When hawk-sized dragonflies ruled the air Almost anyone who has spent time around fresh

Transcription

1 25 When hawk-sized dragonflies ruled the air Almost anyone who has spent time around fresh water ponds is familiar with dragonflies. Their hovering flight, bright colors, and transparent wings stimulate our visual senses on bright summer afternoons as they fly about their business of devouring mosquitoes, mating, and laying their eggs. The largest dragonflies alive today have wingspans that can be covered by a human hand. Three hundred million years ago, however, dragonflies such as Meganeuropsis permiana had wingspans of more than 70 centimeters well over 2 feet, matching or exceeding the wingspans of many modern birds of prey and were the largest flying predators on Earth. No flying insects alive today are anywhere near this size. But during the Carboniferous and Permian geological periods, between 350 and 250 million years ago, many groups of flying insects contained gigantic members. Meganeuropsis probably ate huge mayflies and other giant flying insects that shared their home in the Permian swamps. These enormous insects were themselves eaten by giant amphibians. None of the giant flying insects or amphibians of that time would be able to survive on Earth today. The oxygen concentrations in Earth s atmosphere were about 50 percent higher then than they are now, and those high oxygen levels are thought to have been necessary to support giant insects and their huge amphibian predators. Paleontologists have uncovered fossils of Meganeuropsis permiana in the rocks of Kansas. How do we know the age of these fossils, and how can we know how much oxygen that long-vanished atmosphere contained? The stratigraphic layering of the rocks allows us to tell their ages relative to each other, but it does not by itself indicate a given layer s absolute age. One of the remarkable achievements of twentiethcentury scientists was to develop sophisticated techniques that use the decay rates of various radioisotopes, changes in Earth s magnetic field, and the ratios of certain molecules to infer conditions and events in the remote past and to date them accurately. It is those methods that allow us to age the fossils of Meganeuropsis and to calculate the concentration of oxygen in Earth s atmosphere at the time. The development of the science of biology is intimately linked to changing concepts of Giant Dragonflies Meganeuropsis permiana, shown here in a reconstruction from fossils, dwarfed modern dragonflies (shown in the inset at the same scale) in size. Otherwise, however, the Permian giant was quite similar to modern dragonflies in general appearance.

2 CHAPTER OUTLINE 25.1 How Do Scientists Date Ancient Events? 25.2 How Have Earth s Continents and Climates Changed over Time? 25.3 What Are the Major Events in Life s History? Younger Rocks Lie on Top of Older Rocks In the Grand Canyon, the Colorado River cut through and exposed many strata of ancient rocks. The oldest rocks visible here formed about 540 million years ago. The youngest, at the top, are about 500 million years old. Knowing the ages of rock strata allows scientists to date the fossils found in each stratum. time, especially of the age of Earth. About 150 years ago, geologists first provided solid evidence that Earth is ancient; before 1850, most people believed it was no more than a few thousand years old. For many more years, physicists continued to underestimate Earth s age, until an understanding of radioactive decay was developed. Today we know that Earth is about 4.5 billion years old and that life has existed on it for about 3.8 billion of those years. That means human civilizations have occupied Earth for less than percent of the history of life. Discovering what happened before humans were around is an ongoing and exciting area of science. IN THIS CHAPTER we will examine how biologists assign dates to events in the distant evolutionary past, and how such dating allows us to review the major changes in physical conditions on Earth during the past 4 billion years. We will then look at how these changes in physical conditions have influenced the major patterns in the evolution of life, and describe how scientists organize our knowledge of biological diversity based on the relationships among species. How Do Scientists Date 25.1 Ancient Events? Many evolutionary changes happen rapidly enough to be studied directly and manipulated experimentally. Plant and animal breeding by agriculturalists and insects evolution of resistance to pesticides are examples of rapid, short-term evolution. Other changes, such as the appearance of new species and evolutionary lineages, usually take place over much longer time frames. To understand the long-term patterns of evolutionary change, we must think in time frames spanning many millions of years, and consider events and conditions very different from those we observe today. Earth of the distant past was so unlike the present that it seams like a foreign planet inhabited by strange organisms. The continents were not where they are today, and climates were sometimes dramatically different from those of today. Fossils the preserved remains of ancient organisms can tell us a great deal about the body form, or morphology, of organisms that lived long ago, as well as how and where they lived. Fossils provide a direct record of evolution. But to understand patterns of evolutionary change, we must also understand how Earth has changed over time. Earth s history is largely recorded in its rocks. We cannot tell the ages of rocks just by looking at them, but we can determine the ages of rocks relative to one another. The first person to formally recognize that this could be done was the seventeenthcentury Danish physician Nicolaus Steno. Steno realized that in undisturbed sedimentary rocks (rocks formed by the accumulation of grains on the bottom of bodies of water), the oldest layers of rock, or strata (singular stratum), lie at the bottom; thus successively higher strata are progressively younger. Geologists, particularly the eighteenth-century English scientist William Smith, subsequently combined Steno s insight with their observations of fossils contained in sedimentary rocks. They concluded that: Fossils of similar organisms are found in widely separated places on Earth. Certain fossils are always found in younger rocks, and certain other fossils in older rocks. Organisms found in higher, more recent strata are more similar to modern organisms than are those found in lower, more ancient strata. These patterns revealed much about the relative ages of sedimentary rocks as well as patterns in the evolution of life. But the geologists still could not tell how old the rocks were. A method of dating rocks did not become available until after radioactivity was discovered at the beginning of the twentieth century.

