Evolutionary Radiations and Mass Extinctions

In most cases, the boundaries of the eras and periods within the Phanerozoic eon are marked by the demise, or extinction, of a particular group of organisms, followed by the rise, or radiation, of a new group of organisms. When groups of organisms are no longer able to adapt to changing environmental conditions or compete with more successful groups of organisms, they become extinct. An interval when many groups of organisms become extinct at the same time is called a mass extinction (Figure 11.17) (see Chapter 8). In a few cases, the boundaries of the geologic time scale are marked by environmental catastrophes of truly global magnitude. Radiations are stimulated by the availability of new habitats when a mass extinction eliminates highly competitive and established groups of organisms.

Figure 11.17: The diversity of animal fossils reveals both mass extinctions and radiations. This graph shows the number of “shelly” animal families found in the fossil record during the last 600 million years; each family comprises many species. During a radiation, such as the Cambrian explosion, the number of new families increases. During a mass extinction, such as the one at the end of the Cretaceous period, the number of families decreases. (Ma, million years ago.)

Radiation of Life: The Cambrian Explosion

Perhaps the most remarkable geobiological event in Earth’s history, aside from the origin of life itself, was the sudden appearance of large animals with shells and skeletons at the end of Precambrian time (Figure 11.18). This rapid development of new types of organisms from a common ancestor—what biologists call an evolutionary radiation—had such an extraordinary effect on the fossil record that its culmination 542 million years ago is used to mark the most profound boundary of the geologic time scale: the beginning of the Phanerozoic eon. This boundary also coincides with the start of the Paleozoic era and the Cambrian period (see Chapter 8 and Figure 11.12).

Figure 11.18: Fossils that record the Cambrian explosion. Precambrian organisms such as Namacalathus (left) were the first organisms to use calcite in making shells. These organisms became extinct at the Precambrian-Cambrian boundary. Their extinction paved the way for a strange new group of organisms, including Hallucigenia (center) and the more familiar trilobites (right), that formed weak shells made of organic material similar to fingernails. In each example, the fossils are shown on top and the reconstructed organism is shown on the bottom.

Evolutionary radiations are rapid by nature; if they were not, they would not be noticed in the fossil record. However, the radiation of animals during the early Cambrian, after almost 3 billion years of very slow evolution, was so fast that it is often called the Cambrian explosion, or biology’s Big Bang. Every major animal group that exists on Earth today, as well as a few more that have since become extinct, appeared within less than 10 million years. All the major branches (phyla) on the animal tree of life (Figure 11.19) originated during the Cambrian explosion. Note, however, that as impressive as this tree of animals seems, it is a single, short branch of the universal tree of life (see Figure 11.5).

Figure 11.19: Every major group of animals alive today originated during a great evolutionary radiation in the early Cambrian known as the Cambrian explosion.

Geobiologists have raised two major questions about the Cambrian explosion. First, what allowed these early animals to develop such complex body forms so rapidly, and therefore to become so diverse? Systematic change in organisms over many generations is referred to as evolution. Evolution is driven by natural selection, the process by which populations of organisms adapt to changes in their environment. The theory of evolution by natural selection states that, over many generations, individuals with the most favorable traits are most likely to survive and reproduce, passing those traits on to their offspring. If environmental conditions change over time, the traits that are favored change as well. This process can lead eventually to the emergence of new species.

One hypothesis for the cause of the Cambrian explosion is that the genes of these early animals changed in some way that made it possible for them to exceed some sort of evolutionary barrier. The stage was set by the development of multicellularity in late Precambrian time (Figure 11.20), which opened up new evolutionary possibilities. It is also possible that the ancestral animals had to reach a certain size before they could diversify. Some Precambrian animals, such as the fossil animal embryo shown in Figure 11.20, are so small they can be seen only with a microscope. The development of shells and skeletons might have been an important trigger of further diversification: once one group of animals had evolved hard parts, the others had to as well, or they would have been eliminated through competition.

Figure 11.20: A fossilized animal embryo from the latest part of Precambrian time. Such fossils show that multicellular animals had evolved before the Cambrian period and are the ancestors of the animals that evolved during the Cambrian explosion.

