A New History of Life (34 page)

Since the introduction of the Kump et al. interpretation, others including Tom Algeo of the University of Cincinnati have greatly increased our understanding of the chemical aspects of this particular mass extinction, through numerous references.
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The study of past mass extinctions is not new; in fact it is one of the very first kinds of research that could actually be called “science” when geology was first stirring as a discipline, in the first years of the nineteenth century. What is new about it is our understanding of the role of microbes in causing one or more of the so-called big five mass extinctions of the Phanerozoic.

Yet if extinctions themselves are not a new topic, the opposite side of the coin—the aftermath to mass extinction—has emerged in the past decade as a major new subdiscipline of evolutionary biology and paleobiology alike. We have learned that the more devastating the mass extinction, the more different the world coming after, not only in the immediate aftermath—the first few hundred thousand to million years later—but for subsequent tens of millions of years, and for some biological lineages, for all time.

A previously unrecognized aspect of the changing oxygen levels would be their effect on species migration and gene flow. Mountain ranges in our world are often barriers to gene exchange, producing
different biota on either side of the range. At the end of the Permian, just living at sea level would have been equivalent to breathing at five thousand meters, a height that is greater than that found atop Mount Rainier in Washington State. Thus even low altitudes during the Permian would have exacerbated this, so that even a modest set of hills would have isolated all but the most altitude- or low-oxygen-tolerant animals. The result would be a world composed of numerous endemic centers hugging the sea level coastlines.

Ranges of vertebrate fossils from the Karoo of South Africa, from approximately 260 to 250 million years ago. Each vertical line represents a genus of vertebrate animal (based on fossils recovered from the strata). While most extinctions took place over a fairly narrow interval, this pattern is nothing like that seen at the end of the Cretaceous. The “smeared” appearance of extinctions shown here is characteristic of “greenhouse mass extinctions”—not a single level of extinction, but successive extinctions.

The high plateaus of many continents may have been without animal life save for the most altitude tolerant. This goes against expectation based on continental position: because the continents at this 250-million-year-old time were all merged into one gigantic supercontinent (named Pangaea), we would expect a world where there were very few terrestrial biotic provinces, since animals would be able to walk from one side of the continent to the other without an Atlantic Ocean in the way. But altitude became the new barrier to migration,
and new studies of various vertebrate faunas appear to show a world of many separate biotic provinces, at least on land.

The late twentieth- and early twenty-first-century work of Roger Smith, Jennifer Botha, and coauthor Ward in the Karoo desert, Mike Benton in Russia, and Christian Sidor in Niger
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showed that each of these separate localities in Africa had distinct and largely nonoverlapping faunas. Thus, during times of low oxygen, altitude would have created significant barriers to migration and gene flow.
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Low-oxygen times therefore should have had many separate biotic provinces, at least on land. The opposite occurred during high-oxygen times: there would be relatively few biotic provinces and a worldwide fauna.

The drop in oxygen did more than make mountain ranges barriers to migration. It made most areas higher than a thousand meters uninhabitable during the late Permian through Triassic time interval. This effect, called altitudinal compression, could have had a major impact on Triassic land life in the time of lowest oxygen. The removal of habitat because of altitudinal compression would have caused species from the highlands to migrate toward sea level or die out. Doing so would have increased competition for space and resources, and perhaps would have introduced new predators, parasites, or diseases in the previously populated lowlands, causing some number of species to go extinct. We calculated that by the end of the Permian more than 50 percent of the Earth’s land surface would no longer have been habitable because of altitudinal compression. There may even have been extinction caused by the effects modeled long ago by Robert MacArthur and E. O. Wilson in
The Theory of Island Biogeography
. Those two scientists noted that diversity is related to habitat area, and that when islands or reserves of some sort became smaller, species died out. Altitudinal compression would accomplish the same by making the continental landmasses functionally lower in usable area.

A final aspect of the Permian extinction comes from research not yet published, but because it comes from coauthor Ward it will be reported
here, as it is most pertinent to the topic of the Permian mass extinction. One of Ward’s graduate students, Frederick Dooley, combined with Lee Kump to produce an unexpected discovery. Dooley studies the effect of hydrogen sulfide on plants and some animals; Kump has been modeling ocean conditions at the end of the Permian, including estimated amounts of hydrogen sulfide in the global oceans’ surfaces. Kump arrived at a value that Dooley then used in actual experiments on single-celled oceanic phytoplankton, as well as the most important oceanic zooplankton, the tiny shrimp-like creatures called copepods. The levels were not sufficient to kill the algae, and surprisingly actually made them grow faster. The copepods, on the other hand, died almost instantly. Without copepods to feed on phytoplankton and keep it in check, these tiny plants sink to the sea bottom and rot, removing any last vestige of oxygen. This would produce a great oscillation in the carbon isotope pattern, as well as kill off every marine animal species that has an early life history in the upper water column as temporary plankton. The result would be a planet choked in rotting plants but nearly without animals. This is exactly what happened at the end of the Permian—in the oceans, anyway. On land it would have been very much like some combination of World War I and II combined. Roger Smith of South Africa now has very credible evidence of an extraordinary period of drying and sudden heat in the South Africa of 252 million years ago, while our own work on the vertebrates in the Karoo, published in 2005, remains the best record of land animal extinction across that boundary.
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Roger Smith thinks that the drought and heat alone can account for the extinction of most vertebrates. We maintain that the world war analogy is apt: great armies dying in the desert, and in World War I, killed by poisonous chlorine gas. Long ago it was death in the desert from poisonous hydrogen sulfide in the air and sea.