3 520 CHAPTER 25 HISTORY OF LIFE ON EARTH Radioisotopes provide a way to date rocks Radioactive isotopes of atoms (see Section 2.1) decay in a predictable pattern over long time periods. During each successive time interval, known as a half-life, half of the remaining radioactive material of the radioisotope decays to become a different, stable isotope (Figure 25.1A). To use a radioisotope to date a past event, we must know or estimate the concentration of the isotope at the time of that event. In the case of carbon, the production of new carbon-14 ( 14 C) in the upper atmosphere (by the reaction of neutrons with nitrogen-14) just balances the natural radioactive decay of 14 C to 14 N. Therefore, the ratio of 14 C to its stable isotope, carbon-12 ( 12 C), is relatively constant in living organisms and their environment. As soon as an organism dies, however, it ceases to exchange carbon compounds with its environment. Its decaying 14 C is no longer replenished, and the ratio of 14 C to 12 C in its remains decreases through time. Paleontologists can use the ratio of 14 C to 12 C in fossil material to date fossils that are less than 50,000 years old (and thus the sedimentary rocks that contain those fossils). If fossils are older than that, so little 14 C remains that the limits of detection using this particular isotope are reached. (A) (B) Radioisotope remaining in sample (%) Radioisotope Carbon-14 ( 14 C) Potassium-40 ( 40 K) Uranium-238 ( 238 U) 14 C half-lives (thousands of years) /2 Half-life (years) 1 /4 1/8 1/ Number of half-lives Decay product Useful dating range (years) 5,700 Nitrogen-14 ( 14 N) , billion Argon-40 ( 40 Ar) 10 million 4.5 billion 4.5 billion Lead 206 ( 206 Pb) 10 million 4.5 billion 25.1 Radioactive Isotopes Allow Us to Date Ancient Rocks The decay of radioactive parent atoms into stable daughter isotopes happens at a steady rate known as a half-life. (A) The graph demonstrates the principle of half-life using carbon-14 ( 14 C) as an example. (B) Radioisotopes have different characteristic half-lives that allow us to measure how much time has elapsed since the rocks containing them were laid down. TABLE 25.1 Earth s Geological History RELATIVE TIME SPAN ERA PERIOD ONSET MAJOR PHYSICAL CHANGES ON EARTH Cenozoic Quaternary 2.6 mya Cold/dry climate; repeated glaciations Tertiary 65 mya Continents near current positions; climate cools Cretaceous 145 mya Northern continents attached; Gondwana begins to drift apart; meteorite strikes Yucatán Peninsula Mesozoic Jurassic 200 mya Two large continents form: Laurasia (north) and Gondwana (south); climate warm Triassic 251 mya Pangaea begins to slowly drift apart; hot/humid climate Precambrian Permian 297 mya Extensive lowland swamps; O 2 levels 50% higher than present; by end of period continents aggregate to form Pangaea, and O 2 levels begin to drop rapidly Carboniferous 359 mya Climate cools; marked latitudinal climate gradients Paleozoic Devonian 416 mya Continents collide at end of period; meteorite probably strikes Earth Silurian 444 mya Sea levels rise; two large land masses emerge; hot/humid climate Ordovician 488 mya Massive glaciation, sea level drops 50 meters Cambrian 542 mya O 2 levels approach current levels 900 mya O 2 level at >5% of current level 1.5 bya O 2 level at >1% of current level Precambrian 3.8 bya O 2 first appears in atmosphere 4.5 bya Note: mya, million years ago; bya, billion years ago.

