The Sixth Extinction: An Unnatural History

by Elizabeth Kolbert

there “have been five great mass extinctions during the history of life on this planet.” These extinctions they described as events that led to “a profound loss of biodiversity.”

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The first took place during the late Ordovician period, some 450 million years ago, when living things were still mainly confined to the water.

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The most devastating took place at the end of the Permian period, some 250 million years ago, and it came perilously close to emptying the earth out altogether. (This event is sometimes referred to as “the mother of mass extinctions” or “the great dying.”)

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The most recent—and famous—mass extinction came at the close of the Cretaceous period; it wiped out, in addition to the dinosaurs, the plesiosaurs, the mosasaurs, the ammonites, and the pterosaurs.

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Amphibians are, after all, among the planet’s great survivors. The ancestors of today’s frogs crawled out of the water some 400 million years ago, and by 250 million years ago the earliest representatives of what would become the modern amphibian orders—one includes frogs and toads, the second newts and salamanders, and the third weird limbless creatures called caecilians—had evolved. This means that amphibians have been around not just longer than mammals, say, or birds; they have been around since before there were dinosaurs.

extinction takes place only very rarely, more rarely even than speciation, and it occurs at what’s known as the background extinction rate. This rate varies from one group of organisms to another; often it’s expressed in terms of extinctions per million species-years.

For what’s probably the best-studied group, which is mammals, it’s been reckoned to be roughly .25 per million species-years. This means that, since there are about fifty-five hundred mammal species wandering around today, at the background extinction rate you’d expect—once again, very roughly—one species to disappear every seven hundred years.

Anthony Hallam and Paul Wignall, British paleontologists who have written extensively on the subject, define mass extinctions as events that eliminate a “significant proportion of the world’s biota in a geologically insignificant amount of time.”

Another expert, David Jablonski, characterizes mass extinctions as “substantial biodiversity losses” that occur rapidly and are “global in extent.”

Michael Benton, a paleontologist who has studied the end-Permian extinction, uses the metaphor of the tree of life: “During a mass extinction, vast swathes of the tree are cut short...

A fifth paleontologist, David Raup, has tried looking at matters from the perspective of the victims: “Species are at a low risk of extinction most of the time.” But this “condition of relative safety is punctuated at rare intervals by a vastly higher risk.” The history of life th...

Adapted from David M. Raup and J. John Sepkoski Jr./Science 215 (1982), 1502 The Big Five extinctions, as seen in the marine fossil record, resulted in a sharp decline in diversity at the family level. If even one species from a family made it through, the family cou...

Today, amphibians enjoy the dubious distinction of being the world’s most endangered class of animals; it’s been calculated that the group’s extinction rate could be as much as forty-five thousand times higher than the background rate.

It is estimated that one-third of all reef-building corals, a third of all freshwater mollusks, a third of sharks and rays, a quarter of all mammals, a fifth of all reptiles, and a sixth of all birds are headed toward oblivion.

One theory has it that Bd was moved around the globe with shipments of African clawed frogs, which were used in the nineteen-fifties and sixties in pregnancy tests. (Female African clawed frogs, when injected with the urine of a pregnant woman, lay eggs within a few hours.)

The same was true of the so-called Maastricht animal, whose remains—an enormous, pointy jaw studded with sharklike teeth—had been found in a Dutch quarry. (The Maastricht fossil had recently been seized by the French, who occupied the Netherlands in 1795.)

We chatted about proboscideans for a while. “They’re a fascinating group,” he told me. “For instance, the trunk, which is a change of anatomy in the facial area that is truly extraordinary, it evolved separately five times. Two times—yes, that’s surprising. But it happened five times independently!

About thirty million years ago, the proboscidean line that would lead to mastodons split off from the line that would lead to mammoths and elephants. The latter would eventually evolve its more sophisticated teeth, which are made up of enamel-covered plates that have been fused into a shape a bit like a bread loaf. This arrangement is a lot tougher, and it allowed mammoths—and still allows elephants—to consume an unusually abrasive diet. Mastodons, meanwhile, retained their relatively primitive molars (as did humans) and just kept chomping away.

