When Life Nearly Died
Mass extinction is box office, a darling of the popular press, the subject of cover stories and television documentaries, many books, even a rock song … At the end of 1989, the Associated Press designated mass extinction as one of the “Top 10 Scientific Advances of the Decade.” Everybody has weighed in, from the economist to National Geographic.
—David Raup, 1991
For every problem, there is a solution that is simple, neat and wrong.
—H. L. Mencken
In 1980, a scientific paper hit the professions of geology and paleontology like a blazing comet. After more than a century of speculating about how and why the dinosaurs had vanished, and why other great mass extinctions occurred, the authors had proposed a novel solution. In their scenario, about 65 million years ago, an asteroid 10 km in diameter had slammed into the earth and caused a global “nuclear winter” of cold and dark conditions that decimated life. About 50 percent of the earth’s species had vanished, including dinosaurs, marine reptiles, pterosaurs, and many marine organisms, especially among the plankton and the Nautilus-like ammonites. The paper provided a simple and neat solution to a complex problem, but this was not how the research started. Ironically, the authors were looking for something else altogether and found the evidence of an impact by accident.
Fig. 11.1. The Gubbio boundary layer (dark band with the coin on it, just above the light-colored limestone). (Photo courtesy A. Montanari)
In the late 1970s, a young geologist named Walter Alvarez was busy working on the geology of Italy, especially the Apennine Mountains that run down the spine of the peninsula. (I knew Walter when he was a postdoctoral student at Lamont-Doherty Geological Observatory of Columbia University and I was a graduate student.) Although Walter was primarily concerned with the way in which the mountain belt had grown and been deformed, he was aware that in some outcrops along a highway near Gubbio, Italy, there was an amazingly complete sequence of limestones that spanned the end of the Cretaceous Period (the final period of the Mesozoic or “age of dinosaurs”) and the first few million years of the Tertiary Period (“the age of mammals”). Sandwiched between the limestones at this boundary was a thin layer of clay that represented the time when the extinction took place (fig. 11.1).
Walter took a sample of this Gubbio boundary rock home to his father, Nobel Prize–winning physicist Luis Alvarez, at the University of California, Berkeley. They both were interested in finding a technique that would show how long it took the clay layer to form and, therefore, how rapid the mass extinction event had been. They looked for rare elements found primarily in extraterrestrial matter as a means of detecting how much cosmic dust had accumulated in the sample. If there was a lot of cosmic dust, the sample had accumulated slowly, but if there was very little, the time interval would be very short. They used the rare platinum-group element iridium as their tracer of cosmic dust and gave the samples to nuclear chemists Frank Asaro and Helen Michel to analyze. When the results came back, they were all stunned. The total amount of iridium (the iridium “anomaly”) was much larger than any of them expected and could not be just the product of a simple rain of cosmic dust. Eventually, they concluded that it could have been produced by the impact of an extraterrestrial body, presumably an asteroid, which blasted a huge amount of crustal material into space upon landing and caused a “nuclear winter” of cold and darkness that killed off plants on land and algae in the ocean and so on up the food chain.
Naturally, the geological community immediately challenged such a bold and provocative hypothesis. All ideas in science must undergo the crucible of testing by other scientists and by peer review. At first, scientists thought it might be an artifact of the well-known and peculiar ability of clays to concentrate all sorts of rare materials or an artifact of unusual ocean chemistry, but those ideas were ruled out when the iridium anomaly was found in land sections. As the 1980s progressed, iridium anomalies were found at the boundary around the world, both in deep-sea cores and on marine sections that had been uplifted onto land. As a result, geologists and geochemists jumped on the bandwagon of declaring that it was the impact that killed the dinosaurs. Soon the media jumped into the fray. The May 6, 1985, issue of Time magazine ran an uncritical article and a crowd-pleasing cover of a Tyrannosaurus rex and a plummeting asteroid. The trendy impacts-extinction bandwagon was going full tilt, and many people jumped on for a free ride, funding for their research, and a chance to be part of the hottest idea in the profession.
Naturally, as soon as the impact advocates had argued that an asteroid caused the end-Cretaceous extinctions, other scientists quickly tried to find iridium anomalies and signs of impact at other great mass extinctions. Soon there were claims of iridium anomalies during the extinctions of the late Eocene 35.5 million years ago, the Devonian extinctions 375 million years ago, the late Triassic extinctions 200 million years ago, and the mother of all mass extinctions, the end of the Permian Period about 250 million years ago. By the late 1980s, many geologists were claiming that this “new catastrophism” of the importance of rare extreme events like extraterrestrial impacts was revolutionizing geology. At the International Geologic Congress in Washington, D.C., in 1989, the Canadian paleontologist Digby McLaren (who once suggested that a supernova caused the late Devonian extinctions) said that all mass extinctions were caused by impact, whether or not there was evidence of impact in the fossil record! I was in that audience and remember the stunned reaction of the audience at this untestable statement. The iconoclastic paleontologist David Raup (1991) wrote that all extinctions (even normal background extinctions) might be caused by impacts. With statements such as these, why bother gathering data any more? Impacts occurred, and extinctions occurred; therefore, all extinctions were caused by impacts. I remember receiving reviews by an anonymous scientist for an upcoming revision of my historical geology textbook Evolution of the Earth (written with Bob Dott). This reviewer said we should completely rewrite and reorganize the entire book to emphasize the importance of extraterrestrial impacts in earth history. I also recall having lunch with a major museum director at his swank downtown club to discuss setting up an institute devoted to the study of impacts and extinctions. He must be relieved that he didn’t invest too much of his time and money in the idea, because it would have failed miserably based on advancements over the past 10 years.
