single-celled organisms alive today.

Modern single-celled organisms are classified into two kingdoms: the prokaryotes and protists. Prokaryotes, which include bacteria, are very simple organisms, and lack many of the features seen in animal cells. Protists, on the other hand, are more complex, and their cells contain all the specialized structures, or organelles, found in the cells of animals. One protist group, the choanoflagellates or collar flagellates, contains organisms that bear a striking resemblance to cells that are found in sponges. Most choanoflagellates live on their own, but significantly, some form permanent groups or colonies.

This tendency to form colonies is widely believed to have been an important stepping stone on the path to animal life. The next step in evolution would have involved a transition from colonies of independent cells to colonies containing specialized cells that were dependent on each other for survival. Once this development had occurred, such colonies would have effectively become single organisms. Increasing specialization among groups of cells could then have created tissues, triggering the long and complex evolution of animal bodies.

This conjectural sequence of events probably occurred along several parallel paths. One path led to the sponges, which retain a collection of primitive features that sets them apart from all animals. Another path led to two major subdivisions of the animal kingdom: the protostomes, which include arthropods, annelid worms, mollusks, and cnidarians; and the deuterostomes, which include echinoderms and chordates. Protostomes and deuterostomes differ fundamentally in the way they develop as embryos, strongly suggesting that they split from each other a long time ago.

Animal life first appeared perhaps a billion years ago, but for a long time after this, the fossil record remains almost blank. Fossils exist that seem to show burrows and other indirect evidence for animal life, but the first direct evidence of animals themselves appears about 650 million years ago, toward the end of the Precambrian period. At this time, the animal kingdom stood on the threshold of a great explosion in diversity (see Biodiversity). By the end of the Cambrian Period, 150 million years later, all of the main types of animal life existing today had become established.

Moving onto Land

When the first animals evolved, dry land was probably devoid of any kind of life, except possibly bacteria. Without terrestrial plants, land-based animals would have had nothing to eat. But when plants took up life on land over 400 million years ago, that situation changed, and animals evolved that could make use of this new source of food. The first land animals included primitive wingless insects and probably a range of soft-bodied invertebrates that have not left fossil remains. The first vertebrates to move onto land were the amphibians, which appeared about 370 million years ago.

For all animals, life on land involved meeting some major challenges. Foremost among these were the need to conserve water and the need to extract oxygen from the air. Another problem concerned the effects of gravity. Water buoys up living things, but air, which is 750 times less dense than water, generates almost no buoyancy at all. To function effectively on land, animals needed support.

In soft-bodied land animals such as earthworms, this support is provided by a hydrostatic skeleton, which works by internal pressure. The animal’s body fluids press out against its skin, giving the animal its shape. In insects and other arthropods, support is provided by the exoskeleton (external skeleton), while in vertebrates it is provided by bones. Exoskeletons can play a double role by helping animals to conserve water, but they have one important disadvantage: unlike an internal bony skeleton, their weight increases very rapidly as they get bigger, eventually making them too heavy to move. This explains why insects have all remained relatively small, while some vertebrates have reached very large sizes.

Speciation and Extinction

Galápagos Finches

The fourteen species of finch that inhabit the Galápagos Islands are believed to have evolved from a single species resembling the blue-black grassquit, Volatinia jacarina, abundant in Latin America and the Pacific coast of South America. The ancestral finch, with its short, stout, conical bill specialized for crushing seeds, probably migrated from the mainland to the Galápagos Islands. Its descendants, free to exploit the resources they would otherwise share with warblers, woodpeckers, and other birds, adapted to the available range of habitats (tree, cactus, or ground) and food (seeds, cactus, fruit, or insects). The size and shape of their bills reflect these specializations, an example of adaptive radiation.

Like other living things, animals evolve by adapting to and exploiting their surroundings. In the billion-year history of animal life, this process has created vast numbers of new species, each capable of using resources in a slightly different way. Some of these species are alive today, but these are a minority; an even greater number are extinct, having lost the struggle for survival.

Speciation, the birth of new species, usually occurs when a group of living things becomes isolated from others of their kind (see Species and Speciation). Once this has occurred, the members of the group follow their own evolutionary path and adapt in ways that make them increasingly distinct. After a long period—typically thousands of years—their unique features mean that they can no longer breed with their former relatives. At this point, a new species comes into being.

In animals, this isolation can come about in several different ways. The simplest form, geographical isolation, occurs when members of an original species become separated by a physical barrier. One example of such a barrier is the open sea, which isolates animals that have been accidentally stranded on remote islands. As the new arrivals adapt to their adopted home, they become more and more distinct from their mainland relatives. Sometimes the result is a burst of adaptive radiation, which produces a number of different species. In the Hawaiian Islands, for example, 22 species of honeycreepers have evolved from a single pioneering species of finch-like bird.

Another type of isolation is thought to occur where there is no physical separation. In this case, differences in behavior, such as mate selection, may sometimes help to split a single species into distinct groups. If the differences persist for a long enough time, new species are created.

The fate of a new species depends very much on the environment in which it evolved. If the environment is stable and no new competitors appear on the scene, an animal species may change very little in hundreds of thousands of years. But if the environment changes rapidly and competitors arrive from outside, the struggle for survival is much more intense. In these conditions, either a species changes, or it eventually becomes extinct.

During the history of animal life, on at least five occasions, sudden environmental change has triggered simultaneous extinction on a massive scale. One of these mass extinctions occurred at the end of the Cretaceous Period, about 65 million years ago, killing all dinosaurs and perhaps two-thirds of marine species. An even greater mass extinction took place at the end of the Permian Period, about 200 million years ago. Many biologists believe that we are at present living in a sixth period of mass extinction, this time triggered by human beings.

ANIMALS IN THE BALANCE OF NATURE

Compared to plants, animals make up only a small part of the total mass of living matter on earth. Despite this, they play an important part in shaping and maintaining natural environments.

Many habitats are directly influenced by the way animals live. Grasslands, for example, exist partly because grasses and grazing animals have evolved a close partnership, which prevents other plants from taking hold. Tropical forests also owe their existence to animals, because most of their trees rely on animals to distribute their pollen and seeds. Soil is partly the result of animal activity, because earthworms and other invertebrates help to break down dead remains and recycle the nutrients that they contain. Without its animal life, the soil would soon become compacted and infertile.

By preying on each other, animals also help to keep their own numbers in check. This prevents abrupt population peaks and crashes and helps to give living systems a built-in stability. On a global scale, animals also influence some of the nutrient cycles on which almost all life depends. They distribute essential mineral elements in their waste, and they help to replenish the atmosphere’s carbon dioxide when they breathe. This carbon dioxide is then used by plants as they grow.

Animals and People

African Elephant Killed by Poachers

Elephant populations are on the brink of extinction due to poachers who kill elephants for their ivory tusks. An international ban on ivory trade, instituted in 1989 by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), has diminished the illicit ivory trade and reduced the killing. Over 120 countries support the ban.

Wolfgang Bayer/Bruce Coleman, Inc.

Until relatively recently in human history, people existed as nomadic hunter-gatherers. They used animals primarily as a source of food and also for raw materials that could be used for making tools and clothes. By today’s standards, hunter-gatherers were equipped with rudimentary weapons, but they still had a major impact on the numbers of some species. Many scientists believe, for example, that humans were involved in a cluster of extinctions that occurred about 12,000 years ago in North America. In less than a millennium, two-thirds of the continent’s large mammal species disappeared.

This simple relationship between people and animals changed with domestication, which also began about 12,000 years ago. Instead of being actively hunted, domesticated animals were slowly brought under human control. Some were kept for food or for clothing, others for muscle power, and some simply for companionship.