In one of the most often-quoted passages of On the Origin of Species, Darwin wrote: It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers.

great auks were the original “penguins.” They were called this—the etymology of “penguin” is obscure and may or may not be traced to the Latin pinguis, meaning “fat”—by European sailors who encountered them in the North Atlantic.

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auks and penguins belong to entirely different families. (Penguins constitute their own family, while auks are members of the family that includes puffins and guillemots; genetic analysis has shown that razorbills are the great auk’s closest living relatives.)

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Foraminifera, or “forams” for short, are the tiny marine creatures that create little calcite shells, or tests, which drift down to the ocean floor once the animal inside has died. The tests have a distinctive shape, which varies from species to species; some look (under magnification) like beehives, others like braids or bubbles or clusters of grapes. Forams tend to be widely distributed and abundantly preserved, and this makes them extremely useful as index fossils: on the basis of which species of forams are found in a given layer of rock, an expert like Silva can tell the rock’s age.

Iridium is extremely rare on the surface of the earth but much more common in meteorites. In the form of microscopic grains of cosmic dust, bits of meteorites are constantly raining down on the planet.

On an otherwise ordinary day sixty-five million years ago, an asteroid ten kilometres wide collided with the earth. Exploding on contact, it released energy on the order of a hundred million megatons of TNT, or more than a million of the most powerful H-bombs ever tested. Debris, including iridium from the pulverized asteroid, spread around the globe. Day turned to night, and temperatures plunged. A mass extinction ensued.

In 1841, John Phillips, a contemporary of Lyell’s who succeeded him as president of the Geological Society of London, divided life into three chapters. He called the first the Paleozoic, from the Greek for “ancient life,” the second the Mesozoic, meaning “middle life,” and the third the Cenozoic, “new life.”

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Phillips fixed as the dividing point between the Paleozoic and the Mesozoic what would now be called the end-Permian extinction, and between the Mesozoic and the Cenozoic, the end-Cretaceous event. (In geologic parlance, the Paleozoic, Mesozoic, and Cenozoic are “eras,” and each era comprises several “periods”; the Mesozoic, for example, spans the Triassic, the Jurassic, and the Cretaceous.)

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belemnites—squid-like creatures that left behind fossils shaped like bullet casings.

rudist bivalves—mollusks that formed immense reefs. (Rudists have been described as oysters pretending to be corals.)

The first independent corroboration came in the form of tiny grains of rock known as “shocked quartz.” Under high magnification, shocked quartz exhibits what look like scratch marks, the result of bursts of high pressure that deform the crystal structure. Shocked quartz was first noted at nuclear test sites and subsequently found in the immediate vicinity of impact craters.

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In 1984, grains of shocked quartz were discovered in a layer of clay from the Cretaceous-Tertiary, or K-T, boundary in eastern Montana. (K is used as the abbreviation for Cretaceous because C was already taken by the Carboniferous; today, the border is formally known as the Cretaceous-Paleogene, or K-Pg, boundary.)

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The next clue showed up in south Texas, in a curious layer of end-Cretaceous sandstone that seemed to have been produced by an enormous tsunami. It occurred to Walter Alvarez that if there had been a giant, impact-induced tsunami, it would have scoured away shorelines, leaving behind a distinctive fingerprint in the sedimentary record. He scanned the records of thousands of sediment cores that had been drilled in the oceans, and found such a fingerprint in cores from the Gulf of Mexico.

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Finally, a hundred-and-fifty-kilometre-wide crater was discovered or, more accurately, rediscovered, beneath the Yucatán Peninsula. Buried under a kilometre of newer sediment, the crater had shown up in gravity surveys taken in the nineteen-fifties by Mexico’s state-run oil company.

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The cores were finally located in 1991 and found to contain a layer of glass—rock that had melted, then rapidly cooled—right at the K-T boundary. To the Alvarez camp, this was the clincher, and it was enough to move many uncommitted scientists into the pro-impact column.

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Walter dubbed the formation the “Crater of Doom.” It became more widely known, after the nearest town, as the Chicxulub crater.

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Ammonites floated through the world’s shallow oceans for more than three hundred million years, and their fossilized shells turn up all around the world.