The faddish impacts bandwagon in the 1980s was moving quickly, often far beyond (or despite) the data. As Keith Thomson (1988, 59) put it, “With most subjects there is a silly season, usually of unpredictable duration and with an intensity correlated with the status of the acceptance of the new idea, [including] proposal of ideas even more far out than the original one.” Unfortunately, this describes much of the scientific media storm during the heyday of the impact bandwagon in the 1980s and 1990s. Claim after claim of iridium anomalies at this or that boundary, or some other cool-sounding discovery, were trumpeted in the press and spread through the scientific media. Reporters would parrot the provocative claim without any caveats or criticism, and seldom bothered to ask geologists and paleontologists who were not part of the bandwagon what they thought. Periodically, a reporter would call to ask me to comment on the latest news flash, but my criticisms would be reduced to one or two sentences or ignored completely. After all, the media want a simple, dynamic story, not one muddled by caveats and doubts. Yet even as these ideas were trumpeted in the media, other scientists were dropping their important research to test the ideas, scrutinize the original data, and see whether a different lab could replicate the results. Sure enough, a year or so later, the original “discovery” had been discredited, but the media took no notice. Nobody likes a killjoy. The media only want to feature the next startling claim in science, not a rebuttal of a year-old idea that reveals that their reporting was inadequate and misleading and their enthusiasm was premature. And so it goes—the media write up a flashy idea, most people only remember the initial intense media coverage, and almost no one but the specialists in the field remember that the original idea was debunked a year or two later.
Even though people are fascinated about radical and flashy ideas, other scientists have to buckle down and test these ideas to see whether they meet the critical scrutiny that science requires. Science is not a popularity contest or an academic field driven by fads with no external reality to check them against, like deconstructionism and so many other ideas elsewhere in academia. It is true that scientists are human and have their own biases, and they may follow the current fad like sheep. Eventually, though, ideas must be subjected to critical experiments and abandoned if they fail, no matter how popular they were. As Thomas Henry Huxley (1893, 6:8) put it, this is “the great tragedy of Science—the slaying of a beautiful hypothesis by an ugly fact.”
Such is the case with the impact-extinction bandwagon. End-Cretaceous extinctions were popular with geochemists, geophysicists, planetary geologists, and many other geoscientists, but never widely accepted among paleontologists who knew this extinction event best. Even before the 1980 impact paper, paleontologists were aware that the end-Cretaceous event was a complex story that defied a simple explanation. Once thousands of researchers were working on the problem after the 1980 paper, a much more detailed record of the last few million years of the Cretaceous was collected, studied, and provided with a lot of detail that was not easily explained by the impact model.
In particular, it turned out that the end of the Cretaceous was a complicated time, with many events that could have contributed to the mass extinction. In addition to the impact (which was real, although it took until 1990 to establish this and find the site of the Chicxulub impact crater near Yucatán), there were two other major events happening at the same time. There was a big drop in sea level that dried up the shallow seaways that once flooded the continents (fig. 10.1) and certainly reduced the area of habitat for marine life. Finally, the end of the Cretaceous was marked by one of the biggest flood basalt eruptions in earth history, the Deccan lavas of western India and Pakistan. These eruptions produced more than 10,000 km3 (2,400 cubic miles) of lava flows, with individual flows as thick as 150 m (500 feet), totaling at least 2.4 km (1.5 miles) in thickness. Such huge eruptions of material from the mantle would have filled the atmosphere with thick clouds of ash that carried iridium, as well as gases such as carbon dioxide that would have changed atmospheric and ocean chemistry.
Thus, there are three possible killers: (1) asteroid impact, (2) volcanoes, and (3) sea-level retreat. If the asteroid impact were the only important culprit, then almost all groups of organisms should have been severely affected and the extinction horizon should have been a single sharply defined event with all victims disappearing at the same level in the strata. If falling sea level or the Deccan volcanoes were the killers, then a gradual decline in certain selected groups over the last few million years of the Cretaceous would have been expected, with relatively few victims right at the boundary.
In the marine record, there were a few types of plankton that seemed to die out abruptly (the planktonic foraminifera and the algae known as coccolithophorids), but the rest of the microplankton (the algae known as diatoms, plus the radiolarians and dinoflagellates) showed no effect. Although the squid-like ammonites vanished (either abruptly or slowly is subject to debate), nearly the rest of the groups of marine mollusks underwent only a minor extinction. Some, such as the cone-shaped rudistid clams and the huge, flat inoceramid clams, were both gone long before the impact occurred. The same is true of nearly all the rest of the marine invertebrates, almost none of which show a major abrupt extinction at the end of the Cretaceous (Prothero 2006, chap. 2; Prothero 2009, chap. 5; MacLeod et al. 1997). Likewise, the marine reptiles appear to have been in decline through most of the Cretaceous, and there’s no evidence that any of them were alive to see the impact occur.