The first animal to be domesticated was almost certainly the dog, which was bred from wolves. It was followed by species such as the cat, horse, camel, llama, and aurochs (a species of wild cattle), and also by the Asian jungle fowl, which is the ancestor of today’s chickens. Through selective breeding, each of these animals has been turned into forms that are particularly suitable for human use. Today, many domesticated animals, including chickens, vastly outnumber their wild counterparts. In some cases, such as the horse, the original wild species has died out altogether.

Over the centuries, many domesticated animals have been introduced into different parts of the world only to escape and establish themselves in the wild. Together with stowaway pests such as rats, these feral animals have often had a highly damaging effect on native wildlife. Cats, for example, have inflicted great damage on Australia’s smaller marsupials, and feral pigs and goats continue to be serious problems for the native wildlife of the Galápagos Islands.

Despite the growth of domestication, humans continue to hunt some wild animals. Some forms of hunting are carried out mainly for sport, but others provide food or animal products. Until recently, one of the most significant of these forms of hunting was whaling, which reduced many whale stocks to the brink of extinction. Today, highly efficient sea fishing threatens some species of fish with the same fate (see Fisheries).

Endangered Nēnē Goose

Rats and mongooses introduced in the Hawaiian Islands have found an easy meal in the nēnē goose, one of the many bird species native only to Hawaii. Through captive breeding programs, the population of this endangered bird had rebounded to more than 1,000 by the early 2000s, but today the birds are all genetically similar, creating inbreeding that harms their chances of survival.

James L. Amos/Corbis

Since the beginning of agriculture, the human population has increased by more than two thousand times. To provide the land needed for growing food and housing people, large areas of Earth’s landscapes have been completely transformed. Forests have been cut down, wetlands drained, and deserts irrigated, reducing these natural habitats to a fraction of their former extent.

Some species of animals have managed to adapt to these changes. A few, such as the brown rat, raccoon, and house sparrow, have benefited by exploiting the new opportunities that have opened up and have successfully taken up life on farms, or in towns and cities. But most animals have specialized ways of life that make them dependent on a particular kind of habitat. With the destruction of their habitats, their number inevitably declines.

During the last century or so, animals have also had to face additional threats from human activities. Foremost among these are environmental pollution and the increasing demand for resources, such as timber and fresh water. For some animals, the combination of these changes has proved so damaging that their numbers are now below the level needed to guarantee survival.

Across the world, efforts are currently under way to address this urgent problem (see Endangered Species). In the most extreme cases, gravely threatened animals can be helped by taking them into captivity and then releasing them once breeding programs have increased their number. One species that was restored in this way is the Hawaiian mountain goose or nēnē. In the 1950s, its population had been reduced to about 25 birds. Captive breeding has since helped the population increase, although the nēnē remains on the endangered list.

While captive breeding is a useful emergency measure, it cannot assure the long-term survival of a species. Today animal protection focuses primarily on the preservation of entire habitats, an approach that maintains the necessary links between the different species the habitats support. With the continued growth in the world’s human population, habitat preservation will require a sustained reduction in our use of the world’s resources to minimize our impact on the natural world.

Contributed By:
David Burnie

Anthropology

2009-01-02T16:32:00.000-08:00

Anthropology

INTRODUCTION

Anthropology, the study of all aspects of human life and culture. Anthropology examines such topics as how people live, what they think, what they produce, and how they interact with their environments. Anthropologists try to understand the full range of human diversity as well as what all people share in common.

Anthropologists ask such basic questions as: When, where, and how did humans evolve? How do people adapt to different environments? How have societies developed and changed from the ancient past to the present? Answers to these questions can help us understand what it means to be human. They can also help us to learn ways to meet the present-day needs of people all over the world and to plan how we might live in the future.

KEY CONCEPTS

Much of the work of anthropologists is based on three key concepts: society, culture, and evolution. Together, these concepts constitute the primary ways in which anthropologists describe, explain, and understand human life.

Society and Culture

Two interrelated anthropological concepts, society and culture, are crucial to understanding what makes humans unique. In its general sense, a society consists of any group of interacting animals, such as a herd of bison. But human societies often include millions or billions of people who share a common culture. Culture refers to the ways of life learned and shared by people in social groups. Culture differs from the simpler, inborn types of thinking and behavior that govern the lives of many animals. The people in a human society generally share common cultural patterns, so anthropologists may refer to particular societies as cultures, making the two terms somewhat interchangeable.

Culture is fundamentally tied to people’s ability to use language and other symbolic forms of representation, such as art, to create and communicate complex thoughts. Thus, many anthropologists study people’s languages and other forms of communication. Symbolic representation allows people to pass a great amount of knowledge from generation to generation. People use symbols to give meaning to everything around them, every thought, and every kind of human interaction.

Evolution

Tree of Human Evolution

Fossil evidence indicates that the first humans evolved from ape ancestors at least 6 million years ago. Many species of humans followed, but only some left descendants on the branch leading to Homo sapiens. In this slide show, white skulls represent species that lived during the time period indicated; gray skulls represent extinct human species.

Most anthropologists also believe that an understanding of human evolution explains much about people’s biology and culture. Biological evolution is the natural process by which new and more complex organisms develop over time. Some anthropologists study how the earliest humans evolved from ancestral primates, a broader classification group that includes humans, monkeys, and apes. They also study how humans evolved, both biologically and culturally, over the past several million years to the present.

Humans have changed little biologically for the past 100,000 years. On the other hand, today’s worldwide culture, characterized by the rapid movement of people and ideas throughout the world, is only a few hundred years old. Today’s global-scale culture differs vastly from that of the small-scale societies (nonindustrialized societies, with small populations) in which our ancestors lived for hundreds of thousands of years. Understanding these kinds of societies and their cultures can help us make more sense of how people cope with life in today’s culturally diverse and complex world.

III

FIELDS OF ANTHROPOLOGY

Because anthropology is a very broad field of study, anthropologists focus on particular areas of interest. In the United States, anthropologists generally specialize in one of four subfields: cultural anthropology, linguistic anthropology, archaeology, and physical anthropology. Each of the subfields requires special training and involves different research techniques. Anthropology departments in colleges and universities in the United States usually teach courses covering all of these subfields.

In many other countries it is common for the subfields to be found in their own academic departments and to be known by different names. For example, in Britain and other parts of Europe, what Americans call cultural anthropology is commonly called social anthropology or ethnology. Also in Europe, archaeology and the field of linguistics (including what American anthropologists study as linguistic anthropology) are often considered as fields distinct from anthropology.

Cultural Anthropology

Cultural anthropology involves the study of people living in present-day societies and their cultures. Cultural anthropologists study such topics as how people make their living, how people interact with each other, what beliefs people hold, and what institutions organize people in a society. Cultural anthropologists often live for months or years with the people they study. This is called fieldwork. Some must learn new, and sometimes unwritten languages, and this may require extra training in linguistics (the study of the sounds and grammar of languages). Cultural anthropologists commonly write book-length (and sometimes shorter) accounts of their fieldwork, known as ethnographies.

Linguistic Anthropology

Linguistic anthropology focuses on how people use language in particular cultures. Those who practice this form of anthropology have a substantial amount of training in linguistics. Linguistic anthropologists often work with people who have unwritten (purely spoken, or oral) languages or with languages that very few people speak. Linguistic anthropological work may involve developing a way to write a formerly unwritten language. Cultures often use these written versions to teach their children the language and thus keep it in use. Some linguistic anthropologists specialize in reconstructing dead languages (languages no longer in use) and their connections to living languages, a study known as historical linguistics.

Archaeology

Archaeology focuses on the study of past, rather than living, human societies and culture. Most archaeologists study artifacts (the remains of items made by past humans, such as tools, pottery, and buildings) and human fossils (preserved bones). They also examine past environments to understand how natural forces, such as climate and available food, shaped the development of human culture. Some archaeologists study cultures that existed before the development of writing, a time known as prehistory. The archaeological study of periods of human evolution up to the first development of agriculture, about 10,000 years ago, is also called paleoanthropology. Other archaeologists study more recent cultures by examining both their material remains and written documents, a practice known as historical archaeology.

