BY ELIZABETH KOLBERT

Charles Darwin didn’t believe in catastrophe. The whole idea struck him as implausible, a product of the sort of antiquated thinking that mixed science and religion, Noah’s flood and the ice ages. In

On the Origin of Species

, published in 1859, he heaped scorn on the “catastrophist” approach: “So profound is our ignorance, and so high our presumption, that we marvel when we hear of the extinction of an organic being; and as we do not see the cause, we invoke cataclysms to desolate the world.”

If the fossil record seemed to show that whole groups of organisms had suddenly vanished from the face of the earth all at one time, then that must mean the record was imperfect. At one point in

On the Origin of Species

, he compares the fossil record to a multivolume world-history set no one has bothered to look after. Of the many original books in the set, he writes, “We possess the last volume alone, relating only to two or three countries”; of this volume, “only here and there a short chapter has been preserved.”

By the start of the 20th century, Darwin’s view had become so dominant that to be a scientist meant to see extinction as he did. All the way into 1970s, paleontologists were delivering lectures on “the incompleteness of the fossil record.” But Darwin, it turns out, was wrong.

Over the past half-billion years, there have been at least 20 mass extinctions. Five of these—the so-called Big Five—were so devastating they are usually put in their own category. The first took place during the late Ordovician period, nearly 450 million years ago, when life was still largely confined to the water. Geologic records indicate that more than 80 percent of marine species died out. The fifth occurred at the end of the Cretaceous period, 65 million years ago. The end-Cretaceous event exterminated not only the dinosaurs but 75 percent of all species on earth.

It’s now generally believed that the end-Cretaceous extinction was caused by an asteroid that struck the Yucatán Peninsula. The asteroid, which was about six miles wide, was traveling at a speed of 45,000 miles per hour, and its impact released a billion times more energy than the detonation of the bomb that destroyed Hiroshima. The discovery of the asteroid impact in the early 1980s blasted away, in a manner of speaking, Darwinian skepticism about mass extinction. It also challenged some fundamental notions about the nature of the world. No organism, no matter how well adapted to its environment, can prepare for a catastrophe that occurs only once every 25 million years or so. So is it the fit that survive, or just the lucky?

The biggest of the Big Five extinctions is what is known as the end-Permian. It took place some 250 million years ago and is named after the geologic period—the Permian—that it brought to a disastrous conclusion.

At the time of the end-Permian catastrophe, geologic records indicate that almost all of what we would now call the seven continents were part of one huge landmass, which goes by the name Pangaea. The oceans—or, really, ocean, since all the seas were also smushed together in one super-sea, known as Panthalassa—were filled with reefs, which, while very different from today’s, were home to a wildly diverse cast of corals, sponges, mollusks, brachiopods, and fish. (One place you can see the remains of these reefs is in West Texas; the Guadalupe Mountains are the remnants of an enormous underwater ecosystem.)

Then, all of a sudden, those crowded seas became a lot less crowded. The reefs, everywhere, died out. Rugose corals, which built skeletons that looked like horns, and tabulate corals, which built skeletons that looked like honeycombs, vanished completely, never to reappear. All in all, something like 90 percent of species in the oceans disappeared, and recent research suggests that the devastation was roughly the same on land. It took ten million years for reefs to reappear, and when they did, they were patchy affairs, nowhere near as rich or diverse as those that they replaced. It took tens of millions of years more for life in general to rebound.

The end-Permian has been called the “the great dying,” the “mother of mass extinctions,” and the “greatest unsolved murder mystery of all time.” The mystery is not only why life should have been nearly obliterated but also why, having got so close to the brink, it wasn’t done in entirely. (In that case, of course, we wouldn’t be here now to wonder about the event.)

Following the discovery of the asteroid impact that ended the Cretaceous, many scientists expected to find evidence of similar impacts coinciding with other mass extinctions. But this hasn’t happened. There are impact craters all over the planet, but the timing just doesn’t jibe with the chronology of the extinctions. In recent years, scientists studying the end-Permian have focused on a series of gigantic volcanic eruptions that occurred in what is now Siberia. The eruptions created a formation known as the Siberian Traps—essentially a million cubic miles’ worth of lava. The process threw into the air huge amounts of carbon dioxide and also may have released large amounts of chlorine, fluorine, and sulfur dioxide.

How the eruptions would have produced a mass extinction is not known exactly, but there are some plausible theories. Then, as now, the added CO2 in the air would have led to significant warming. Meanwhile, the chlorine and fluorine would have damaged the ozone layer, and the sulfur dioxide would have produced acid rain. It is disconcerting to realize that all of the proposed triggers for the event—the release of large amounts of CO2, chlorine, fluorine, and sulfur dioxide—are things that are occurring today.

One of the few clear and unambiguous facts about the end-Permian is that the oceans became anoxic, or devoid of oxygen. (This signal can be found in rocks from the period.) Most organisms obviously can’t survive such conditions, but some forms of bacteria, which, in effect, “breathe” sulfur, can. These bacteria, which nowadays tend to be found tucked away in the bottom of lakes, seem to have thrived at the time of the extinction; if they had released large amounts of hydrogen sulfide into the water, they could potentially have poisoned most organisms in the sea and also on land.

What about the organisms that made it through? Were they special in any way? Among the relatively few land animals to survive was a reptile known as

Lystrosaurus. Lystrosaurus

—the name means “shovel lizard”—had a flat face, a waddling gait, and two pointy teeth and was about the size of a pig. It went on to dominate the wasted landscape—for a time, it was the largest creature on Earth—and in some areas makes up 95 percent of the animal fossils from the period right after the extinction. Various explanations have been offered to account for

Lystrosaurus

’s good fortune, none of which has proved convincing. As Michael Benton, a paleontologist at Britain’s University of Bristol, has put it, the survivors of mass extinctions appear to be “more lucky than specially adapted.”

Right now, as it happens, another mass extinction is taking place—this one entirely of our making. It is sometimes referred to, for dramatic effect, as the Sixth Extinction, though it is still unclear whether things will get bad enough to justify that title.

Currently, a third of all amphibian species, nearly a third of reef-building corals, a quarter of all mammals, and an eighth of all birds are classified as “threatened with extinction.” These estimates do not include the species that humans have already wiped out—a list that starts with the mastodon and extends beyond the passenger pigeon—or the species for which there are insufficient data. If you go to the Hall of Biodiversity at the American Museum of Natural History and look down, you will see a tiny exhibit set into the floor. The exhibit shows the fossils of various lineages that died out in each of the previous mass extinctions. “We are in the midst of the Sixth Extinction,” it says, “this time caused solely by mankind’s transformation of the ecological landscape.” Visitors can walk right over the exhibit, and when I visited the museum a few months ago, that’s exactly what they were doing. I didn’t see anyone in the hall pause long enough to figure out what they were stepping on.