Even more striking is the evidence from terrestrial life (Archibald 1996). Yes, the dinosaurs vanished (except for those that evolved into birds). Some say that they did so abruptly, although there are no dinosaur fossils within the last 3 m (10 feet) of rock below the iridium layer, and dinosaurs were already in a long-term decline anyway. There was some change in the land flora, and a big “spike” of fern spores at the iridium anomaly, suggesting unusual conditions during that time. But virtually no other group of terrestrial animals was affected by this supposed “nuclear winter” catastrophe. Huge crocodilians and pond turtles all breezed through with no effect, as did bony fish, insects, and birds. There are only slight changes in the mammal fauna from marsupial dominated to placental dominated.
Nor was there any effect on the amphibians. Interestingly, amphibians provide an important test of a faddish idea: impact acid rain. Impact enthusiasts postulated that a large amount of acid rain produced from the debris was scattered by the impact. If that scenario actually happened, neither frogs nor salamanders would be alive today, because they are extremely sensitive to the acidity of fresh waters in which they live, and they breathe through their porous skins. Today, amphibians are vanishing from many places because of the tiny amount of human-induced acid rain in their habitats. If there were really global clouds of darkness, there would be no tropical bees left today, because they cannot survive more than a few days without flowers and are sensitive to cold. Some impact advocates have tried ad hoc explanations to salvage these inconvenient truths, such as claiming that the survivors were all aquatic or burrowing and hid out during the hellish days after the impact. That doesn’t work for many of them (especially crocodilians, amphibians, birds, and insects). The terrestrial record does not support the “hell-on-earth” scenario that impact proponents promote. Yes, a rock from space clobbered Yucatán, but its effects must not have been as big or catastrophic as long asserted.
You would never know this by reading the media accounts or popular books on the topic, because they are uniformly biased toward the simplistic, spectacular scenario of asteroid impact. The impact model is still supported among geochemists and planetary geologists, who consider it “case closed,” but not so with the paleontologists, who know the fossil record best. Several books by top-notch paleontologists (Keller and Macleod 1996; Archibald 1996; Hallam and Wignall 1997; Dingus and Rowe 1998) dispute the importance of the impact. A distinguished panel of 22 British paleontological specialists in nearly every group of marine fossils (MacLeod et al. 1997) argued against the impact scenario causing marine extinctions. In 2004, a poll (Brysse 2004) was conducted of the membership of the Society of Vertebrate Paleontology. Of those surveyed, 72 percent felt that the extinctions were caused by gradual processes followed by an impact. Only 20 percent felt that the impact was the sole cause. The other 8 percent had no opinion or questioned whether it was a mass extinction at all. After 30 years of the Cretaceous impact scenario, there is no clear consensus about the cause of the extinction, despite media reports.
Although most people care only about dinosaurs and have never heard about any other kinds of prehistoric life, the end-Cretaceous extinctions were not even the biggest mass extinction in earth history. That honor goes to the end-Permian extinction 250 million years ago, which has been nicknamed “The Mother of All Mass Extinctions” (in reference to Saddam Hussein threatening the United States with the “Mother of all Battles” if they invaded Iraq in 1991). By various estimates, as much as 95 percent of all marine species, and a comparable percentage of land animals, died out at this event. The extinction decimated nearly every group of marine animals that had dominated the Paleozoic seafloor since the late Cambrian, more than 500 million years ago. Trilobites vanished, as did all the corals known at the time, and the tremendously abundant fusulinids, a group of amoeba-like protozoans with shells shaped like rice grains. Most of the other dominant Paleozoic groups (the brachiopods, or “lamp shells”; the bryozoans, or “moss animals”; the crinoids, or “sea lilies”; as well as the early ammonoids and most other mollusks) were reduced to a few surviving lineages to repopulate the planet in the Mesozoic. It was such a profound extinction that the entire composition of marine life was rearranged from a typical “Paleozoic fauna” to what is known as the “Modern fauna,” which has ruled the oceans for the past 250 million years. Most land animals suffered severely, especially the once-dominant lineages of synapsids (once misleadingly called “mammal-like reptiles,” although they are not really reptiles). These animals were pruned down to a few survivors, some of which evolved into the first true mammals by 200 million years ago.
Naturally, a mass extinction event as big as this has generated many possible explanations. During the 1970s and 1980s, a number of novel ideas were proposed. For example, the diversity of life is strongly controlled by the amount of habitable area for animals to live on. Geologists pointed out that the shallow marine shelf area was greatly reduced in the Permian because all the continents had coalesced to form Pangea. Unfortunately for this idea, it turns out that Pangea had already assembled by the early Permian, too early to have anything to do with the mass extinction. Likewise, geologists pointed to the great Permian ice cap (fig. 9.8A) and thought that the extinction might be due to global cooling. This idea was shot down because the ice sheet was already large by the Carboniferous Period and actually disappearing at the end of the Permian. In fact, all the recent data showed that the end of the Permian was marked by a “super-greenhouse” global warming event and not by cooling after all.