4 25.2 HOW HAVE EARTH S CONTINENTS AND CLIMATES CHANGED OVER TIME? 521 Radioisotope dating methods have been expanded and refined MAJOR EVENTS IN THE HISTORY OF LIFE Humans evolve; many large mammals become extinct Diversification of birds, mammals, flowering plants, and insects Dinosaurs continue to diversify; mass extinction at end of period ( 76% of species disappear) Diverse dinosaurs; radiation of ray-finned fishes; first fossils of flowering plants Early dinosaurs; first mammals; marine invertebrates diversify; mass extinction at end of period ( 65% of species disappear) Reptiles diversify; giant amphibians and flying insects present; mass extinction at end of period ( 96% of species disappear) Extensive fern forests; first reptiles; insects diversify Fishes diversify; first insects and amphibians; mass extinction at end of period ( 75% of species disappear) Jawless fishes diversify; first ray-finned fishes; plants and animals colonize land Mass extinction at end of period ( 75% of species disappear) Rapid diversification of multicellular animals; diverse photosynthetic protists Ediacaran fauna; earliest fossils of multicellular animals Eukaryotes evolve Origin of life; prokaryotes flourish Sedimentary rocks are formed from materials that existed for varying lengths of time before being transported, sometimes over long distances, to the site of their deposition. Therefore, the inorganic isotopes in a sedimentary rock do not contain reliable information about the date of its formation. Dating rocks more ancient than 50,000 years requires estimating isotope concentrations in igneous rocks rocks formed when molten material cools. To date older sedimentary rocks, geologists search for places where sedimentary rocks show igneous intrusions of volcanic ash or lava flows. Apreliminary estimate of the age of an igneous rock determines which isotope is used to date it (Figure 25.1B). The decay of potassium-40 (which has a half-life of 1.3 billion years) to argon-40 has been used to date many of the ancient events in the evolution of life. Fossils in the adjacent sedimentary rock that are similar to those in other rocks of known ages provide additional clues. Radioisotope dating of rocks, combined with fossil analysis, is the most powerful method of determining geological age. But in places where sedimentary rocks do not contain suitable igneous intrusions and few fossils are present, paleontologists turn to other methods. One method, known as paleomagnetic dating, relates the ages of rocks to patterns in Earth s magnetism, which change over time. Earth s magnetic poles move and occasionally reverse themselves. Because both sedimentary and igneous rocks preserve a record of Earth s magnetic field at the time they were formed, paleomagnetism helps determine the ages of those rocks. Other dating methods use information about continental drift, sea level changes, and molecular clocks (the last of which is described in Section 22.3). Using these methods, geologists divided the history of life into eras, which in turn are subdivided into periods (Table 25.1). The boundaries between these time frames are based on striking differences scientists have observed in the assemblages of fossil organisms contained in successive layers of rocks. Geologists defined and named these divisions before they were able to establish the ages of fossils, adding and refining the time scales as new methods for geological dating were developed RECAP Fossils in sedimentary rocks enabled geologists to determine the relative ages of organisms, but absolute dating was not possible until the discovery of radioactivity. Geologists divide the history of life into eras and periods, based on assemblages of fossil organisms found in successive layers of rocks. What observations about fossils suggested to geologists that they could be used to determine the relative ages of rocks? See p. 519 How is the rate of decay of radioisotopes used to estimate the absolute ages of rocks? See p. 520 and Figure 25.1 The scale at the left of Table 25.1 gives a relative sense of geological time, especially the vast expanse of the Precambrian era, during which early life evolved amid stupendous physical changes of Earth and its atmosphere. During the Precambrian to Cambrian transition, an explosion of new life forms took place as representatives of many of the major multicellular groups of life evolved. Earth continued to undergo massive physical changes that influenced the evolution of life, and these events and important milestones are listed in the table. In the next two sections we ll discuss the most important of these changes. How Have Earth s Continents and 25.2 Climates Changed over Time? The globes and maps that adorn our walls, shelves, and books give an impression of a static Earth. It would be easy for us to assume that the continents have always been where they are, but we would be wrong. The idea that Earth s land masses have changed position over the millennia, and that they continue to do so, was first put forth in 1912 by the German meteorologist

5 522 CHAPTER 25 HISTORY OF LIFE ON EARTH 25.2 Plate Tectonics and Continental Drift The heat of Earth s core generates convection currents (arrows) in the magma that push the lithospheric plates, along with the land masses lying on them, together or apart. When lithospheric plates collide, one often slides under the other. The resulting seismic activity can create mountains and deep rift valleys (the latter known as trenches when they occur under ocean basins). Cooling magma forms crust. Convection currents in liquid magma generate pressure that pushes the plates apart, forming ocean basins. Lithospheric plate Magma Mantle Where two plates collide, one is pushed under the other, generating seismic activity, mountains, rift valleys, and oceanic trenches. Melting lithosphere provides magma that fuels volcanoes. and geophysicist Alfred Wegener. His book The Origin of Continents and Oceans was initially met with skepticism and resistance. By the 1960s, however, physical evidence and increased understanding of the geophysics of plate tectonics the study of movement of major land masses had convinced virtually all geologists of the reality of Wegener s vision. Earth s crust consists of several solid plates approximately 40 kilometers thick, which collectively make up the lithosphere. The lithospheric plates float on a fluid layer of molten rock, or magma (Figure 25.2). Heat produced by radioactive decay deep in Earth s core sets up convection currents in the fluid magma, which then rises and exerts tremendous pressure on the solid plates. When the pressure of the rising magma pushes plates apart, ocean basins may form between them. When plates are pushed together, they either move sideways past each other or one plate slides under the other, pushing up mountain ranges and carving deep rift valleys. When they occur under the water of ocean basins, rift valleys are known as trenches. The movement of the lithospheric plates and the continents they contain is known as continental drift. We now know that at times the drifting of the plates has brought continents together and at other times has pushed them apart (these movements are depicted in Figure 25.12). The positions and sizes of the continents influence oceanic circulation patterns, global climates, and sea levels. Major drops in sea level have usually been accompanied by massive extinctions particularly of marine organisms, which could not survive the exposure of vast areas of the continental shelves and the disappearance of the shallow seas that covered them (Figure 25.3). yourbioportal.com GO TO Animated Tutorial 25.1 Evolution of the Continents 25.3 Sea Levels Have Changed Repeatedly Most mass extinctions of marine organisms (indicated by asterisks) have coincided with periods of low sea levels. High Sea level Low Asterisks indicate times of mass extinctions of marine organisms, most of which occurred when sea levels dropped. Precambrian * Cambrian Ordovician Silurian Devonian * * Carboniferous Permian Triassic Jurassic Cretaceous Tertiary P a l e o z o i c M e s o z o i c Cenozoic * * Quaternary Present Millions of years ago (mya)