Physical Anthropology

Forensic Anthropology

Forensic anthropologists specialize in the analysis of human corpses or remains for legal investigations. In this photo, a forensics team working for the International Criminal Tribunal for the Former Yugoslavia examine human remains on a hillside near Srebrenica in northeastern Bosnia. Forensic analysis helps investigators determine how large numbers of civilians died in the Yugoslav Wars of Succession (1991-1995), information needed to convict those responsible for the killings.

Corbis

Physical anthropology, also known as biological anthropology, concentrates on the connections between human biology and culture. Some physical anthropologists, like some archaeologists, study human evolution. But physical anthropologists focus on the evolution of human anatomy and physiology, rather than culture. Areas of particular interest include the evolution of the brain, especially the areas of the brain associated with speech and complex thought; of the vocal apparatus necessary for speech; of upright posture; and of hands capable of making and using tools. Physical anthropologists work from the belief that humans are primates. Primatology, the study of the behavior and physiology of nonhuman primates, is a specialized area of interest within physical anthropology.

Some physical anthropologists specialize in forensic science, the study of scientific evidence for legal cases. Forensic anthropologists, with their knowledge of human anatomy, sometimes get called upon by law enforcement officials to identify the sex, age, or ancestry of human remains found at crime scenes or uncovered by excavations. Forensic anthropologists also have exhumed mass graves in cases of genocide, the crime of mass murder usually associated with wars. In some cases, anthropologists have provided evidence used in war crimes trials to convict guilty parties.

ANTHROPOLOGY AND OTHER SOCIAL SCIENCES

Anthropology shares certain interests and subjects of study with other fields of social science, especially sociology, psychology, and history, but also economics and political science. Anthropology also differs from these fields in many ways.

Like sociology, anthropology involves the study of human society and culture. But anthropology began as the study of small-scale tribal societies, large-scale chiefdoms, and ancient civilizations, and later moved to include global-scale societies. Sociology, on the other hand, has always emphasized the study of modern and urbanized societies. Anthropology involves the comparison of different societies in order to understand the scope of human cultural diversity. Sociology, on the other hand, frequently examines universal patterns of human behavior.

Anthropology also examines certain aspects of human psychology. Anthropology studies how people become enculturated—shaped by their culture as they grow up in a particular society. Through enculturation, people develop culturally accepted ideas of what behavior is normal or abnormal and of how the world works. Anthropology examines how people’s patterns of thought and behavior are shaped by culture and how those patterns vary from society to society. By contrast, psychology generally focuses on the universal characteristics of human thought and behavior, and studies these characteristics in individual people.

The study of history is also a part of anthropology. In its formal sense, the term history refers only to periods of time after the invention of writing. Anthropologists often study historical documents to learn more about the past of living peoples. Historical archaeologists, who specialize in the study of historical cultures, also study written documents. But all anthropologists primarily study people, their societies, and their cultures. Historians, on the other hand, primarily study written records of the past—from which they cannot learn about human societies that had or have no writing. See also History and Historiography.

In addition, anthropology examines some topics also studied in economics and political science. But anthropologists focus on how aspects of economics and politics relate to other aspects of culture, such as important rituals. Anthropologists who specialize in the study of systems of exchange in small-scale societies may refer to themselves as economic anthropologists.

UNDERSTANDING HUMAN DIVERSITY

Anthropologists have particular ways of approaching their studies. They compare differences among human societies to get an appreciation of cultural diversity. They also study the full breadth of human existence, past and present. In addition, anthropologists try to appreciate all peoples and their cultures and to discourage judgments of cultural superiority or inferiority.

Making Comparisons

Most anthropological studies involve making comparisons. Only through comparison can anthropologists learn about the uniqueness of particular cultures as well as the characteristics that people in all cultures share.

For example, comparison has helped anthropologists learn about the variety of ways in which people classify their kinship relations. People of European descent, as well as various Eskimo and Inuit groups, regard all children of their parents’ siblings as “cousins.” But in many other cultures, people may regard some of those same relations as the equivalent of a European or Eskimo “brother” or “sister.” See also Kinship and Descent.

Anthropologists also study how culture has evolved, and continues to evolve, by comparing cultural traits among different groups of people, both past and living. Patterns of similarity and increasing complexity over time can be seen in such cultural traits as forms of language or types of tools. These patterns indicate when and where cultural innovation has occurred and how ideas and people have moved around the world.

A linguistic anthropologist, for instance, might trace the development and spread of new words or forms of grammar through history. A cultural anthropologist might look for the same kinds of trends and changes in the organization of families in societies of different scale or economic system. Archaeologists, as well, often study trends of styles in artifacts, such as types of pottery.

By comparing humans with other animals, and particularly other primates, anthropologists can learn about the uniqueness of humans as a species. For instance, unlike other primates, humans commonly use language; use fire; adorn themselves with clothing, jewelry, or body markings; manufacture and decorate objects; and have beliefs about the supernatural.

Comparison also reveals what humans and nonhuman primates have in common. Most primates, including humans, share many biological characteristics, such as relatively large brains, grasping hands, acute vision and depth perception, and teeth designed to eat a variety of foods. Many primates, particularly our closest biological relatives, the chimpanzees, are highly intelligent and social animals like people. Anthropologists believe that many of the characteristics shared by humans and nonhuman primates, but not found in other animals, were probably also shared by our earliest ancestors.

Some physical anthropologists study human genetics, the science of biological heredity. By comparing genetic differences among contemporary human populations, anthropologists try to understand when various populations branched off from a common ancestor, and how each population has adapted to its environment (see Race). For instance, anthropological research suggests that highly pigmented, or dark, skin evolved in the tropics as a protection against intense sunlight. Lighter, unpigmented skin most likely evolved in temperate climates to absorb more light, which is crucial for the body’s ability to make vitamin D.

Comparative genetic research has also shown that despite genetic differences, all humans are extremely closely related. Such research suggests that all humans probably share a common ancestor who lived as recently (in evolutionary terms) as 150,000 to 200,000 years ago.

A cross-cultural perspective allows anthropologists to step back and view human cultural and biological development with relative detachment. As recently as the late 19th century, sociologists and early anthropologists believed that cultural development meant progress—a series of improvements in human life marked by inventions and discoveries. However, as anthropologists studied more cultures, their research suggested that cultural developments are not always advantageous, but that every cultural group lives in a way that works well for many of its people.

For example, anthropological research has revealed how the technology of food production changed over the past 15,000 years. All people once made their living by hunting and foraging using tools of stone, wood, and bone. Subsequently, some societies moved to gardening and herding, then to plow agriculture using metal tools, and then to industrial factory production using machinery powered by internal combustion engines.

Many people think of the evolution of food production as a story of progress and improvement. But archaeological evidence shows that the first development of agriculture, as early as 9000 bc in the Middle East, may have hurt people's health. These early farmers, who settled in villages, became dependent on a very limited diet of harvested crops as opposed to the varied and nutritious diet available to them as nomadic foragers.

Examining Many Perspectives

Because anthropology examines human culture from so many perspectives, anthropologists commonly characterize their discipline as holistic, meaning all-encompassing. The holistic approach of anthropological research can provide insight into complex contemporary problems.

Studies of the connections among human ecology, biology, and culture in small-scale societies have given anthropologists insights on large-scale, even worldwide, problems. Anthropologists have studied how small-scale hunter-gatherer, gardening, and farming societies manage to make a living without destroying species of plants or animals, or ruining the soil or water. Their findings may provide new approaches to urgent global environmental problems, such as deforestation and the loss of biological diversity. Anthropologists have learned, for instance, about gardening methods that allow patches of forest to grow back after land has been used for planting and harvesting crops.