The second riddle of the Cambrian explosion is why these animals differentiated when they did. Geobiologists have puzzled over the timing of the Cambrian explosion for more than 150 years. Back in the days of Charles Darwin, it wasn’t clear whether the Cambrian explosion represented the origin of life itself. But the sudden appearance of complex and diverse animal fossils in the geologic record presented a challenge to Darwin’s theory of natural selection. His theory predicted slow changes in the form and function of organisms; hence, it predicted that less complex life-forms should have occurred before the first animals, and it could not easily accommodate these complex creatures that apparently had no simpler ancestors. Therefore, Darwin hypothesized that the expected ancestors must be absent from the record because the rocks containing the Cambrian fossils must lie above an unconformity. He predicted that rocks from the time of the proposed unconformity would eventually be discovered, and that those rocks would contain the “missing” ancestors. Darwin turned out to be right, but it was only in the past several decades that geobiologists discovered the fossils described earlier in this chapter, proving that animals did indeed originate before the Cambrian explosion.

So it seems clear that the Cambrian animals did have ancestors, perhaps lurking between tiny grains of sand at the bottom of shallow seas. However, isotopic dating techniques show that these tiny animals were probably less than 100 million years older than their Cambrian descendants. Other dating techniques, based on studies of the genes of modern organisms, suggest that the origin of animals may have predated the Cambrian explosion by several hundred million years. But even these estimates hardly matter compared with the billions of years that passed before the Cambrian explosion occurred.

Most geobiologists agree that once animals had evolved, they could have radiated at any time. Why, then, did they radiate about 542 million years ago and not at some other time? Perhaps the timing of the Cambrian explosion was driven by the dramatic environmental changes that occurred near the end of Precambrian time. To human eyes, Earth at that time would have seemed a very strange place: long chains of great mountains were forming as the pieces of the giant continent Gondwana were being fused together, and the climate was in turmoil, flipping between frigid periods when the entire Earth may have been covered in ice and extremely warm, ice-free periods (see Chapter 21). Oxygen concentrations in the oceans and atmosphere were increasing as erosion of the rising mountains produced sediments, which buried the organic matter whose decomposition would otherwise have consumed that oxygen. This last change may have been the most important. Without sufficient oxygen, animals simply cannot grow large.

Whatever the ultimate cause of the Cambrian explosion, one point stands clear: evolutionary radiations are the result of genetic possibility combined with environmental opportunity. The radiation of organisms is not just the result of having the right genes, and it is not just the result of living in the right environment. Organisms must take advantage of both to evolve.

Tail of the Devil: The Demise of Dinosaurs

The mass extinction that marks the Cretaceous-Tertiary boundary and the end of the Mesozoic era (about 65 million years ago; see Figures 8.11 and 8.15) represents one of the greatest such events in Earth’s history. Entire global ecosystems were obliterated, and about 75 percent of all species on Earth, both on land and in the ocean, were extinguished forever. The dinosaurs are only one of several groups that became extinct at the end of the Cretaceous period, but they are certainly the most prominent. Other groups, such as ammonites, marine reptiles, certain types of clams, and many types of plants and plankton, also perished.

In contrast to the Cambrian explosion, almost all scientists agree on the cause of the Cretaceous-Tertiary mass extinction. We are now virtually certain that the cause was a gigantic asteroid impact. In 1980, geologists discovered a thin layer of dust containing iridium—an element that is typical of extraterrestrial materials—in sediments deposited at the end of the Cretaceous in Italy (Figure 11.21).

Figure 11.21: The pocket knife marks a light-colored layer of clay, containing both extraterrestrial materials and materials from local rocks at the Chicxulub impact site, which accumulated in the Raton Basin of the southwestern United States. Such deposits have been found worldwide.

This extraterrestrial dust was subsequently found at many other locations around the world, on every continent and in every ocean, but always exactly at the Cretaceous-Tertiary boundary. The geologists argued that the accumulation of this much iridium-bearing dust would require an asteroid about 10 km in diameter to hit Earth, explode, and send its cosmic detritus across the globe. Publication of this hypothesis spurred a search for the impact crater. That search was bound to be difficult for two reasons. First, most of Earth’s surface is covered by oceans, so the crater could easily have been under water. Second, since the crater would be 65 million years old, it could have been eroded or filled in with sediments and sedimentary rock. In the early 1990s, however, geologists found a huge crater, almost 200 km in diameter and 1.5 km deep, buried under sediments near a town on Mexico’s Yucatán Peninsula, called Chicxulub.