6 (A) (B) The layers are formed as biofilms of cyanobacteria die and others take their place. Living cyanobacteria are found in the upper parts of these structures. 12 cm 30 cm 25.4 Stromatolites (A) A vertical section through a fossil stromatolite. (B) These rocklike structures are living stromatolites that thrive in the very salty waters of Shark Bay in western Australia. Layers of cyanobacteria are found in the uppermost parts of the structures. 523 stromatolites, which are abundantly preserved in the fossil record. Cyanobacteria are still forming stromatolites today in a few very salty places on Earth (Figure 25.4). Cyanobacteria liberated enough O 2 to open the way for the evolution of oxidation reactions as the energy source for the synthesis of ATP (see Section 9.1). The evolution of life thus irrevocably changed the physical nature of Earth. Those physical changes, in turn, influenced the evolution of life. When it first appeared in the atmosphere, O 2 was poisonous to the anaerobic prokaryotes that inhabited Earth at the time. Over millennia, however, prokaryotes that evolved the ability to metabolize O 2 not only survived but gained several advantages. Aerobic metabolism proceeds more rapidly and harvests energy more efficiently than anaerobic metabolism (see Section 9.4), and organisms with aerobic metabolism replaced anaerobes in most of Earth s environments. An atmosphere rich in O 2 also made possible larger cells and more complex organisms. Small unicellular aquatic organisms can obtain enough O 2 by simple diffusion even when O 2 concentrations are very low. Larger unicellular organisms have lower surface area-to-volume ratios (see Figure 5.2); to obtain enough O 2 by simple diffusion, they must live in an environment with a relatively high oxygen concentration. Bacteria can thrive on 1 percent of the current atmospheric O 2 levels; eukaryotic cells require levels that are at least 2 3 percent of current concentrations. (For concentrations of dissolved O 2 in the oceans to reach these levels, much higher atmospheric concentrations were needed.) Probably because it took many millions of years for Earth to develop an oxygenated atmosphere, only unicellular prokaryotes lived on Earth for more than 2 billion years. About 1.5 bya, atmospheric O 2 concentrations became high enough for large eukaryotic cells to flourish (Figure 25.5). Further increases in atmospheric O 2 levels 750 to 570 million years ago (mya) enabled several groups of multicellular organisms to evolve. Oxygen concentrations in Earth s atmosphere have changed over time As the continents have moved over Earth s surface, the world has experienced other physical changes, including large increases and decreases in atmospheric oxygen. The atmosphere of early Earth probably contained little or no free oxygen gas (O 2 ). The increase in atmospheric O 2 came in two big steps more than a billion years apart. The first step occurred at least 2.4 billion years ago (bya), when certain bacteria evolved the ability to use water as the source of hydrogen ions for photosynthesis. By chemically splitting H 2 O, these bacteria generated atmospheric O 2 as a waste product. They also made electrons available for reducing CO 2 to form organic compounds (see Section 10.3). One group of O 2 -generating bacteria, the cyanobacteria, formed rocklike structures called O 2 in atmosphere (%) First life First photosynthetic First bacteria eukaryotes First aerobic bacteria O 2 levels almost 50% higher than present. First multicellular organisms First chordates Rapid drop of O 2 levels at end of the Permian Giant flying insects Invasion of land First flowering plants O 2 levels 25 40% lower than present. 4,000 3,000 2,000 1, Present Millions of years ago (mya) 25.5 Larger Cells, Larger Organisms Need More Oxygen Changes in oxygen concentrations have strongly influenced, and been influenced by, the evolution of life. (Note that the horizontal axis of the graph is on a logarithmic scale.)

7 524 CHAPTER 25 HISTORY OF LIFE ON EARTH CONCLUSION INVESTIGATING LIFE 25.6 Rising Oxygen Levels and Body Size in Insects In this experiment, flies were raised under hyperbaric conditions (increased atmospheric pressure), thus increasing the partial pressure of O 2 in a manner that simulated the greater levels of atmospheric O 2 characteristic of the Carboniferous and Permian. Robert Dudley asked if flies raised in hyperbaric conditions would grow larger than their normal counterparts. HYPOTHESIS Under increased atmospheric pressure, the increased partial pressure of O 2 will allow directional selection for increased body size in flying insects. METHOD RESULTS Body mass (mg) Divide a population of fruit flies (Drosophila melanogaster) into two lines. 2. Raise one line (the control) at current atmospheric oxygen conditions. Raise the experimental line in hyperbaric conditions (increased partial pressure of O 2, simulating increased atmospheric oxygen concentrations). Continue for 5 generations. 3. Raise the F 6 offspring of both lines under identical environmental conditions. 4. Weigh all the F 6 individuals and test for statistical differences in the average body mass of the flies in each population. The average body mass of F 6 individuals of both sexes in the experimental line was significantly (p < ) greater than that of insects in the control line. Males Females Normal atmosphere (control) Hyperbaric conditions In at least some flying insects, increased concentrations of oxygen could lead to a long-term evolutionary trend toward increased body size. FURTHER INVESTIGATION: How would you confirm that the change in average body size is related to increased partial pressure of O 2, and not to other aspects of overall increased atmospheric pressure? Go to yourbioportal.com for original citations, discussions, and relevant links for all INVESTIGATING LIFE figures. O 2 concentrations increased again during the Carboniferous and Permian periods because of the evolution of large vascular plants in the expansive lowland swamps that existed then (see Table 25.1). These swamps resulted in extensive burial of plant debris from vascular plants, which led to the formation of Earth s vast coal deposits. As the buried organic material was not subject to oxidation, and the living plants were producing large quantities of O 2, atmospheric O 2 increased to concentrations that have not been reached again in Earth s history (see Figure 25.5). As mentioned in the opening of this chapter, high concentrations of atmospheric O 2 allowed the evolution of giant flying insects and amphibians that could not survive in today s atmosphere. The drying of the lowland swamps at the end of the Permian reduced global organic burial, and also the production of atmospheric O 2, so O 2 concentrations dropped rapidly. Over the past 200 million years, with the diversification of flowering plants, O 2 concentrations have again increased, but not to the levels that characterized the Carboniferous and Permian periods. Biologists have conducted experiments that demonstrate the changing selective pressures that can accompany changes in O 2 levels. In experimental conditions, an increase in O 2 concentration can be simulated by increasing atmospheric pressure in a hyperbaric chamber. Increasing atmospheric pressure increases the partial pressure of oxygen (see Chapter 49) in a manner that simulates an increase in O 2 concentration at normal atmospheric pressure. When lines of fruit flies (Drosophila) are raised in artificial hyperbaric atmospheres (which have higher partial pressure of O 2 ), they quickly evolve larger body sizes over just a few generations (Figure 25.6). The current levels of atmospheric O 2 appear to constrain body size evolution of these flying insects; increases in O 2 appear to relax these constraints. This demonstrates that the stabilizing selection on body size at present O 2 concentrations can quickly switch to directional selection (see Section 21.3) for a change in body size in response to a change in O 2 levels. Directional selection over a period of millions of years would be sufficient to account for giant insects such as Meganeuropsis, described at the beginning of this chapter. Many physical conditions on Earth have oscillated in response to the planet s internal processes, such as volcanic activity and continental drift. Extraterrestrial events, such as collisions with meteorites, have also left their mark. In some cases, as we saw earlier and will see again in this chapter, changing physical parameters caused mass extinctions, during which a large proportion of the species living at the time disappeared. After each mass extinction, the diversity of life rebounded, but recovery took millions of years. Earth s climate has shifted between hot/humid and cold/dry conditions Through much of its history, Earth s climate was considerably warmer than it is today, and temperatures decreased more gradually toward the poles. At other times, Earth was colder than it is today. Large areas were covered with glaciers near the end of the Precambrian and Ordovician, and during parts of the Carboniferous and Permian periods. These cold periods were separated by long periods of milder climates (Figure 25.7). Because we are living in one of the colder periods in Earth s history, it is difficult for us to imagine the mild climates that were found at high latitudes during much of the history of life. During the