Studies of small-scale societies have also provided much information about the importance of various species of plant and animal life to human survival. For instance, anthropologists with knowledge of entomology (the study of insects) have learned how people in small-scale societies have developed food production techniques that allow them to grow healthy crops without artificial fertilizers or pesticides. These techniques benefit insect species that help fertilize plants and help eliminate unwanted animal pests.

Physical anthropologists, along with physicians and other researchers, have also conducted health and nutritional surveys on many relatively self-sufficient societies. For instance, they have analyzed the health of peoples living throughout the Amazon rain forest. This research has consistently shown that people native to the Amazon typically are in excellent physical condition and eat a varied and nutritious diet.

Anthropological studies of hunter-gatherers, such as the San people of the Kalahari Desert, has revealed that they enjoy great amounts of leisure time, despite their need to provide themselves daily with food, shelter, and other basic necessities. Anthropologists have made similar findings in studies of people in other small-scale societies. Such people appear to have far more leisure time than do most people living in urban, industrialized societies.

Anthropological research has also shown that the key to people’s well-being in most small-scale societies centers on their relationship with their environments. For instance, anthropologists trained in botany and linguistics have found that individuals living in many small groups throughout the Amazon use hundreds of rain forest plants for medicine, food, and cosmetics. These societies have long maintained a successful way of life, satisfying their needs according to what the forest can sustainably provide.

Drawing on their knowledge of small-scale societies, anthropologists also now study large-scale urban societies in an attempt to understand the long-term significance and potential impacts of cultural change. Paleoanthropological research has shown that all people lived in small-scale societies for about 99 percent of human existence. With their holistic perspective on cultural evolution and diversity, anthropologists question the ability of rapidly growing urban, industrialized societies to manage the growth of human populations and the potential overuse of natural resources.

Avoiding Cultural Bias

An anthropologist tries to understand other cultures from the perspective of an insider—that is, as someone living within the culture. This technique, known as cultural relativism, helps anthropologists to understand why people in different cultures live as they do. Anthropologists work from the assumption that a culture is effective and adaptive for the people who live in it. In other words, a culture structures and gives meaning to the lives of its members and allows them to work and prosper.

Assuming the insider’s perspective presents a challenge, because most people, including anthropologists, harbor some ethnocentrism, the belief that their own culture makes the most sense or is superior. Ethnocentrism somewhat resembles and sometimes occurs with racism, the belief that some groups of people are genetically superior to others. Ethnocentrism and racism make it difficult to view other people and cultures objectively, according to their own merits. By trying to break the barriers of culturally and racially bound perspectives, anthropologists aim to reduce ethnocentrism and racism and the misunderstandings that they cause.

Anthropological research gives a view of human physical and cultural development that challenges many people’s common beliefs. For example, research by physical anthropologists demonstrates conclusively that humans do not fall into sharply defined races. Although many people have tried to identify the characteristics of pure human races, anthropologists have shown that all human populations contain variability and that all people differ from each other very little genetically. In addition, the most easily observed physical variations—in skin color, facial features, and body form—are only a miniscule portion of the almost endless variety of differences that make every person unique.

RESEARCH METHODS

Anthropologists use both objective (scientific) and subjective (interpretive) methods in their research. As scientists, anthropologists systematically collect information to answer specific research questions. They also document their work so that other researchers can duplicate it. But many anthropologists also conduct informal kinds of research, including impromptu discussions with and observations of the peoples they study. Some of the more common types of anthropological research methods include (1) immersion in a culture, (2) analysis of how people interact with their environment, (3) linguistic analysis, (4) archaeological analysis, and (5) analysis of human biology.

Cultural Immersion

Bronislaw Malinowski

Bronislaw Malinowski, a Polish-born British anthropologist, established methods of modern cultural anthropological research in his study of the people of the Trobriand Islands, near Papua New Guinea. An actor recites an excerpt from Malinowski’s 1922 book about these people, Argonauts of the Western Pacific.

Researchers trained in cultural anthropology employ a variety of methods when they study other cultures. Traditionally, however, much anthropological research involves long-term, direct observation of and participation in the life of another culture. This practice, known as participant observation, gives anthropologists a chance to get an insider’s view of how and why other people do what they do.

Polish-born British anthropologist Bronislaw Malinowski was the first anthropologist to document a detailed method of participant observation. Malinowski spent two years living with the people of the Trobriand Islands, part of Papua New Guinea, between 1915 and 1918. He learned the Trobriand language and explored the people’s religion, magic, gardening, trade, and social organization. He later published a series of books describing all aspects of Trobriand life. Malinowski's work became a model of research methods for generations of anthropologists.

Just as Malinowski did, most anthropologists today learn local languages to help them gain an insider’s view of a culture. Anthropologists commonly collect information by informally asking questions of the people with whom they live.

Often anthropologists will find individuals within the society being studied who are especially knowledgeable and who are willing to become so-called informants. Informants typically enjoy talking with a sympathetic outsider who wishes to interpret and record their culture. Informants and anthropologists may also form teams in which the informants work as anthropologists. While informants often provide much useful information, anthropologists also have to take into account the biases that people typically have in explaining their own cultures.

In some cases, anthropologists may use interviews to record extensive life histories of individuals with whom they have good relationships. Older people usually volunteer to tell their life stories, often because they have seen many changes since their youth and enjoy telling of past experiences and lessons learned. Such stories can provide valuable insights on how cultures change.

Anthropologists also commonly construct genealogies (diagrams of kinship relations) and maps to show how the people in communities are related to one another, how people organize themselves in groups, and how people and groups interact with each other. These research tools can provide a way for anthropologists to see cultural patterns and complexities of daily life that would otherwise be difficult to discern or comprehend.

Human Ecology

Many anthropologists combine cultural research with studies of the environments in which people live. Human ecology examines how people interact with their natural environments, such as to make a living. Anthropologists may collect large amounts of data about features of a culture’s environment, such as types of plants and animals, the chemical and nutritional properties of medicines and foods, and climate patterns. This information can provide explanations for some characteristics of a people’s culture.

For instance, in the 1960s American anthropologist Roy Rappaport analyzed the ecological significance of a ritual cycle of peace and warfare among the Tsembaga people of Papua New Guinea. Rappaport found that the Tsembaga and neighboring groups would maintain peace for periods of between 12 and 20 years. During these periods, the people would grow sweet potato gardens and raise pigs. The people would also guard areas of land they had previously gardened but which were now unused and believed to be occupied by ancestor spirits. When the presence of too many pigs rooting up gardens and eating sweet potato crops became a nuisance, the Tsembaga would feast on the pigs, perform a ritual to remove spirit ancestors from old gardens, and then lift the ban on warfare. The lifting of the ban allowed the Tsembaga to capture abandoned lands from other groups. This regulation of warfare coincided with the amount of time it took for abandoned gardens to regain their fertility, and so made good ecological sense.

Linguistic Analysis

Linguistic anthropologists, as well as many cultural anthropologists, use a variety of methods to analyze the details of a people’s language. The practice of phonology, for example, involves precisely documenting the sound properties of spoken words. Many linguistic anthropologists also practice orthography, the technique of creating written versions of spoken languages. In addition, most study the properties of grammar in languages, looking for the rules that guide how people communicate their thoughts through strings of words.

Language reveals much about a people’s culture. Anthropologists have studied such topics as how different languages assign gender to words, shape the ways in which people perceive the natural and supernatural worlds, and create or reinforce divisions of rank and status within societies.

For instance, many of the peoples native to North America conceive of time as a continual cycle of renewal, a concept quite different from the European belief that time only moves forward in a progression from the past to the future. Linguists have found that many Native American languages, such as that of the Hopi of the North American Southwest, include grammatical constructions for saying that something exists in a state of “becoming,” even though it does not yet actually exist. English and other European languages cannot as easily express such an idea, nor can most Europeans or Americans of European descent truly understand it.