In the 1990s, research intensified, and soon there were excellent sections of rock (especially in China) that yielded highly detailed records of the end of the Permian. When the time ranges of fossils were plotted, the Permian extinction suddenly looked more abrupt than had ever been expected. This led to a revival of the impact hypothesis, which had been proposed for the Permian event in the 1980s and then abandoned. Luann Becker and her colleagues (2001) have repeatedly pushed the idea that a large impact that produced Bedout Crater in Australia (Becker et al. 2004) was the culprit, and naturally, the press carried the story for months. Unfortunately, this exciting idea falls apart when the data are examined. No other laboratory can replicate Becker’s claims of impact-derived chemicals, such as iridium, unusual helium, and “buckyballs” (the 60-carbon molecules known as buckminsterfullerenes) at the Permian-Triassic boundary (Farley and Mukhopadhyay 2001; Braun et al. 2001). To top it off, Bedout Crater is the wrong size and age to have anything to do with the end of the Permian (Glikson 2004; Koeberl et al. 2002; Renne et al. 2004; Wignall et al. 2004).
If not impacts, what could be the cause of “the Mother of All Mass Extinctions”? Currently, several good candidates have survived the gauntlet of testing and peer review. The most important is that some of the largest eruptions in earth history, the Siberian lavas, were spewing gigantic amounts of lava and greenhouse gases in a true “supervolcano” eruption at the end of the Permian (see chapter 3). These stacks of lava flows are up to 6.5 km (4 miles) in total thickness in 11 discrete eruptive sequences and cover a total of about 7 million square kilometers (2.7 million square miles), an area equivalent to the size of the continental United States (Erwin 2006). The ages of these flows have been recently redated between 252.2 million and 251.1 million years old, the same as the dates from the Permian-Triassic boundary in China. As mentioned earlier, geochemical evidence exists of runaway global warming, possibly triggered by carbon dioxide released from the Siberian eruptions. In addition, geochemical evidence from marine sediments suggests that the oceans were highly over-saturated in carbon dioxide (hypercapnia) and depleted in oxygen, a condition that would have been fatal to most kinds of marine organisms.
After the end-Permian and end-Cretaceous mass extinctions, the third biggest mass extinction occurred during the last two stages of the late Devonian, about 375 million years ago, when 75 percent of the marine species died out. Impacts were blamed at first, and iridium anomalies were reported. However, closer scrutiny shows that these iridium anomalies are at the wrong time, and the evidence for impacts (if it is real) is not correlated with the several pulses of geochemical changes and extinctions in the late Devonian (McGhee 1996).
The same can be said of the fifth largest mass extinction in earth history at the end of the Triassic Period about 200 million years ago. This event caused a significant extinction in marine life (especially certain types of brachiopods and most of the Triassic ammonoid lineages) and helped change the land fauna by wiping out archaic groups of reptiles and amphibians, replacing them with the newly evolved dinosaurs. Naturally, if geologists could tie the end-Cretaceous extinctions to impacts, they would try to do so with the end-Triassic extinctions that helped the dinosaurs take over the planet. Several geologists pointed to the huge Manicouagan Crater (fig. 11.2) in Quebec as the culprit (Olsen et al. 1987, 2002). This monstrous hole, which shows up as a huge ring on the satellite images of Quebec, is about 100 km in diameter, not much smaller than the Chicxulub crater in Yucatán associated with the Cretaceous impact. Unfortunately, recent dating of the crater at 214 million years ago is not even close to the age of the Triassic-Jurassic boundary at 201 million years, or near the age of any other mass extinction (Palfy et al. 2000). Shocked quartz and iridium have also been claimed for this boundary, but further scrutiny has shown that they are in such small concentrations that they were unlikely to be related to the extinction (Hallam 1990, 2004; Hallam and Wignall 1997; Tanner et al. 2004). Most recent research has focused on the huge eruptions of the Central Atlantic Magmatic Province (CAMP) basalts, which occurred when the Atlantic was ripping apart as Pangea broke up. However, many paleontologists do not regard the end-Triassic as a real “mass extinction event” but simply as an artifact of the compilation of the data (Tanner et al. 2004). If you plot all the true time ranges of organisms at very coarse resolution (as if they died only at the boundaries and not within the interval), you will get artificially high apparent “extinction rates” at the boundaries.
Fig. 11.2. The Manicouagan Crater in Quebec, photographed from the space shuttle. The impact that formed it was once blamed for the end-Triassic extinction, but it is now known to be an impact from a different time period. (Courtesy NASA)
The most recent proposition that impacts cause extinctions was the claim by Firestone et al. (2007) that the extinction of ice age “megamammals” (large mammals more than 40 kg in weight) was due to the impact of an extraterrestrial object about 12,900 years ago. At first, the media had a field day with this proposition, and almost no dissenters or critics were heard from at all. Some geology textbooks even inserted this untested idea into their new editions without waiting for confirmation. As with other half-baked ideas from impact advocates, the “late Pleistocene impact” scenario has been discredited by a range of observations.
The late Pleistocene impact hypothesis was born from observations that there was a distinctive “black mat” organic layer in several localities across the southwestern United States, immediately above the last appearance of some ice age megamammal fossils. The victims of this megamammal extinction include not only huge mammoths and mastodons but also ground sloths, horses, camels, two genera of peccaries, giant beavers, and predators, such as short-faced bears, dire wolves, and saber-toothed cats but not bison, deer, pronghorns, and a number of other large mammals still found in North America today. The “black mat” is also above the first known artifacts of the Clovis culture, which were thought to be the first human arrivals from Eurasia and allegedly responsible for overhunting the megamammals to extinction. Firestone et al. (2007) also claimed to have found “nanodiamonds,” iridium, helium-3, buckyballs, and many other geochemical and mineralogical “impact indicators” in the black mat layer and then painted a variety of different (and conflicting) scenarios about the impacting object (they are not consistent about whether it is a comet or an asteroid) hitting near the Carolina Bays region. This supposedly affected the Laurentide ice sheet in the northeastern part of North America and triggered the Younger Dryas cooling event at 12,900 years ago.