8 25.2 HOW HAVE EARTH S CONTINENTS AND CLIMATES CHANGED OVER TIME? 525 High Earth s mean temperature Low Cold/dry Hot/humid Cold/dry Hot/humid Large areas of Earth s surface were covered by glaciers during these periods. Pangaea Cold/dry Hot/humid Cold/dry Precambrian Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Tertiary P a l e o z o i c M e s o z o i c Cenozoic Quaternary Present Millions of years ago (mya) 25.7 Hot/Humid and Cold/Dry Conditions Have Alternated over Earth s History Throughout Earth s history, periods of cold climates and glaciations (white depressions) have been separated by long periods of milder climates. Quaternary period there has been a series of glacial advances, interspersed with warmer interglacial intervals during which the glaciers retreated. Weather refers to daily events, such as individual storms. Climate refers to long-term average expectations of the various seasons at a given location. Weather often changes rapidly; climates typically change slowly. Major climatic shifts have taken place over periods as short as 5,000 to 10,000 years, primarily as a result of changes in Earth s orbit around the sun. A few climatic shifts have been even more rapid. For example, during one Quaternary interglacial period, the ice-locked Antarctic Ocean became nearly ice-free in less than 100 years. Such rapid changes are usually caused by sudden shifts in ocean currents. Some climate changes have been so rapid that the extinctions caused by them appear to be nearly instantaneous in the fossil record. We are currently living in a time of rapid climate change thought to be caused by a buildup of atmospheric CO 2, primarily from the burning of fossil fuels. We are reversing the process of organic burial that occurred (especially) in the Carboniferous and Permian, but we are doing so over a few hundred years rather than the many millions of years over which these deposits accumulated. The current rate of increase of atmospheric CO 2 is unprecedented in Earth s history. A doubling of the atmospheric CO 2 concentration which may happen during the current century is expected to increase the average temperature of Earth, change rainfall patterns, melt glaciers and ice caps, and raise sea level. The possible consequences of such climate changes are discussed in Chapters 58 and 59. Volcanoes have occasionally changed the history of life Most volcanic eruptions produce only local or short-lived effects, but a few large volcanic eruptions have had major consequences for life. When Krakatoa erupted in Indonesia in 1883, it ejected more than 25 cubic kilometers of ash and rock, as well as large quantities of sulphur dioxide gas (SO 2 ). The SO 2 was ejected into the stratosphere and then moved by high-level winds around the planet. This led to high concentrations of sulphurous acid (H 2 SO 3 ) in high-level clouds, which meant less sunlight got through to Earth s surface. Global temperatures dropped by 1.2 C in the year following the eruption, and global weather patterns showed strong effects for another 5 years. This was all the result of a single volcanic eruption. The collision of continents during the Permian period (about 275 mya) formed a single, gigantic land mass (Pangaea) and caused many massive volcanic eruptions. These eruptions resulted in considerable blockage of sunlight, contributing to the glaciations of that time (see Figure 25.7). Massive volcanic eruptions occurred again as the continents drifted apart during the late Triassic and at the end of the Cretaceous. Extraterrestrial events have triggered changes on Earth At least 30 meteorites between the sizes of baseballs and soccer balls hit Earth each year. Collisions with large meteorites or comets are rare, but such collisions have probably been responsible for several mass extinctions. Several types of evidence tell us about these collisions. Their craters, and the dramatically disfigured rocks that resulted from their impact, are found in many places. Geologists have also discovered compounds in these rocks that contain helium and argon with isotope ratios characteristic of meteorites, which are very different from the ratios found elsewhere on Earth. A meteorite caused or contributed to a mass extinction at the end of the Cretaceous period (about 65 mya). The first clue that a meteorite was responsible came from the abnormally high concentrations of the element iridium in a thin layer separating rocks deposited during the Cretaceous from those deposited during the Tertiary (Figure 25.8). Iridium is abundant in some meteorites, but it is exceedingly rare on Earth s surface. Scientists discovered a circular crater 180 kilometers in diameter buried beneath the northern coast of the Yucatán Peninsula of