Geologic evidence from Chicxulub, as well as from the surrounding region and around the world, has allowed geologists to paint a picture of what happened there. The name Chicxulub means “tail of the devil” in the local Mayan language, and the immediate aftermath of the impact would have been hellish indeed. The asteroid struck Chicxulub at Mach 40, coming in from the south at an angle of about 20° to 30° from the horizontal. Its explosion would have produced a blast 6 million times more powerful than the 1980 eruption of Mount St. Helens. It would have created winds of unimaginable fury and a tsunami as high as 1 km (100 times higher than the great Indian Ocean tsunami of 2004). The sky would have turned black with massive amounts of dust and vapor. A global firestorm may have resulted as the flaming fragments from the blast fell back to Earth (Figure 11.22).

Figure 11.22: An artist’s rendition of the Cretaceous-Tertiary scene after the asteroid impact.

Materials from the impact crater spread out in a radial kill zone focused toward western and central North America. Creatures around at that time, assuming they weren’t in the kill zone, might have witnessed the following events: a brilliant flash as the asteroid rammed into Chicxulub, vaporizing Earth’s upper crust at temperatures up to 10,000°C; an arc of flaming hot rocks that bolted across the sky at speeds of up to 40,000 km/hour, then crashed into North America; and a plume of debris, gas, and molten material that heated part of the atmosphere to several hundred degrees, punched into space, and then collapsed back to Earth. Over the next several days or weeks, the finer materials in this plume would have settled across Earth’s entire surface.

The direct effects of the impact would have been devastating for many organisms. But worse yet would have been the aftermath for months and years to come, which scientists think led to the actual mass extinction. The high concentration of debris in the atmosphere would have blocked out the Sun, vastly reducing the light available for photosynthesis. In addition to solid particles of debris, poisonous sulfur- and nitrogen-bearing gases would have been injected into the atmosphere, where they would have reacted with water vapor to form toxic sulfuric and nitric acids that would have rained down on Earth. The combination of these two effects, and others, would have been devastating to plants and other photosynthetic autotrophs, and thus to both marine and terrestrial ecosystems that depended on them as the base of the food chain. Heterotrophs, including the dinosaurs, would have been next; once their food sources died off, they would have died off as well. A cascading series of such effects leading to the collapse of ecosystems was probably the ultimate cause of the mass extinction.

Global Warming Disaster: The Paleocene-Eocene Mass Extinction

The mass extinction at the Paleocene-Eocene boundary (about 55 million years ago; see Figure 8.11) was not one of the largest such events. It was an important event in the evolution of life, however, because it paved the way for the mammals, including the primates, to radiate into an important group. Unlike the mass extinction that wiped out the dinosaurs, it had no extraterrestrial cause. Instead, it was caused by abrupt global warming. Earth scientists are very interested in the details of what happened because global warming—this time produced by human activities—may threaten ecosystems in the coming decades (as we will see in Chapter 23).

We now believe that the global warming at the end of the Paleocene epoch occurred when the oceans suddenly belched an enormous amount of methane—a potent greenhouse gas—into the atmosphere. The resulting global warming was the primary cause of the mass extinction. But where did all that methane come from? To unravel this mystery, we must weave together many of the processes we have learned about in this chapter, including microbial metabolism, biogeochemical cycles, and the global behavior of the biosphere.

Microorganisms Sow The Seeds of Disaster

The story begins with the biogeochemical cycle of carbon, which will be described in more detail in Chapter 15. Normally, carbon is removed from the atmosphere by photoautotrophs, including algae and cyanobacteria in the oceans. After these marine organisms die, they slowly settle to the seafloor, where they accumulate as organic debris. Some of this carbon-rich debris is buried in sediments, but some is consumed by heterotrophic microorganisms as food. As you may recall, some heterotrophic microorganisms that live in anaerobic environments produce methane as a by-product of respiration. The methane produced by these anaerobes accumulates in the pores of seafloor sediments. If the seafloor is as cold as it is in our present climate (about 3°C), the methane combines with water to form a frozen solid (a methane-water ice), which remains within the sediments. Geologists searching for oil and natural gas have found layers with abundant methane-water ices in the upper 1500 m of sediments along many continental margins. If temperatures rise by even a few degrees, however, the methane-water ices melt, and the methane is quickly transformed into a gas.

The Oceans Bubble Methane

At the end of the Paleocene epoch, average temperatures in the deep sea may have risen by as much as 6°C. Once the first methane-water ices thawed and were transformed back into gases, they bubbled up through the oceans and entered the atmosphere, where they reinforced the greenhouse effect. This effect raised temperatures on the seafloor even further, which accelerated the rate of thawing. These positive feedbacks eventually resulted in a sudden—and catastrophic—release of methane that caused average global temperatures to rise dramatically. As much as 2 trillion tons of carbon, in the form of methane, may have escaped to the atmosphere over a period as short as 10,000 years or less!