The entire scenario has been completely demolished by a number of lines of evidence. As Pinter and Ishman (2008) showed, there is no evidence of an impact in the Carolina Bays, and most of the alleged “impact evidence” is questionable when analyzed by other labs. Firestone et al. (2007) argued that the impact was an airburst, since there is no crater, no tektites, no shocked quartz, or no other high-pressure minerals, which are the best indicators of a true impact. Most of the material that was allegedly impact derived (nanodiamonds, iridium, helium-3, buckyballs, and so on) has been discredited by further testing or is also consistent with the normal rain of micrometeorites and not abundant enough to be evidence of an impact.
The claim that the black mat was an impact layer has been debunked. It likely indicates a high water table and wetter conditions associated with the abrupt Younger Dryas cooling event (Haynes 2008). The supposed “instantaneous” extinction of megamammals at this horizon has also been debunked because extinctions were scattered across a wide geographic area with different genera vanishing locally at different times (Grayson and Meltzer 2003; Fiedel 2009; Scott 2010). That mammoths, mastodons, giant deer (“Irish elk”), ground sloths, and many other megamammals did not die out at 12,900 years ago but survived in most cases to 10,000 to 11,000 years ago, discredits the idea that a single impact killed them off. In fact, none of the well-dated extinctions occur at 12,900 years ago. Most extinctions are either significantly younger than that interval (examples given earlier), or there are no good final dates for their last appearance. Little appears to have happened to the megamammals at precisely 12,900 years ago.
Particularly striking is the persistence of mammoths and ground sloths well into the Holocene (only 6,000 years ago), and, of course, the bison, deer, grizzly bear, cougars, peccaries, and pronghorns still with us; while elk and moose (which were not wiped out) came to North America at this time. The impact hypothesis does not explain the selectivity of this extinction. In addition, the South American, Australian, and Eurasian-African megafaunal extinctions are not synchronous with the alleged “impact,” so it does nothing to explain their demise.
The claim that the “impact” had a severe effect on human cultures has been completely shot down as well (Buchanan et al. 2008), because there is no evidence that human cultures changed dramatically at this time or that there was a major population decline. Clovis culture was gradually transformed into Folsom, Dalton, and eastern U.S. Paleoindian cultures, and they apparently spread widely at this time, rather than declining. In early 2010, Jacquelyn Gill of the University of Wisconsin presented a paper at the Ecological Society of America meeting analyzing the details of lake sediments from the Northeast, which preserve a high-fidelity record of that time. She found no evidence of the impact debris that was supposed to be common, and her data were gathered even closer to the alleged impact site than the evidence garnered from the western United States. Nor was there any great shift in vegetation, pollen, spores, or other biotic signal that would be consistent with the impact hypothesis.
Finally, if the authors of the Pleistocene impact scenario had paid any attention to the past decade of research on impacts and extinctions (as discussed in this chapter), they would have realized that the “impacts cause extinction” notion is passé. As we have already shown, none of the great extinctions of the past (except possibly the end-Cretaceous event) are associated with impacts. In the next section, we will see that the largest impacts to hit the earth, other than Chicxulub, caused no extinction. It feels like the Firestone et al. (2007) impact scenario is a bad rehash of the debates from the 1980s. Apparently, the authors are still stuck on a bandwagon that has long since ground to a halt, except in the popular media. As Barnosky et al. (2004) and Scott (2010) showed, the causes of the late Pleistocene megafaunal extinctions are complicated and probably involve both human overhunting and climatic change. Impact, however, doesn’t seem to be relevant.
The perfect counterexample for all these trendy mass extinction scenarios comes from the interval of time I have studied for the past 30 years. Although these extinctions were not part of the “Big Five” mass extinctions, extinctions at the end of the Eocene and early Oligocene were significant, especially because so many archaic groups of the Eocene greenhouse world (see chapters 9 and 10) died out.
When the Alvarez et al. (1980) paper was first published and blamed the extinction of the dinosaurs on an impact, there was a rush to find evidence of iridium at other extinction horizons. Sure enough, they found evidence of iridium in the late Eocene (Asaro et al. 1982; Alvarez et al. 1982; Ganapathy 1982; Glass et al. 1982). They crowed that the discovery of iridium “solved” the mystery of the Eocene extinctions, and then they moved on to other research, believing the issue was resolved.