9 526 CHAPTER 25 HISTORY OF LIFE ON EARTH What Are the Major Events 25.3 in Life s History? Iridium-rich layer at the Cretaceous-Tertiary (K/T) boundary 25.8 Evidence of a Meteorite Impact The white layers of rock are Cretaceous in age; the layers at the upper left were deposited in the Tertiary. Between the two is a thin, dark layer of clay that contains large amounts of iridium, a metal common in some meteorites but rare on Earth. Its high concentration in sediments deposited about 65 million years ago suggests the impact of a large meteorite. Mexico. When it collided with Earth, the meteorite released energy equivalent to that of 100 million megatons of high explosives, creating great tsunamis. A massive plume of debris swelled to a diameter of up to 200 kilometers, spread around Earth, and descended. The descending debris heated the atmosphere to several hundred degrees, ignited massive fires, and blocked the sun, preventing plants from photosynthesizing. The settling debris formed the iridium-rich layer. About a billion tons of soot, which has a composition that matches smoke from forest fires, was also deposited. Many fossil species (particularly dinosaurs) that are found in Cretaceous rocks are not found in the Tertiary rocks of the next layer RECAP Conditions on Earth have changed dramatically over time. Changes in atmospheric concentrations of O 2 and in Earth s climate have had major effects on biological evolution. Continental drift, volcanic eruptions, and large meterorite strikes have contributed to climatic changes during Earth s history. Describe how increases in atmospheric concentrations of O 2 affected the evolution of multicellular organisms. See pp and Figure 25.5 How have volcanic eruptions and meteorite strikes influenced the course of life s evolution? See p. 525 The many dramatic physical events of Earth s history have influenced the nature and timing of evolutionary changes among Earth s living organisms. We now will look more closely at some of the major events that characterize the history of life on Earth. Life first evolved on Earth about 3.8 bya. By about 1.5 bya, eukaryotic organisms had evolved (see Table 25.1). The fossil record of organisms that lived prior to 550 mya is fragmentary, but it is good enough to show that the total number of species and individuals increased dramatically in late Precambrian times. As discussed above, pre-darwinian geologists divided geological history into eras and periods based on their distinct fossil assemblages. Biologists refer to the assemblage of all organisms of all kinds living at a particular time or place as a biota. All of the plants living at a particular time or place are its flora; all of the animals are its fauna. Table 25.1 describes some of the physical and biological changes, such as mass extinctions and dramatic increases in the diversity of major groups of organisms, associated with each unit of time. About 300,000 species of fossil organisms have been described, and the number steadily grows. The number of named species, however, is only a tiny fraction of the species that have ever lived. We do not know how many species lived in the past, but we have ways of making reasonable estimates. Of the present-day biota, nearly 1.8 million species have been named. The actual number of living species is probably well over 10 million, and possibly much higher, because many species have not yet been discovered and described by biologists. So the number of described fossil species is only about 3 percent of the estimated minimum number of living species. Life has existed on Earth for about 3.8 billion years. Many species last only a few million years before undergoing speciation or going extinct; therefore, Earth s biota must have turned over many times during geological history. So the total number of species that have lived over evolutionary time must vastly exceed the number living today. Why have only about 300,000 of these tens of millions of species been described from fossils to date? Several processes contribute to the paucity of fossils Only a tiny fraction of organisms ever become fossils, and only a tiny fraction of fossils are ever discovered by paleontologists. Most organisms live and die in oxygen-rich environments in which they quickly decompose. They are not likely to become fossils unless they are transported by wind or water to sites that lack oxygen, where decomposition proceeds slowly or not at all. Furthermore, geological processes often transform rocks, destroying the fossils they contain, and many fossil-bearing rocks are deeply buried and inaccessible. Paleontologists have studied only a tiny fraction of the sites that contain fossils, but they find and describe many new ones every year. The fossil record is most complete for marine animals that had hard skeletons (which resist decomposition). Among the nine major animal groups with hard-shelled members, approximately 200,000 species have been described from fossils roughly twice the number of living marine species in these same groups. Paleontologists lean heavily on these groups in their interpretations of the evolution of life. Insects and spiders are also relatively well represented in the fossil record, because they are numerically abundant and have hard exoskeletons