Archaeological Analysis

Archaeologists use specialized research methods and tools for the careful excavation and recording of the buried remains of past cultures. Remote sensing involves the use of airplane photography and radar systems to find buried sites of past human cultures. Rigorous methods of excavation allow archaeologists to map the precise locations of remains for later analysis. Seriation, the practice of determining relative age relationships among different types of artifacts based on their shapes and styles, helps archaeologists learn how past cultures changed and evolved. Archaeologists also use a variety of dating methods involving chemical and other types of scientific analysis to reveal the age of buried objects up to millions of years old.

In addition, some archaeologists have training in cultural anthropology, and they may use cultural research to help them interpret what they find buried in the ground. For example, people in many small-scale societies continued to make tools of stone into the 20th century, and some still know how. By watching these people make their tools, archaeologists have learned how to interpret patterns of chipped pieces of stone buried in the ground.

Physical Anthropological Research

Physical anthropologists often rely on rigorous medical scientific methods for at least part of their research, in addition to more general observational methods. All physical anthropologists have detailed knowledge of human skeletal anatomy. Paleoanthropologists and forensic anthropologists can construct extremely detailed descriptions of people’s lives from only measurements of bones and teeth. These researchers typically analyze the chemical or cellular composition of bones and teeth, patterns of wear or injury, and placement in or on the ground. Such analyses can reveal information about the sex, age, work habits, and diet of a person who died long ago.

Some physical anthropologists specialize in epidemiology, the study of disease and health among large groups of people. In addition to studying diseases themselves, physical anthropologists focus on cultural causes and preventions of disease. They may study such specific medical topics as nutrition and gastrointestinal function, human reproduction, or the effects of drugs on brain and body function. For instance, physical anthropologists working in San Francisco, California, studied how the beliefs and practices of homosexual and bisexual men factored into the spread of the AIDS (acquired immunodeficiency syndrome) virus in the 1980s. This information helped in the design of effective health education programs to reduce the spread of the disease.

Physical anthropologists studying human genetics use sophisticated laboratory techniques to analyze human chromosomes and DNA (deoxyribonucleic acid), the structures through which people inherit traits from their parents. With these techniques, researchers have identified human populations that have genetic predispositions to specific diseases, such as types of cancer. This knowledge has promoted increased focus on the use of preventive measures among people with higher risk for disease.

VII

DOCUMENTING AND PRESENTING RESEARCH

Whatever kind of work they do, anthropologists share an interest in making the findings of anthropological research available as widely as possible. Many anthropologists work as professors in colleges and universities. In addition to teaching, they publish results of their research in scholarly books and journals. Others write popular books and magazine articles, produce films, lecture to nonacademic audiences, or work in museums organizing exhibits and maintaining collections.

Academic anthropologists often present their work in a highly technical style, narrowly focused for specialists in the particular subfields of anthropology. Historically, anthropologists conducted field research in order to produce an ethnography, a book or long article that describes many aspects of a particular culture.

Early ethnographies attempted to describe entire cultures. For example, in 1946 American anthropologists Clyde Kluckhohn and Dorothea Leighton published a study on the culture of the Navajo (also spelled Navaho), Native Americans of the Southwestern United States. The book, called The Navajo, covered a wide variety of topics about the Navajo, including their prehistory, history, economic activities, physique, clothing, housing, health, kinship, religious life, language, worldview, and relations with outsiders.

Ethnographies also sometimes focus on a single aspect of a culture. Bronislaw Malinowski's ethnography Argonauts of the Western Pacific (1922) dealt primarily with the interisland trading system of the Trobriand Islanders. Malinowski demonstrated, in great detail, how the ritual exchange of items such as jewelry, food, clothing, and weapons among trading partners was central to the entire culture.

Some ethnographies written between the 1920s and the 1960s discussed the history of a culture and described how it changed over time. But many classic anthropological texts of this period were written in a timeless ethnographic present, describing a culture as though it had always existed in the same way, and always would. This style represented a trend in anthropology known as functionalism, in which anthropologists analyzed cultures as if all the parts of a culture fit and worked neatly together. The functionalist model of cultural integrity portrayed cultures as being stable and unchanging.

Later anthropologists became more concerned with the dynamics of culture change. It became clear by the 1960s that the world and all its cultures were changing in dramatic ways. Contemporary ethnographies often focus on change, especially changes brought about by global cultural contact, urbanization, and people’s increasing exposure to and dependence on mass-produced goods, services, and images (as from films or advertisements).

A contemporary anthropologist may write an ethnography from the perspective of a single individual within a culture. Others may write stories or poems. Many try to write using the voices of people they study, and some encourage informants to write their own ethnographies. Anthropologists always give copies of their books or articles to the people they study.

VIII

ETHICAL CONCERNS

Often, the people that anthropologists study have strong feelings about how they are portrayed to the rest of the world. Professional anthropologists must therefore exercise great care in how they conduct and present their work. Anthropological research also has the potential to disrupt a people’s way of life and bring problems into their societies. Anthropologists try to avoid introducing new ideas, technologies, or even food items into the societies they study, because to do so can make people want things that cannot be readily obtained.

Anthropologists also have ethical obligations to those who fund their research activities as well as to students and the interested public who may want to learn from their work. As a basic rule, anthropologists only conduct research openly, honestly, and with the approval of the people they study. In the United States, federally funded projects and research conducted through a public university might face a formal review procedure to make sure that the rights and safety of human subjects are protected.

Today, anthropologists are also obliged to share their research results with the people who helped produce it and to acknowledge the assistance those people give. Anthropologists do not normally pay for specific information, but they may compensate some of the people they study for their time and effort put in as field assistants or informants.

In rare cases a researcher might decide not to work with a particularly isolated and self-sufficient group because to do so might unavoidably introduce disease and open the way for exploitation by other outsiders. Small, self-sufficient societies may have difficulty defending themselves against more powerful groups. For example, information from anthropological work can familiarize governments and businesses with small-scale societies living in remote regions. This information can convince state and business interests to negotiate with the people of such societies about using their land for such projects as road or dam building, mining, or large-scale farming. These so-called development projects can cause great hardships for people who live off the land.

Anthropologists must practice particularly great care if they work directly for governmental or commercial agencies whose political or economic interests could conflict with the interests of the people being studied. For example, in the 1970s and 1980s the Brazilian government hired anthropologists to pacify people who lived in the rain forest and who were being forcibly relocated to make way for the Trans-Amazon Highway. While some anthropologists considered this work unethical, others felt they could help negotiate with the government to minimize damage to the peoples living in the highway’s future path.

Most anthropologists take a position of cultural relativism when making decisions on issues of ethics and rights. This position calls for respect for all cultural differences and opposes culture change imposed on one society by another. Anthropologists know that people derive their individual identity and sense of dignity from their own cultures. This ethical stance reflects the 1948 United Nations Universal Declaration of Human Rights and the United Nations Declaration on the Rights of Indigenous Peoples (drafted in 1994), both of which recognize cultural practices as basic human rights.

This does not mean, however, that anthropologists believe all cultural practices are necessarily good. Extreme relativism, which anthropologists avoid, could condone such acts as the Holocaust or other instances of mass ethnocide (the killing of people of a particular ethnic group). Many cultures may foster practices that clearly harm some individuals. Such practices include infanticide (the killing of infants), the burning of people thought to be witches, and the surgical modification of women’s sexual organs (known as female genital mutilation). Anthropologists might speak out against such practices, but generally they believe that change should come from within a culture and not be imposed from outside it.

Archaeologists have other ethical concerns to consider. Archaeological excavations may unearth sensitive or sacred remains of past cultures with living descendants. Such remains might include the bones of dead ancestors or ancient religious offerings.

Archaeologists respect the claims of cultural groups to ownership of their ancestors’ cultural and physical remains, and work to prevent unauthorized removal of such materials by commercial collectors. They also commonly hand over most or all of their finds to the rightful owners or to museums of the countries in which excavations took place. Sometimes, however, an archaeologist may argue that certain excavated materials have such great scientific importance that they should be analyzed before being returned or reburied.