Those of us who have spent long years in the trenches, working on the details of the Eocene-Oligocene transition knew the story was not quite so simple. First, the iridium anomalies occurred in the middle of the late Eocene, 35.5–36.0 million years ago, but they are too young for the major extinction event at the end of the middle Eocene (37 million years ago) and too old for the early Oligocene glaciation event 33 million years ago. Second, there were almost no species known to die out at the time of the iridium anomaly (except for a handful of species of plankton known as radiolarians), and more than 99 percent of all life on the planet appeared to be unaffected by the impact event. The impact advocates had found iridium close to the Eocene-Oligocene boundary, but close doesn’t cut it. Close only works in horseshoes and hand grenades. Thousands of feet of strata full of fossils lie between the 37-million-year-old extinction and the impact horizon, and thousands of feet more above the impact horizon and below the 33 million year event. This kind of slapdash, sloppy science is unacceptable and should never have been published in such prestigious scientific journals, except that every article on impacts was accepted without any serious criticism during the heyday of the impact bandwagon.
The story became more interesting when impact debris of the same age were found in oceanic cores, and finally when the craters themselves were identified (fig. 11.3). One of them is a huge buried crater responsible for the Chesapeake Bay (Poag 1999), and there is a slightly smaller one on the Atlantic continental shelf near Toms Canyon. The third crater, known as Popigai, was found in northeastern Siberia. These craters were truly impressive in size. The Chesapeake crater is almost 100 km (62 miles) in diameter, and the Popigai crater is about the same size; both are just slightly smaller than the 180 km (112 mile) diameter Chicxulub crater in Yucatán associated with the end-Cretaceous extinction. As the study of the Chesapeake crater continued, impact advocates were embarrassed to learn that such a large impact had almost zero effect on life in the late Eocene. Some had predicted that impact should produce a ring of dust and debris and another “nuclear winter” scenario, but the evidence showed that the earth responded with warming in the latest Eocene instead. As discussed in Prothero (2009), the evidence for what caused the late Eocene extinctions is complex, but impacts had nothing to do with them, even though extraterrestrial objects did strike the planet.
Fig. 11.3. The location of the late Eocene impact craters and their debris fields. (After Poag 1997, Palaios; used by permission)
During the height of the impact bandwagon, geologists were predicting that every impact should cause an extinction, and David Raup (1991) drafted a “kill curve” that would predict what percentage of species would go extinct for an impact of a given size (fig. 11.4). Raup had originally fit a simple curve to one data point, the end-Cretaceous event (dashed line in fig. 11.4). But with the addition of the nonextinction from the Chesapeake and Popigai craters, the “curve” changes shape radically (solid line in fig. 11.4). In its current incarnation, the Popigai-Chesapeake data in the Raup “kill curve” say that no extinction can be expected unless the impact is the size of Chicxulub or larger. This is borne out by studies that document thousands of impacts through earth history (Prothero 2004, 2007), but so far, only one—Chicxulub—is even potentially associated with a mass extinction. You can look up these thousands of impacts, their sizes, and estimated ages, on the online impact database (www.unb.ca/passc/ImpactDatabase/). As Prothero (2004) and Alroy (2002) both showed, there is no statistical correlation between any impacts or even the size of impacts, and the frequency of extinction.
Fig. 11.4. The Raup “kill curve” as modified by Poag. (1997, Palaios; used by permission)
The impact bandwagon seems to have run out of steam. In its heyday during the 1980s, planetary geologists received lots of publicity by claiming that asteroids were always threatening the earth and that we needed to spend millions on detecting them and thinking up ways to destroy or deflect any that might approach the earth. Big-budget Hollywood disaster films like Deep Impact and Armageddon made millions exaggerating this scenario. I do not question the basic idea that there are many rocks in space that could hit us, but what the geologic record shows is that virtually no impact causes mass extinction. Even impacts that hit during prehistoric times (including Meteorite Crater in Arizona, which hit 40,000 years ago) had no apparent effect on life except possibly in the area around the impact (Prothero 2004, 2007). It now seems bizarre and anachronistic to hear planetary geologists repeatedly raising this false alarm about the danger of impacts to drum up funding for their asteroid searches.
As Ward (2007) and many other former impact advocates now admit, the asteroid bandwagon is now passé, and geologists no longer maintain that impacts have an effect on life at all. They are now focusing on important changes in the atmosphere, especially “super-greenhouse” climates of high carbon dioxide and low oxygen, as more likely culprits for the end-Permian, end-Triassic, and several other major extinctions. This explanation (like the impact scenario and the massive volcanism scenario) fails for the late Eocene-Oligocene extinctions. That event is a perpetual “fly in the ointment” of every grandiose attempt to blame all mass extinctions on a simple single cause. It refuses to follow those simplistic scenarios that seduce some scientists.
In the early days of the impact-extinction frenzy, the late Jack Sepkoski of the University of Chicago compiled a huge database of the time range of all known families and genera of marine fossil. Dave Raup and Jack analyzed this data set with a number of computer techniques and found there was a general “background” rate of extinction (Raup and Sepkoski 1984). Five huge “mass extinction” events clearly stood above the background and were not just products of normal extinction intensities. We have already mentioned most of them. Number one was the end-Permian extinction 250 million years ago, when 95 percent of marine life vanished. The end-Cretaceous event that wiped out the dinosaurs and ammonites 65 million years ago was the second. The late Devonian event (375 million years ago), the late Ordovician event (450 million years ago), and the end-Triassic event (200 million years ago) rounded out the top five. The late Eocene extinctions were not among the “Big Five” events but came close to being the sixth largest extinction in earth history.