10 25.3 WHAT ARE THE MAJOR EVENTS IN LIFE S HISTORY? 527 For most of its history, life was confined to the oceans, and all organisms were small. Over the long ages of the Precambrian era more than 3 billion years the shallow seas slowly began to teem with life. For most of the Precambrian, life consisted of microscopic prokaryotes; eukaryotes evolved about twothirds of the way through the era (Figure 25.10). Unicellular eukaryotes and small multicellular animals fed on floating photosynthetic microorganisms. Small floating organisms, known collectively as plankton, were strained from the water and eaten by slightly larger filter-feeding animals. Other animals ingested sediments on the seafloor and digested the remains of organisms within them. By the late Precambrian ( mya), many kinds of multicellular soft-bodied animals had evolved. Some of them were very different from any animals living today, and may be members of groups that have no living descendants (Figure 25.11). Life expanded rapidly during the Cambrian period Solenopsis sp Insect Fossils Chunks of amber fossilized tree resin often contain insects that were preserved when they were trapped in the sticky resin. This fire ant fossil is some 30 million years old. (Figure 25.9). The fossil record, though incomplete, is good enough to document clearly the factual history of the evolution of life. By combining information about geological changes during Earth s history with evidence from the fossil record, scientists have composed portraits of what Earth and its inhabitants may have looked like at different times. We know in general where the continents were and how life changed over time, but many of the details are poorly known, especially for events in the more remote past. The Cambrian period ( mya) marks the beginning of the Paleozoic era. The oxygen concentration in the Cambrian atmosphere was approaching its current level, and the land masses had come together to form several large continents. A geologically rapid diversification of life took place that is sometimes referred to as the Cambrian explosion (although in fact it began before the Cambrian, and the explosion took millions of years). Several of the major groups of animals that have species living today first evolved during the Cambrian. An overview of the continental and biotic shifts that characterized the Cambrian and subsequent periods is shown in Figure on the following pages. For the most part, fossils tell us only about the hard parts of organisms, but in three known Cambrian fossil beds the Burgess Shale in British Columbia, Sirius Passet in northern Greenland, and the Chengjiang site in southern China the soft parts of many animals were preserved. Crustacean arthropods (crabs, shrimps, and their relatives) are the most diverse group Precambrian life was small and aquatic A Sense of Life s Time The top timeline shows the 4.5 billion year history of life on Earth. Most of this time is accounted for by the Precambrian, a 3.4 billion year era that saw the origin of life and the evolution of cells, photosynthesis, and multicellularity. The final 600 million years are expanded in the second timeline and detailed in Figure Formation of the Earth First oceans Origin of life Origin of photosynthesis First eukaryotes First photosynthetic eukaryotes First fossils of multicellular animals BILLIONS OF YEARS AGO P r e c a m b r i a n MILLIONS OF YEARS AGO Precambrian Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Tertiary P a l e o z o i c M e s o z o i c Cenozoic Quaternary Present

11 528 CHAPTER 25 HISTORY OF LIFE ON EARTH Spriggina floundersi Mawsonites spriggi Dickinsonia costata Precambrian Life These fossils of soft-bodied invertebrates, excavated at Ediacara in southern Australia, were formed about 600 million years ago. Very different from later life forms, they illustrate the diversity of life at the end of the Precambrian era. in the Chinese fauna; some of them were large carnivores. Multicellular diversity was largely or completely aquatic during the Cambrian. If there was life on land at this time, it was probably restricted to microbial organisms. Many groups of organisms that arose during the Cambrian later diversified Geologists divide the remainder of the Paleozoic era into the Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. Each period is characterized by the diversification of specific groups of organisms. Mass extinctions marked the ends of the Ordovician, Devonian, and Permian. THE ORDOVICIAN ( MYA) During the Ordovician period, the continents, which were located primarily in the Southern Hemisphere, still lacked multicellular plants. Evolutionary radiation of marine organisms was spectacular during the early Ordovician, especially among animals, such as brachiopods and mollusks, that lived on the seafloor and filtered small prey from the water. At the end of the Ordovician, as massive glaciers formed over the southern continents, sea levels dropped about 50 meters and ocean temperatures dropped. About 75 percent of the animal species became extinct, probably because of these major environmental changes. THE SILURIAN ( MYA) During the Silurian period, the continents began to merge together. Marine life rebounded from the mass extinction at the end of the Ordovician. Animals able to swim in open water and feed above the ocean bottom appeared for the first time. Jawless fishes diversified, and the first ray-finned fishes evolved. The tropical sea was uninterrupted by land barriers, and most marine organisms were widely distributed. On land, the first vascular plants evolved late in the Silurian (about 420 mya). The first terrestrial arthropods scorpions and millipedes evolved at about the same time. THE DEVONIAN ( MYA) Rates of evolutionary change accelerated in many groups of organisms during the Devonian period. The major land masses continued to move slowly toward each other. In the oceans there were great evolutionary radiations of corals and of shelled, squidlike cephalopod mollusks. Fishes diversified as jawed forms replaced jawless ones and as heavy armor gave way to the less rigid outer coverings of modern fishes. Terrestrial communities changed dramatically during the Devonian. Club mosses, horsetails, and tree ferns became common; some attained the size of large trees. Their roots accelerated the weathering of rocks, resulting in the development of the first forest soils. The ancestors of gymnosperms the first plants to produce seeds appeared in the Devonian. The first known fossils of centipedes, spiders, mites, and insects date to this period, and fishlike amphibians began to occupy the land. A massive extinction of about 75 percent of all marine species marked the end of the Devonian. Paleontologists are uncertain about its cause, but two large meteorites that collided with Earth at that time (one in present-day Nevada and the other in western Australia) may have been responsible, or at least a contributing factor. The continued coalescence of the continents, with the corresponding reduction in continental shelves, may have also contributed to this mass extinction event. THE CARBONIFEROUS ( MYA) Large glaciers formed over high-latitude portions of the southern land masses during the Carboniferous period, but extensive swamp forests grew on the tropical continents. These forests were not made up of the kinds of trees we know today, but were dominated by giant tree ferns and horsetails with small leaves. Fossilized remains of those forests formed the coal we now mine for energy. In the seas, A Brief History of Life on Earth The geologically rapid explosion of life during the Cambrian saw the rise of several animal groups that have representatives surviving today. The following three pages depict life s history from the Cambrian forward. Movements of the major continents during the past half-billion years are shown in the maps of Earth, and associated biotas for each time period are depicted. The artists reconstructions are based on fossils such as those shown in the photographs.