Like nautiluses, to whom they were distantly related, ammonites constructed spiral shells divided into multiple chambers. The animals themselves occupied only the last and largest chamber; the rest were filled with air, an arrangement that might be compared to an apartment building in which just the penthouse is rented. The walls between the chambers, known as septa, were fantastically elaborate, folded into intricate ruffles, like the edges of a snowflake.

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(Individual species can be identified by the distinctive patterns of their pleats.) This evolutionary development allowed ammonites to build shells that were at once light and robust—capable of withstanding many atmospheres’ worth of water pressure. Most ammonites could fit in a human hand; some grew to be the size of kiddie pools.

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Based on the number of teeth ammonites had—nine—it’s believed that their closest living kin are octopuses. But since ammonites’ soft body parts are virtually never preserved, what exactly the animals looked like and how they lived are largely matters of inference. It’s probable, though not certain, that they propelled themselves by shooting out a jet of water, which means that they could only travel backward.

Owing to the composition of the Yucatán Peninsula, the dust thrown up was rich in sulfur. Sulfate aerosols are particularly effective at blocking sunlight, which is the reason a single volcanic eruption, like Krakatoa, can depress global temperatures for years. After the initial heat pulse, the world experienced a multiseason “impact winter.” Forests were decimated.

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Palynologists, who study ancient spores and pollen, have found that diverse plant communities were replaced entirely by rapidly dispersing ferns. (This phenomenon has become known as the “fern spike.”) Marine ecosystems effectively collapsed, and they remained in that state for at least half a million, and perhaps as many as several million, years. (The desolate post-impact sea has been dubbed the “Strangelove ocean.”)

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Everything (and everyone) alive today is descended from an organism that somehow survived the impact. But it does not follow from this that they (or we) are any better adapted. In times of extreme stress, the whole concept of fitness, at least in a Darwinian sense, loses its meaning: how could a creature be adapted, either well or ill, for conditions it has never before encountered in its entire evolutionary history?

“Though the world does not change with a change of paradigm, the scientist afterward works in a different world” is how Kuhn put it.

In this sense the reigning paradigm is neither Cuvierian nor Darwinian but combines key elements of both—“long periods of boredom interrupted occasionally by panic.”

graptolites, a once vast and extremely diverse class of marine organisms that thrived during the Ordovician and then, in the extinction event, were very nearly wiped out. To the naked eye, graptolite fossils look like scratches or in some cases tiny petroglyphs. (The word “graptolite” comes from the Greek meaning “written rock”; it was coined by Linnaeus, who dismissed graptolites as mineral encrustations trying to pass themselves off as the remnants of animals.)

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Viewed through a hand lens, they often prove to have lovely, evocative shapes; one species suggests a feather, another a lyre, a third the frond of a fern. Graptolites were colonial animals; each individual, known as a zooid, built itself a tiny, tubular shelter, known as a theca, which was attached to its neighbor’s, like a row house. A single graptolite fossil thus represents a whole community, which drifted or more probably swam along as a single entity, feeding off even smaller plankton.

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The end-Permian or Permo-Triassic extinction was the biggest of the Big Five, an episode that came scarily close to eliminating multicellular life altogether.

The current theory is that the end-Ordovician extinction was caused by glaciation.

The word “Anthropocene” is the invention of Paul Crutzen, a Dutch chemist who shared a Nobel Prize for discovering the effects of ozone-depleting compounds. The importance of this discovery is difficult to overstate; had it not been made—and had the chemicals continued to be widely used—the ozone “hole” that opens up every spring over Antarctica would have expanded until eventually it encircled the entire earth. (One of Crutzen’s fellow Nobelists reportedly came home from his lab one night and told his wife, “The work is going well, but it looks like it might be the end of the world.”)

Most significantly, Crutzen said, people have altered the composition of the atmosphere. Owing to a combination of fossil fuel combustion and deforestation, the concentration of carbon dioxide in the air has risen by forty percent over the last two centuries, while the concentration of methane, an even more potent greenhouse gas, has more than doubled.

(To geologists, an epoch is a subdivision of a period, which, in turn, is a division of an era: the Holocene, for instance, is an epoch of the Quaternary, which is a period in the Cenozoic.)