A mass extinction event is happening now, and threatens to dwarf even the biggest mass extinctions of the past. In The Sixth Extinction, Richard Leakey and Roger Lewin (1995) call this mass extinction event “the sixth extinction” to reflect that, over the past few centuries, humans are wiping out animals and plants at an unprecedented rate. Nobody knows exactly how many species have vanished and how fast extinction is occurring. No matter what estimates you read, the numbers are staggering. Biologists estimate that in the twentieth century between 20,000 and 2 million species went extinct. Current estimates place the rate of extinction at about 140,000 species per year. This is faster than any rate known in the geologic past.
Although we may not know the direct causes of any of the “Big Five” mass extinctions, there is no question about the culprit in the Sixth Extinction: Homo sapiens. Through either hunting or (more often) through habitat destruction and the introduction of diseases, pests, domesticated animals, and exotic invader species, humans have become the most dangerous and destructive species to inhabit the planet. No asteroid impact, gigantic volcanic eruption, or oversupply of carbon dioxide has ever been as destructive to life as humans have been. As Niles Eldredge (1991, 217) put it, “We are like loose cannons, able to wreak great damage on our own, and particularly dangerous if our effects happen to coincide with the physically induced changes that are also causing extinctions.”
The single biggest factor in terrestrial extinction is our wholesale destruction of the world’s rain forests. Each day 80 square miles of rain forest are lost, mostly in Africa, South America, and Southeast Asia. Each year an area of rain forest the size of Maine or Indiana vanishes. The cutting and burning of the rain forest not only destroys the habitats of more than half of the species on the planet but also contributes significantly to global warming. When the rain forest is cut down by slash-and-burn methods, the land is briefly turned into pastures for cattle to feed our fast-food obsession with hamburgers. Yet the rain forests’ nutrients are locked in the trees, not the soil, so just a few years after the area is logged, it becomes a barren wasteland with poor soil that supports almost no life and quickly erodes to flood the rivers with sediment. Damage is now irreparable, and the rain forest cannot grow back. Meanwhile, poor subsistence farmers can no longer make a living on this wasteland and must move on and cut down more rain forest to survive.
People love to watch video footage of wild animals in nature, but most of the world’s wild places are rapidly vanishing or have disappeared because of the invasion of humans and their domesticated animals. In his sad book African Silences, zoologist Peter Matthiessen (1991) surveyed tropical Africa and returned with grim news. Most of the great forests of West Africa were gone and what remains is a wasteland filled with starving people. What little remains of these rich forests (even within the supposed wildlife refuges), which once supported the bulk of Africa’s wildlife, is now heavily poached by starving hunters looking for anything to eat as “bush meat.” As Matthiessen put it, “The great silence that resounds from the wild land without a sign of human life, from which all the great animals are gone, is something ominous. Mile after mile, we stare down in disbelief” (52). The same has happened in the Congo Basin. We hear about the struggles to keep poachers from wiping out the last few populations of mountain gorillas studied by Dian Fossey, or chimpanzees studied by Jane Goodall, but they are just the tip of the iceberg. Most species in the African jungle are not cute, glamorous, or closely related to us, but they are already extinct or endangered. Beasts popularized in the movie Born Free and the TV show Daktari or from the explorations of Stanley and Livingstone are nearly gone, replaced by humans and their cattle. IMAX movies and documentaries on the Animal Planet channel have made us more conscious and sympathetic to the world of wild animals, but ironically, the real animals and their habitat have nearly vanished in the wild.
The plight of the oceans is a parallel tragedy. Marine biologists around the world have warned that the oceanic ecosystem is on the verge of collapse. Those who used to dive in the lush beauty of the tropical coral reefs are now reporting that most reefs are dead, destroyed by a combination of overfishing, pollution, and warming oceans, which kills off coral at an alarming rate. Nearly 75 percent of the world’s fisheries are overexploited or at risk, and many fish (like the once-abundant Atlantic cod) are no longer fished because there are virtually none left. A 2003 study showed that 90 percent of the top predator species of the ocean have vanished, again largely due to overfishing. Sharks in particular display a catastrophic decline in numbers, because they reproduce slowly and do not recover quickly from excessive fishing. The 89 percent decline in hammerhead sharks is a typical number. Sharks are particularly vulnerable because they are accidentally caught in nets intended for other fish. The principal culprit has been the killing of sharks for their fins, since a single bowl of shark fin soup may sell for more than $100.
Some decline in marine species is due to disturbance of their habitat, especially where they breed and spawn, and the effects of warming and acidification of the oceans due to excess atmospheric carbon dioxide. The principal factor in the decline of fishing is the use of highly efficient, large-scale trawlers and fish-finding sonar, which can wipe out huge schools of fish in a single trawl. After this technology was introduced, catches nearly tripled from 1960 to 1992 and then plummeted when nearly all the major stocks of fish had been depleted.