HISTORY OF ANTHROPOLOGY

Origins

Anthropology traces its roots to ancient Greek historical and philosophical writings about human nature and the organization of human society. Anthropologists generally regard Herodotus, a Greek historian who lived in the 400s bc, as the first thinker to write widely on concepts that would later become central to anthropology. In the book History, Herodotus described the cultures of various peoples of the Persian Empire, which the Greeks conquered during the first half of the 400s bc. He referred to Greece as the dominant culture of the West and Persia as the dominant culture of the East. This type of division, between white people of European descent and other peoples, established the mode that most anthropological writing would later adopt.

The Arab historian Ibn Khaldun, who lived in the 14th century ad, was another early writer of ideas relevant to anthropology. Khaldun examined the environmental, sociological, psychological, and economic factors that affected the development and the rise and fall of civilizations. Both Khaldun and Herodotus produced remarkably objective, analytic, ethnographic descriptions of the diverse cultures in the Mediterranean world, but they also often used secondhand information.

During the Middle Ages (5th to 15th centuries ad) biblical scholars dominated European thinking on questions of human origins and cultural development. They treated these questions as issues of religious belief and promoted the idea that human existence and all of human diversity were the creations of God.

Beginning in the 15th century, European explorers looking for wealth in new lands provided vivid descriptions of the exotic cultures they encountered on their journeys in Asia, Africa, and what are now the Americas. But these explorers did not respect or know the languages of the peoples with whom they came in contact, and they made brief, unsystematic observations.

The European Age of Enlightenment of the 17th and 18th centuries marked the rise of scientific and rational philosophical thought. Enlightenment thinkers, such as Scottish-born David Hume, John Locke of England, and Jean-Jacques Rousseau of France, wrote a number of humanistic works on the nature of humankind. They based their work on philosophical reason rather than religious authority and asked important anthropological questions. Rousseau, for instance, wrote on the moral qualities of “primitive” societies and about human inequality. But most writers of the Enlightenment also lacked firsthand experience with non-Western cultures.

Imperialism and Increased Contact with Other Cultures

Origins of Anthropology

The modern study of anthropology had its origins in the European exploration and colonization of lands in the Americas, Asia, Africa, and the Pacific. European contacts with vastly different peoples sparked an interest in understanding and explaining human diversity, the goals of anthropology.

Collection Viollet/Liaison Agency

With the rise of imperialism (political and economic control over foreign lands) in the 18th and 19th centuries, Europeans came into increasing contact with other peoples around the world, prompting new interest in the study of culture. Imperialist nations of Western Europe—such as Belgium, the Netherlands, Portugal, Spain, France, and England—extended their political and economic control to regions in the Pacific, the Americas, Asia, and Africa.

The increasing dominance of global commerce, capitalist (profit-driven) economies, and industrialization in late-18th-century Europe led to vast cultural changes and social upheavals throughout the world. European industries and the wealthy, elite classes of people who owned them looked to exotic foreign lands for sources of labor and goods for manufacturing. In addition, poorer Europeans, many of whom were displaced from their land by industrialization, tried to build new lives abroad. Several European countries took over the administration of foreign regions as colonies (see Colonialism and Colonies). See also Capitalism.

Europeans suddenly had a flood of new information about the foreign peoples encountered in colonial frontiers. The colonizing nations of Europe also wanted scientific explanations and justifications for their global dominance. In response to these developments, and out of an interest in new and strange cultures, the first amateur anthropologists formed societies in many Western European countries in the early 19th century. These societies eventually spawned professional anthropology.

Anthropological societies devoted themselves to scientifically studying the cultures of colonized and unexplored territories. Researchers filled ethnological and archaeological museums with collections obtained from the new empires of Europe by explorers, missionaries, and colonial administrators. Physicians and zoologists, acting as novice physical anthropologists, measured the skulls of people from various cultures and wrote detailed descriptions of the people’s physical features.

Toward the end of the 19th century anthropologists began to take academic positions in colleges and universities. Anthropological associations also became advocates for anthropologists to work in professional positions. They promoted anthropological knowledge for its political, commercial, and humanitarian value.

The Beginnings of Modern Anthropology

In the 19th century modern anthropology came into being along with the development and scientific acceptance of theories of biological and cultural evolution. In the early 19th century, a number of scientific observations, especially of unearthed bones and other remains, such as stone tools, indicated that humanity’s past had covered a much greater span of time than that indicated by the Bible (see Creationism).

In 1836 Danish archaeologist Christian Thomsen proposed that three long ages of technology had preceded the present era in Europe. He called these the Stone Age, Bronze Age, and Iron Age. Thomsen's concept of technological ages fit well with the views of Scottish geologist Sir Charles Lyell, who proposed that the earth was much older than previously believed and had changed through many gradual stages.

Evolutionary Theory

Caricature of Charles Darwin

When Charles Darwin published The Descent of Man in 1871, he challenged the fundamental beliefs of most people by asserting that humans and apes had evolved from a common ancestor. Many critics of Darwin misunderstood his theory to mean that people had descended directly from apes. This caricature of Charles Darwin as an ape appeared in the London Sketch Book in 1874.

Mary Evans Picture Library/Science Source/Photo Researchers, Inc.

In 1859 British naturalist Charles Darwin published his influential book On the Origin of Species. In this book, he argued that animal and plant species had changed, or evolved, through time under the influence of a process that he called natural selection. Natural selection, Darwin said, acted on variations within species, so that some variants survived and reproduced, and others perished. In this way, new species slowly evolved even as others continued to exist. Darwin’s theory was later supported by studies of genetic inheritance conducted in the 1850s and 1860s by Austrian monk Gregor Mendel. Evolutionary theory conflicted with established religious doctrine that all species had been determined at the creation of the world and had not changed since.

English social philosopher Herbert Spencer applied a theory of progressive evolution to human societies in the middle 1800s. He likened societies to biological organisms, each of which adapted to survive or else perished. Spencer later coined the phrase 'survival of the fittest' to describe this process. Theories of social evolution such as Spencer’s seemed to offer an explanation for the apparent success of European nations as so-called advanced civilizations.

Anthropological Evolutionary Theories

Edward Tylor

Sir Edward Burnett Tylor was a pioneer of cultural anthropology in Britain. Tylor gave one of the first anthropological definitions of culture in his book Primitive Culture (1871). Here an actor recites Tylor’s definition of culture.

During the late 1800s many anthropologists promoted their own models of social and biological evolution. Their writings portrayed people of European descent as biologically and culturally superior to all other peoples. The most influential anthropological presentation of this viewpoint appeared in Ancient Society, published in 1877 by American anthropologist Lewis Henry Morgan.

Morgan argued that European civilization was the pinnacle of human evolutionary progress, representing humanity’s highest biological, moral, and technological achievement. According to Morgan, human societies had evolved to civilization through earlier conditions, or stages, which he called Savagery and Barbarism. Morgan believed these stages occurred over many thousands of years and compared them to geological ages. But Morgan attributed cultural evolution to moral and mental improvements, which he proposed were, in turn, related to improvements in the ways that people produced food and to increases in brain size.

Morgan also examined the material basis of cultural development. He believed that under Savagery and Barbarism people owned property communally, as groups. Civilizations and political states, he said, developed together with the private ownership of property. States thus protected people’s rights to own property. Morgan's theories coincided with and influenced those of German political theorists Friedrich Engels and Karl Marx. Engels and Marx, using a model like Morgan’s, predicted the demise of state-supported capitalism. They saw communism, a new political and economic system based on the ideals of communality, as the next evolutionary stage for human society.

Like Morgan, Sir Edward Tylor, a founder of British anthropology, also promoted the theories of cultural evolution in the late 1800s. Tylor attempted to describe the development of particular kinds of customs and beliefs found across many cultures. For example, he proposed a sequence of stages for the evolution of religion—from animism (the belief in spirits), through polytheism (the belief in many gods), to monotheism (the belief in one god).

