How Life on Earth Nearly Ended 250 Million Years Ago
By Douglas H. Erwin
Princeton Univ. 296 pp. $24.95
The last time Earth experienced a mass extinction, some 65 million years ago, at the end of the Cretaceous period, there is little doubt about what happened. A humongous meteor slammed into the Yucatan Peninsula, incinerating everything around for thousands of miles. Plumes of vaporized rock blanketed the planet in a layer of thick ash, blocking the sun and choking off photosynthesis. The entire global ecosystem virtually collapsed in a geological eye-blink.
Though the dinosaurs might find it crass to say so, the late Cretaceous cataclysm that did them in was a planetary bad hair day compared to the mass extinction that occurred some 250 million years ago, at the end of the Permian period. The Permian event is probably the closest that life on Earth ever came to being completely extinguished. Around 95 percent of marine species and 70 percent of terrestrial vertebrates were wiped out -- a greater percentage of the Earth's species than the next two largest mass extinctions combined. The break in the fossil record at the Permian boundary is so severe that 19th-century geologists saw it as evidence of two completely separate creations of life.
Just what caused this apocalypse is one of science's great unsolved riddles. Over the years, a cottage industry of Permian speculators has pointed the finger at just about every conceivable culprit. The list of indicted suspects includes -- take a deep breath -- plate tectonics, volcanoes, glaciation, a meteor, a supernova, a massive methane burp from the depths of the sea, oxygen-deprived oceans, an overly complex global ecosystem that collapsed under its own weight and, most fantastic of all, a buildup of cancer-inducing dark matter in the Earth's core. Dream up a way of killing off life on Earth, and chances are some reputable scientist has already proposed it as a cause of the Permian extinction.
Douglas H. Erwin, a Smithsonian paleobiologist and one of the leading experts on the Permian extinction, has meticulously sifted through the evidence for each of these hypothetical culprits. His accessible new book, Extinction -- written, it seems, both to persuade his colleagues and to educate a lay audience -- is told from the perspective of a forensic scientist trying to piece together a quarter-billion-year-old crime scene from an impossibly scant body of clues. It unfolds as a sort of geological mystery story.
An extraterrestrial impact, like the one that killed off the dinosaurs, is perhaps the most attractive hypothesis, one favored by many scientists because it's both elegant and plausible. But no one has ever found the telltale signs of impact -- a spike in iridium (a metal rare on Earth but common in meteorites) and shocked quartz crystals -- that are present in rocks dating back to the Cretaceous extinction. Unlike the meteor that annihilated the dinosaurs, whatever killed off the Permian fauna apparently left no smoking gun.
But it left some suggestive pieces of circumstantial evidence. The Permian extinction happens to have coincided with the million-year-long eruption of the Siberian flood basalts, one of the most massive volcanic events in the last 600 million years. Those eruptions spilled magma across an area larger than the continental United States -- in some places as deep as six kilometers. It's hard to say what volcanism on that scale would do to the planet or what might have caused it, but the large amounts of dust and carbon dioxide vented into the atmosphere would probably have created a very unpleasant environment. The problem facing geologists is how to distinguish cause from coincidence.
The Permian also happens to have coincided with the formation of the massive supercontinent Pangea. Species that had previously existed in isolation suddenly had to compete against one another, and, according to one theory, this competition forced much of the planet's biodiversity out of business. Other scientists speculate that the formation of Pangea could have severely affected the global climate. Proximity to water moderates the weather, which means that a supercontinent with vast, land-locked spaces would have had nasty seasons -- think Siberia, but much, much worse. But nasty enough to knock out most of the life on Earth?
For a long time, scientists assumed the Permian extinction was a drawn-out affair. Recently, however, a consensus has emerged that the extinction probably occurred in two waves, separated by about 10 million years, and that the second, more brutal wave probably happened over no more than 180,000 years and possibly much faster -- practically a split second in geological time.
The speed of the extinction would seem to rule out gradual processes like plate tectonics as the culprit, but with the Permian extinction, it's hard to rule out anything. The mystery is too deep, the evidence too scarce, the plausible causes too many. Unlike some of his colleagues, Erwin is humble enough not to pretend to know what happened at the end of the Permian. His whodunit ends the way it begins -- not with a conviction but a question mark. ·
Joshua Foer is working on a book about the science of memory.