Nothing encapsulates this story better than the plight of the bluefin tuna, which has long been considered a great delicacy among sushi aficionados. It has been so overfished that it is virtually extinct, and a single bluefin tuna can sell for as much as $173,000. Japanese fishing companies have already deep-frozen about 30,000 tons of bluefin, worth about $10 billion to $20 billion, and are holding on to it like any other precious commodity that will appreciate in value. Normally, it would be in our best interest to leave the fish alone and give its populations a chance to recover. Instead, its rarity makes it even more in demand, so it has become a status item like owning an expensive car or boat—if you’re rich enough to afford bluefin sushi, you’re at the top of the heap. Richard Ellis, author of Tuna: A Love Story, wrote the following:
People believe in their hearts that a piece of raw fish is worth $600. And one of the main reasons that it’s worth $600 is because you can’t afford it and I can’t, but they can. That makes it very special, and it makes people who eat it special. Any kind of luxury goods largely come from that sort of statement: I can afford it, and you can’t. I’ll drive a Maserati, even if I can’t drive it faster than 65 miles per hour in most of the United States. I can afford a $280,000 car, and you’re stuck with a Dodge Neon. I can fly a private jet, drive a Maserati, do anything I bloody well please, including having a $600 piece of fish. And you can’t. And this is the brutal truth: bluefin, which beyond their intrinsic value as living creatures happen to be one of the universe’s more majestic species, a Platonic ideal of oceanic speed and grace, aren’t being extinguished by our greed. They’re being sacrificed to our vanity, pretension, and ostentation—the most pathetic of our vices. (Hive Mind 2009)
We are appalled by these stories of greed and stupidity or how overpopulation (especially the Third World) is driving most nondomesticated animals into extinction. Many people are highly motivated to do something to save the planet or to support efforts to reduce rain forest destruction or overexploitation of wild animals. Many people still react with, “Why should I care? Why should I worry about other species when humans are at risk too? After all, 99 percent of animals that have ever lived on this planet are extinct, so there’s no denying the inevitable.”
There are a number of answers to these questions, some practical and others moral and philosophical. Practical answers point out that wild nature provides huge benefits to us as humans. Many drugs come from rare tropical plants. Nature is essential to our food supply and for providing other needs— we could not survive without the wild kingdom. If we wipe out biodiversity before it has a chance to be studied, we miss the opportunity to identify and analyze tropical plants with the potential to provide valuable medicine, important natural pesticide, or other chemicals we require. The crash in the honeybee populations around the world threatens to destroy nearly all our agricultural crops because most depend on bees for pollination. As James Leape, the director general of the World Wildlife Federation, put it,
Reduced biodiversity means millions of people face a future where food supplies are more vulnerable to pests and disease and where water is in irregular or short supply. No one can escape the impact of biodiversity loss because reduced global diversity translates quite clearly into fewer new medicines, greater vulnerability to natural disasters and greater effects from global warming. The industrialised world needs to be supporting the global effort to achieve these targets, not just in their own territories where a lot of biodiversity has already been lost, but also globally. (Lovell 2008)
What is the big deal if we wipe out a few species here and there? Surely that won’t cause the world to collapse or affect humans? In their 1981 book Extinction, Paul and Anne Ehrlich posed an interesting analogy. Suppose you were flying in a jetliner and looked out the window to see a rivet pop out of the wing. Then you saw the loss of another rivet, then another rivet, one after another. Perhaps one or two rivets will not damage the structural integrity of the aircraft and would not cause alarm. How many rivets are required before the wing falls apart and you crash? Would you be willing to perform this experiment and take the chance that you would die? That is comparable to the unintentional experiment we are performing on nature. Every species lost is another rivet that holds the world’s ecosystems together. One or two species here or there may not make a big difference, but we really don’t know how many can be lost before the entire planetary ecosystem collapses. With the dying coral reefs and the vanishing rain forest, we may already be beyond the turning point.
These are all practical reasons why we must strive to keep our fellow inhabitants of this small blue planet from vanishing. Many people regard conservation as an issue of philosophy, morality, and ethics. In their minds, humans suffer from too much hubris and anthropocentrism and treat the planet as though we were the only species that matters. They would argue that we have no more right to this planet than any other species, nor do we have the moral right to drive other species to extinction because we have the power to do so.
Finally, there are simple issues of esthetics as well. The earth is a beautiful place, and its many creatures are wonderful in their own right. It is tragic when we destroy them through our own carelessness, greed, apathy, and selfishness. The world without pandas, polar bears, and many other amazing creatures is truly a tragic hollow shell of its former self. In 1973, I traveled to Kenya, Tanzania, and Madagascar to see the amazing wildlife. Now, only 28 years later, nearly all that wildlife has been exterminated, and the African game parks have been depleted of rhinos and elephants, the favorites of poachers. My youngest son, Gabriel, is now 6 years old and is already a fanatic about animals, as I was at his age. He loves seeing them on TV and in his books, and he wants to go to the zoo as often as possible. I despair that he will ever get a chance to see most of the earth’s great creatures in the wild because by the time he is old enough to see these creatures in their native habitat, most of them will be extinct in the wild.
In 1990, Mark Carwardine and the late Douglas Adams (author of the classic science fiction satire Hitchhiker’s Guide to the Galaxy) published an amazing book, Last Chance to See. Adams and Carwardine visited many of the rarest animals on the planet before they vanished forever. In the process, they documented the efforts of a handful of dedicated wildlife biologists and conservationists as they risked their lives to save these rare creatures. As they put it in their book, “There is one last reason for caring, and I believe no other is necessary. It is certainly the reason why so many people have devoted their lives to protecting the likes of rhinos, parakeets, kakapos, and dolphins. And it is simply this: the world would be a poorer, darker, lonelier place without them” (Adams and Carwardine 1990, 211).
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