In 1871 Tylor also wrote a still widely quoted definition of culture, describing it as “that complex whole that includes knowledge, belief, art, morals, law, custom and any other capabilities and habits acquired by man as a member of a society.” This definition formed the basis for the modern anthropological concept of culture.

Cultural Evolution, Colonialism, and Social Darwinism

The colonial nations of Europe used ethnocentric theories of cultural evolution to justify the expansion of their empires. Writings based on such theories described conquered peoples as “backward” and therefore unfit for survival unless colonists “civilized” them to live and act as Europeans did. This application of evolutionary theory to control social and political policy became known as social Darwinism.

Theories of cultural evolution in the 19th century took no account of the successes of small-scale societies that had developed long-term adaptations to particular environments. Nor did they recognize any shortcomings of European civilization, such as high rates of poverty and crime.

Furthermore, while many proponents of cultural evolution suggested that the people in small-scale societies were biologically inferior to people of European descent, no evidence actually supported this position. But not all anthropologists believed in this type of cultural evolution. Many actually rejected all evolutionary theory because others misused and abused it.

New Directions in Theory and Research

Anthropology emerged as a serious professional and scientific discipline beginning in the 1920s. The focus and practice of anthropological research developed in different ways in the United States and Europe.

The Influence of Boas

Franz Boas

German-American anthropologist Franz Boas, a professor at Columbia University in New York City for 37 years, helped pioneer modern anthropology. He advocated the theories that there is no pure race and that no race is superior to any other.

Corbis

In the 1920s and 1930s anthropology assumed its present form as a four-field academic profession in the United States under the influence of German-born American anthropologist Franz Boas. Boas wanted anthropology to be a well-respected science. He was interested in all areas of anthropological research and had done highly regarded fieldwork in all areas except archaeology. As a professor at Columbia University in New York City from 1899 until his retirement in 1937, he helped define the discipline and trained many of the most prominent American anthropologists of the 20th century. Many of his students—including Alfred Kroeber, Ruth Benedict, and Margaret Mead—went on to establish anthropology departments at universities throughout the country.

Margaret Mead

American anthropologist Margaret Mead spent many years studying how culture influences individual personality. Mead lived among the Samoan people during 1925 and 1926 to observe their way of life and the types of personalities common in their cultural group. Her 1928 book, Coming of Age in Samoa, provoked a great debate among sociocultural anthropologists regarding the proper method and interpretation of field research. Mead’s approach to studying groups of people, which focused on the individual people and groups with whom she lived, earned her much criticism from anthropologists who believed that research must rely more directly on statistical research and the incorporation of cross-cultural and testable hypotheses.

Boas stressed the importance of anthropologists conducting original fieldwork to get firsthand experiences with the cultures they wished to describe. He also opposed racist and ethnocentric evolutionary theories. Based on his own studies, including his measurement of the heads of people from many cultures, Boas argued that genetic differences among human populations could not explain cultural variation.

Boas urged anthropologists to do detailed research on particular cultures and their histories, rather than attempt to construct grand evolutionary stages for all of humankind in the tradition of Morgan and Tylor. Boas’s theoretical approach became known as historical particularism, and it forms the basis for the fundamental anthropological concept of cultural relativism.

Functionalism

Émile Durkheim

Émile Durkheim, one of the fathers of sociology, utilized scientific methods to approach the study of society and social groups. His work influenced the school of anthropology known as functionalism. Durkheim believed that individuals should be considered within the context of the society in which they live.

THE BETTMANN ARCHIVE

Many other anthropologists working in Boas’s time, mostly in Europe, based their research on the theories of 19th-century French sociologist Émile Durkheim. Like Sir Edward Tylor, Durkheim was interested in religions across cultures. But he was not interested in the evolution of religion. Durkheim instead proposed that religious beliefs and rituals functioned to integrate people in groups and to maintain the smooth functioning of societies.

Durkheim’s ideas were expanded upon by Bronislaw Malinowski and A. R. Radcliffe-Brown, two major figures in the development of modern British anthropology beginning in the 1920s and 1930s. Their approach to understanding culture was known as structural functionalism, or simply functionalism.

A typical functionalist study analyzed how cultural institutions kept a society in working order. For example, many studies examined rites of passage, such as initiation ceremonies. Through a series of such ceremonies, groups of children of the same age would be initiated into new roles and take on new responsibilities as they grew into adults. According to functionalists, any unique characteristics of the rites of passage of a particular society had to do with how initiation ceremonies worked in the function of that society.

Functionalists based their approach to doing fieldwork on their theories. They lived for long periods with the people they studied, carefully recording even very small details about a people’s culture and social life. The resulting ethnographies portrayed all aspects of culture and social life as interdependent parts of a complex model. Functionalist research methods became the blueprint for much anthropological research throughout the 20th century.

During the first half of the 20th century, many anthropologists conducted functionalist ethnographic studies in the service of colonial governments. This research allowed colonial administrators to predict what would happen to an entire society in response to particular colonial policies. Administrators might want to know, for instance, what would happen if they imposed taxes on households or on individuals.

Structuralism

Claude Lévi-Strauss

French anthropologist Claude Lévi-Strauss based his understanding of culture on studies of people’s languages and recurring patterns of thought and behavior. His cultural theories are associated with the anthropological movement known as structuralism.

Keystone Pressedienst GmbH

In the 1950s French anthropologist Claude Lévi-Strauss developed an anthropological theory and analytic method known as structuralism. He was influenced by the theories of Durkheim and one of Durkheim’s collaborators, French anthropologist Marcel Mauss. Lévi-Strauss proposed that many common cultural patterns—such as those found in myth, ritual, and language—are rooted in basic structures of the mind.

He wrote, for instance, about the universal tendency of the human mind to sort things into sets of opposing concepts, such as day and night, black and white, or male and female. Lévi-Strauss believed such basic conceptual patterns became elaborated through culture. For example, many societies divide themselves into contrasting but complementary groups, known as moieties (from the French word for “half”). Each moiety traces its descent through one line to a common ancestor. In addition to many shared ritual functions, moieties create a system for controlling sex and marriage. A person from one moiety may only marry or have sexual relations with a person from the other moiety.

Cultural Materialism and Cultural Ecology

In the 1960s, American anthropologists such as Julian Steward, Roy Rappaport, and Marvin Harris began to study how culture and social institutions relate to a people’s technology, economy, and natural environment. All of these factors together define a people’s patterns of subsistence—how they feed, clothe, shelter, and otherwise provide for themselves.

Economic and ecological approaches to understanding culture and societies are known as cultural materialism or cultural ecology. Harris, for instance, analyzed the religious practice in India of regarding cows as sacred. He suggested that this religious practice developed as a cultural response to the value of cows as work animals for farming and other essential tasks and as a source of dung, which is dried as fuel.

Symbolic Anthropology

In the 1970s many anthropologists, including American ethnologist Clifford Geertz and British ethnologist Victor Turner, moved away from ecological and economic explanations of people’s cultures. Instead, these anthropologists looked for the meanings of particular cultural symbols and rituals within cultures themselves, an approach known as symbolic anthropology.

Symbolic anthropological studies often focus on one particularly important ritual or symbol within a society. Anthropologists using this approach attempt to demonstrate how this one symbol or ritual shapes or reflects an entire culture. Geertz, for example, attempted to show how the culture of the people of Bali, Indonesia, could be understood by examining the important Balinese ritual of staging and betting on cockfights.

ANTHROPOLOGY TODAY

By the early 1990s anthropology had become a very diverse field with numerous areas of specialization. For example, the American Anthropological Association, one of the discipline’s most important professional organizations in the United States, includes sections focused on such specific topics as agriculture, consciousness, education, the environment, feminism, film and photography, museums, nutrition, politics and law, psychology, urban issues, and work. Other groups focus on geographic areas, including Africa, Europe, Latin America, the Middle East, and North America. Specialization within anthropology has become so important that many academic departments have begun questioning the need to teach about the original four subfields.