Because methane easily reacts with oxygen to produce carbon dioxide, the release of methane also caused oxygen concentrations in the oceans to plummet. Marine organisms were essentially suffocated when oxygen concentrations dropped below a critical level. The oxygen decrease and temperature rise were devastating to seafloor ecosystems, and up to 80 percent of bottom feeders, such as clams, became extinct.

Recovery and The Evolution of Modern Mammals

Following the catastrophe, it took about 100,000 years for Earth to return to its previous state. During this time, temperatures remained unusually high until Earth was able to absorb all the extra carbon that had been released into the atmosphere. The warmer temperatures allowed rapid expansion of forests into higher latitudes. Redwoods—related to the giant sequoias of California—grew as far north as 80°, rain forests were widespread in Montana and the Dakotas, and tropical palms flourished near London, England. Primitive mammals rapidly evolved into the ancestors of today’s modern mammals, which adapted to cope with the high temperatures of that time. One particular group of mammals—the primates—eventually gave rise to humans.

Methane Deposits Today: A Ticking Time Bomb?

Could we see a repeat of the Paleocene-Eocene global warming disaster today? In the frozen tundra of northern Canada and other Arctic regions of the world, there may be as much as half a trillion tons of frozen methane, and deep-sea sediments around the world contain much more. The global inventory of methane deposits is estimated to be 10 trillion to 20 trillion tons of carbon present as methane, far more than what was released to cause the Paleocene-Eocene mass extinction. Human activities are adding greenhouse gases to the atmosphere at an unprecedented rate, causing the climate to warm significantly. If this trend continues and the oceans warm up, it is possible that those methane deposits could thaw. We would be wise to pay attention to the lessons of our geologic history.

The Mother of All Mass Extinctions: Whodunit?

The Cretaceous-Tertiary and Paleocene-Eocene extinctions are clear-cut examples of dramatic changes in Earth’s environment that caused the catastrophic collapse of ecosystems and led to mass extinction. Those events were big, but not the biggest. In the mass extinction that marked the end of the Permian period and the Paleozoic era (see Figure 11.17), 95 percent of all species on Earth became extinct.

In this case, it seems unlikely that something as straightforward as an asteroid impact could explain how almost every species on Earth was killed. Not surprisingly, the absence of clear-cut evidence for any single cause has resulted in a long list of hypotheses, as we saw in Chapter 1. Some scientists point to extraterrestrial events, such as a comet impact or an increase in the solar wind. Others argue for events generated by Earth itself, such as an increase in volcanism, depletion of oxygen in the oceans, or a sudden release of carbon dioxide from the oceans. As in the Paleocene-Eocene extinction, a sudden release of methane from the oceans has also been proposed.

Recently, it has been shown that the mass extinction at the end of the Permian occurred exactly 251 million years ago. Perhaps it is no coincidence that the age of an enormous deposit of flood basalts in Siberia is also 251 million years. Flood basalts, as we will see in Chapter 12, are extrusive igneous rocks formed from huge volumes of lava that pour out across the surface of Earth in a relatively short time. In Siberia, volcanic fissures spewed out some 3 million cubic kilometers of basaltic lava, covering an area of 4 million square kilometers, almost twice the size of Alaska. Isotopic dating of the basalt shows that all of it was formed within 1 million years or less. It is hard to escape the conclusion that the Permian mass extinction was somehow related to this catastrophic eruption, which would have injected enormous amounts of carbon dioxide and sulfur dioxide gases into the atmosphere. Carbon dioxide contributes to global warming, and sulfur dioxide is the principal source of acid rain. Both are harmful to life if atmospheric concentrations get too high.

More work is required to test all these hypotheses. For example, the Deccan basalts of India are about 65 million years old, and it is possible that the massive outpouring of lava that formed them enhanced the Cretaceous-Tertiary mass extinction. However, equally large outpourings have occurred at other times in Earth’s history without such apparently devastating effects.

Whatever the cause of the Permian mass extinction, one point is clear: just as in the Cretaceous-Tertiary and Paleocene-Eocene mass extinctions, the ultimate cause was the collapse of ecosystems. We know that this collapse occurred, although we don’t know exactly how. The message that we should take away from this history lesson is that history may repeat itself. The environmental changes that humans are making today will inevitably influence ecosystems—we just don’t know exactly how, at least not yet.