One of the great joys of academics is the sense of community among the faculty, be it a community college or the most high-powered research institution in the land. Much of this comes from the very nature of the American university system, which requires a six- or seven-year trial period, followed by tenure. Permanence. Perhaps more than in any other profession, university faculties have a high stability, and compared to most other professions, a relatively low rate of turnover. The result is that relationships can literally last for appreciable parts of one’s lifetime. In this the university faculty systems are indeed much like the system they were spawned from, the cloistered seminaries where monks would start as young men and then pass through life with others of their kind. And as was true in the old abbeys, with age and wisdom one learns to respect those with even greater experience—and listen to them.

In the year 2000 or thereabouts, the authors of this book were at lunch with several of the eldest of the science faculty of the California Institute of Technology. One of these elder statesmen was the great Sam Epstein, one of the most distinguished professors of geochemistry, perhaps of all time. Sam was present in the halcyon days at the University of Chicago, when Nobel laureate chemist Harold Urey discovered a way to measure the temperature at which ancient carbonate rocks were formed by comparing the isotopes of oxygen found in the precipitated carbonate rock. The ratio of the isotope O
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varied with the much more rare isotope O
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in proportion to the temperature of formation.

Sam eventually moved to Caltech and spent his career making high-precision measurements of many kinds of samples, using many different methodologies. But his first love seemed to be ancient temperatures. After a wonderful lunch, he took Kirschvink and Ward to his downstairs lab, which was in the process of being dismantled.
The geochemical equipment of the 1950s and 1960s, Sam’s heyday, was mainly composed of handmade and hand-blown glassware, walls of thin tubes spiraling, crisscrossing, making spider webs of glass interrupted by strange flask-like shapes, rubber tubing coming and going, greased glass stopcocks of exquisite manufacture—everything custom made by the artisans who kept science in those days going, the skilled technicians now banished by budget cuts and the new generations of solid-state technology.

We walked through the lab, and conversation moved on to a topic then of our keen interest: the Permian mass extinction and its possible causes. At that moment the impact hypothesis was still viewed as the probable cause. Sam, however, would have none of it. He turned to us with a smile and told us the following short story. In his younger days he had taken samples of marine limestone that dated to the earliest Triassic, samples that had probably been formed in a very shallow seaway somewhere near the Permian equator in what is now Iran. On a whim, or because that is what he loved most, Sam began analyzing these samples for their ancient temperatures. He was stunned, he said, to find that all had been formed in temperatures above 40°C, with some of the temperatures exceeding 50°C—from 104°F to over 120°F! The samples had come from ancient corals, creatures that need water of normal salinity.

Such temperatures can be found in the stagnant pools and lagoons. But brachiopods do not live in such places. The temperatures found by Sam Epstein could not have been formed anywhere on our Earth. They spoke of a postextinction world of unreal water temperature in the main ocean.

Sam, then in his eighties and with only another year ultimately to live, smiled a sad smile. He told us that he never had the guts to publish these data. Any paleotemperature analysis requires really pristine samples to be accurate, and quite often samples looking as if they had not been reheated or exposed to groundwater or chemically changed in any obvious way had, in fact, had their oxygen isotope temperatures “reset,” and such resets were normally to produce what looked like abnormally high temperatures. This process becomes ever more
common the older the sample. But Sam was quite convinced that he had proof of ocean water temperatures above 100°F in the first million years after the Permian extinction—in the first million years of the Triassic.

Several years later, in analyzing paleotemperatures from a different, lower Triassic site, we too found what looked like one-hundred-degree-plus water. This time the depth was even greater than the estimated ancient water depths where Sam Epstein’s Triassic brachiopods had grown so long ago. Like Sam Epstein, we did not publish these results.

The prize never goes to the faint of heart. In 2012 a joint Chinese-American research team, trying to understand why it took so long for life to recover in the seas after the Permian extinction, published an amazing paper.
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Their findings: water temperature of 104°F in the sea, and a blistering 140°F on land! Unlike the work of Epstein, this study involved the analysis of over fifteen thousand samples, making it the most detailed and painstaking look yet at the environmental conditions in the aftermath of the Permian extinction.

The scientists completing this study allowed themselves to speculate about what that ancient hot world would have been like. Most marine organisms die above the plus-100°F level found by the investigators; in fact photosynthesis essentially stops at temperatures much above this. In that world, the entire zone of the topics would have been devoid of animals, and complex life would have hung on only at high latitudes. Land animals would have been rare even in the mid-latitudes. In such heat there would have been enormous volumes of moisture in the air, and the topics would have been wet year-round. But it might have been a wet desert, with no plant life at all.

Ever better geochronology now shows that this time of high temperature extended at least for the first 3 million years of the Triassic, and indeed may have been climbing ever higher during that time, with a maximum temperature occurring during a time interval known as the Smithian stage (a million-year time interval of around 247 million years ago) having the highest of all known temperatures since the time when animals first occurred. Sam Epstein was right.
Our data from Opal Creek
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were right. We were wrong in not publishing those data.