New research agendas have also emerged, and several new trends in world culture have dramatically changed anthropology. Independent, self-sufficient cultures—the focus of traditional anthropology—have virtually disappeared. In addition, the world faces increasing problems of poverty, violence, and environmental degradation. In response to these trends, many anthropologists have shifted their attention to studying urban culture and the workings of global culture. Much new research examines the dynamics of global commerce and the international exchange of ideas, beliefs, and cultural practices.

Beginning in the 1980s a series of new ideas, collectively called postmodernism, also raised questions about some of anthropology’s fundamental methods and objectives. As a result, some anthropologists have moved into a new area of research sometimes known as cultural studies. Others have continued to use more traditional anthropological research methods to solve problems associated with cross-cultural conflicts. This type of work is known as applied anthropology.

Postmodernism and Cultural Studies

Postmodernism describes the philosophy of examining the nature of meaning and knowing, although academics in many fields have debated over its precise definition. Postmodernists question the validity of the faith in science and rationalism that originated during the Enlightenment and that became associated with the philosophy known as modernism. They also question whether anthropology is, or should be, a science. Because all knowledge is necessarily shaped by culture, they argue, anthropologists cannot be objective in their research.

In response to this argument, some anthropologists have turned to simply studying and writing about the effects of the influence of culture on their own perspectives, and on the perspectives of all people. While much of this work is still done in anthropology departments, it has also become a distinct area of research known as cultural studies. Some see cultural studies as a new discipline, separate from anthropology. Others regard it as the newest phase of anthropological theory.

Critics of traditional anthropology view it as a form of colonialism and exploitation. This notion has gained ground as anthropologists have studied the history of their own discipline and reexamined the relationship between the development of anthropology and colonialism. Moreover, traditional anthropology has always been dominated by the ideas, research, and writing of white Europeans and Americans. This, too, is changing, as increasing numbers of people from diverse cultural backgrounds are working in anthropology and cultural studies.

Researchers working in cultural studies have also redefined culture. They tend to view culture as something that people continually negotiate over with each other, rather than as something they share. This view makes sense to a generation of anthropologists who grew up in the 1960s in the United States and Europe. During that time, young people challenged the cultural traditions of their parents and questioned such important problems as racism, sexism, and the violence of modern warfare. They also began to view some of the world’s worst problems—such as ethnic violence, poverty, and environmental destruction—as legacies of the colonial era that also gave rise to anthropology.

Many researchers in cultural studies have worked to deconstruct (take apart to analyze and critique) traditional ethnographies and other types of anthropological research. Their analyses demonstrate that a good deal of this older research might have misrepresented or negatively affected the cultures described. The practice of critiquing early anthropological work requires no special anthropological training or fieldwork. Thus, the field of cultural studies includes people schooled in such diverse topics as literature, gender studies, sociology, and history.

Some anthropologists have reacted against the antiscience critiques of postmodernism. They reject the position that scientific research cannot teach us anything about the nature of the world or humanity. But critiques of traditional anthropological practices may improve the quality of anthropological work by making researchers even more conscious about the methods they use.

Applied Anthropology

World Congress of Indigenous Peoples

In 1992 indigenous peoples from around the world gathered in the town of Kari-Oca outside of Rio de Janeiro, Brazil, at the World Congress of Indigenous Peoples. They drafted documents and signed petitions stating their shared views on respect for land and natural resources, world economic development, and the rights of indigenous cultural groups to determine their own futures.

Antonio Ribeiro/Liaison Agency

Since the 1960s, anthropologists have increasingly applied their special research skills and cross-cultural insights to try to solve important world problems. Applied anthropology involves helping cultural groups, organizations, businesses, and governments solve a wide range of problems.

Applied anthropology developed with the end of colonialism. Many colonies gained their independence within two decades after the end of World War II in 1945. International political and economic agencies began employing anthropologists to promote the development of new forms of industrial and agricultural production in these newly independent countries. This work, known as development anthropology, often involved helping small, self-sufficient societies adjust to the changes brought by development projects.

Many small societies of indigenous peoples who were threatened by development projects began to organize themselves collectively. The term indigenous peoples refers to those who have inhabited and made their living directly off the same land for hundreds or thousands of years. By the 1970s, indigenous groups had begun to come together in order to defend their rights to land and natural resources.

In response, many anthropologists shifted from being advocates for development to providing support for indigenous groups. People who were once the subjects of anthropological study now hire anthropologists to work for them. For example, Native American tribes and nations have employed archaeologists, linguistic anthropologists, and cultural anthropologists to help them document and protect their cultural heritage. Some Native Americans have also become anthropologists themselves to help their own tribal groups.

Archaeological analysis can help support people’s claims to land and natural resources by demonstrating that their ancient ancestors lived, hunted, fished, or buried their dead in a particular place. Cultural anthropologists and archaeologists may also provide testimony in legal cases to defend the integrity of indigenous groups. Linguistic anthropologists can prepare teaching materials and texts for previously unwritten languages. These materials can help teach children to continue to speak their native languages in the face of cultural change.

Anthropologists have also become increasingly interested in examining and trying to lessen the causes and consequences of injustice, violence, and poverty wherever it occurs. For instance, physical anthropologists have supported international human rights organizations by helping to excavate and identify the remains of the victims of political and ethnic mass killings. They have also helped to identify the perpetrators of such killings in a number of countries, including Argentina, Chile, El Salvador, Guatemala, Rwanda, and the former Yugoslavia.

Governments in many parts of the world support the business of large agricultural companies that convert subsistence farmers into wageworkers to produce crops for export. Cultural anthropologists and physical anthropologists specializing in nutrition and health have gathered evidence showing that these changes have led to increased rates of poverty, malnutrition, and infant mortality. In the United States, anthropologists have examined the human impacts of factory closings and wage reductions as companies have shifted their operations overseas (see Multinational Corporation). Anthropologists hope the results of this research will convince governments and businesses to consider the potential negative effects of their actions.

As commerce and cross-cultural exchange create a new global-scale culture, anthropologists hope to learn how social power and decision making are organized around the world. They want to ensure that people remain free to live according to unique cultural beliefs and practices, safe from the control of powerful commercial and political interests.

Contributed By:
John H. Bodley

Astronomy

2009-01-02T16:27:00.000-08:00

Astronomy

INTRODUCTION

Horsehead Nebula

The Horsehead Nebula, located over 1,000 light-years away in the constellation Orion, is an enormous interstellar cloud of gas and dust. This dark nebula is visible from Earth only because it blocks light emanating from young stars located behind the nebula.

Canada-France-Hawaii Telescope/J.-C.Cuillandre/Coelum/2001

Astronomy, study of the universe and the celestial bodies, gas, and dust within it. Astronomy includes observations and theories about the solar system, the stars, the galaxies, and the general structure of space. Astronomy also includes cosmology, the study of the universe and its past and future. People who study astronomy are called astronomers, and they use a wide variety of methods to perform their research. These methods usually involve ideas of physics, so most astronomers are also astrophysicists, and the terms astronomer and astrophysicist are basically identical. Some areas of astronomy also use techniques of chemistry, geology, and biology.

Astronomy is the oldest science, dating back thousands of years to when primitive people noticed objects in the sky overhead and watched the way the objects moved. In ancient Egypt, the first appearance of certain stars each year marked the onset of the seasonal flood, an important event for agriculture. In 17th-century England, astronomy provided methods of keeping track of time that were especially useful for accurate navigation. Astronomy has a long tradition of practical results, such as our current understanding of the stars, day and night, the seasons, and the phases of the Moon. Much of today's research in astronomy does not address immediate practical problems. Instead, it involves basic research to satisfy our curiosity about the universe and the objects in it. One day such knowledge may well be of practical use to humans. See also History of Astronomy.