12 Rapid increase of multicellular organisms (Cambrian explosion ) Major radiation of several marine groups Cambrian MILLIONS OF YEARS AGO First jawed fishes; First vascular plants many animal groups and terrestrial radiate; forests appear arthropods evolve on land Ordovician Silurian Devonian Precambrian P a l e o z o i c % of all animals go extinct as sea levels drop by 50 meters Cambrian 75% of marine species go extinct Devonian Phacops ferdinandi Ottoia sp. Orthoconic nautiloid Marrella splendens Anomalocaris canadensis (claw only) Codiacrinus schultzei Eridophyllum sp.

13 Extensive swamp forests produce coal; origin of amniotes; great increase in terrestrial animal diversity Giant amphibians and flying insects; ray-finned fishes abundant in freshwater Carboniferous On land, conifers become dominant plants; frogs and reptiles begin to diversify Permian Dinosaurs, pterosaurs, ray-finned fishes diversify; first mammals appear Triassic First known flowering plant fossils Jurassic P a l e o z o i c M e s o z o i c Extinction of 96% of Earth s species; oxygen levels drop rapidly L A U R A S I A P A N G O N D W G A E A Permian A N A Triassic Coelophysis bauri Phlebopteris smithii Walchia piniformis Cacops sp.

14 25.3 Flowering plants diversify Many radiations of animal groups, on both land and sea Flowering plants dominate on land; rapid radiation of mammals Cretaceous Grasslands spread as climates cool WHAT ARE THE MAJOR EVENTS IN LIFE S HISTORY? Four major ice ages; evolution of Homo Tertiary M e s o z o i c 531 Quaternary C e n o z o i c 100 Present Mass extinction event, including loss of most dinosaurs Cretaceous Tertiary Hyracotherium leporinum Gryposaurus sp. Magnolia sp. Plesiadapis fodinatus (jaw)

15 532 CHAPTER 25 HISTORY OF LIFE ON EARTH Evidence of Insect Diversification The margins of this fossil fern leaf from the Carboniferous have been chewed by insects. crinoids (sea lilies and feather stars) reached their greatest diversity, forming meadows on the seafloor. The diversity of terrestrial animals increased greatly during the Carboniferous. Snails, scorpions, centipedes, and insects were abundant and diverse. Insects evolved wings, becoming the first animals to fly. Flight gave herbivorous insects easy access to tall plants; plant fossils from this period show evidence of chewing by insects (Figure 25.13). The terrestrial vertebrate lineage split, and amphibians became larger and better adapted to terrestrial existence, while the sister lineage led to the amniotes, vertebrates with well-protected eggs that can be laid in dry places. THE PERMIAN ( MYA) During the Permian period, the continents coalesced completely into the supercontinent Pangaea. Permian rocks contain representatives of many major groups of insects we know today. By the end of the period, the reptiles split from a second amniote lineage (which would lead to the mammals). Ray-finned fishes became common in the fresh waters of Pangaea. Conditions for life deteriorated toward the end of the Permian. Massive volcanic eruptions resulted in outpourings of lava that covered large areas of Earth. The ash and gases produced by the volcanoes blocked sunlight and cooled the climate, resulting in the largest glaciers in Earth s history. Atmospheric oxygen concentrations gradually dropped from about 30 to 15 percent. At such low concentrations, most animals would have been unable to survive at elevations above 500 meters; thus about half of the land area would have been uninhabitable at the end of the Permian. The combination of these changes resulted in the most drastic mass extinction event in Earth s history. Scientists estimate that about 96 percent of all species became extinct at the end of the Permian. Geographic differentiation increased during the Mesozoic era The few organisms that survived the Permian mass extinction found themselves in a relatively empty world at the start of the Mesozoic era (251 mya). As Pangaea slowly began to break apart, the oceans rose and once again flooded the continental shelves, forming huge, shallow inland seas. Atmospheric oxygen concentrations gradually rose. Life once again proliferated and diversified, but different groups of organisms came to the fore. The three groups of phytoplankton (floating photosynthetic organisms) that dominate today s oceans dinoflagellates, coccolithophores, and diatoms became ecologically important at this time; their remains are the primary origin of the world s oil deposits. Seed-bearing plants replaced the trees that had ruled the Permian forests. The Mesozoic era is divided into three periods: the Triassic, Jurassic, and Cretaceous. The Triassic and Cretaceous were terminated by mass extinctions, probably caused by meteorite impacts. THE TRIASSIC ( MYA) Pangaea began to break apart during the Triassic period. Many invertebrate groups became more species-rich, and many burrowing animals evolved from groups living on the surfaces of seafloor sediments. On land, conifers and seed ferns were the dominant trees. The first frogs and turtles appeared. A great radiation of reptiles began, which eventually gave rise to crocodilians, dinosaurs, and birds. The end of the Triassic was marked by a mass extinction that eliminated about 65 percent of the species on Earth. THE JURASSIC ( MYA) During the Jurassic period, Pangaea became fully divided into two large continents: Laurasia drifted northward and Gondwana drifted south. Ray-finned fishes rapidly diversified in the oceans. The first lizards appeared, and flying reptiles (pterosaurs) evolved. Most of the large terrestrial predators and herbivores of the period were dinosaurs. Several groups of mammals made their first appearance, and the earliest known fossils of flowering plants are from late in this period. THE CRETACEOUS ( MYA) By the early Cretaceous period, Laurasia and Gondwana had begun to break apart into the con-