The Sixth Extinction: An Unnatural History

by Elizabeth Kolbert

“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.

Wake and Vredenburg argued that, based on extinction rates among amphibians, an event of a similarly catastrophic nature was currently under way.

Amphibians emerged at a time when all the land on earth was part of a single expanse known as Pangaea.

Their extended evolutionary history means that even groups of amphibians that, from a human perspective, seem to be fairly similar may, genetically speaking, be as different from one another as, say, bats are from horses.

Once chytrid swept through El Valle, it didn’t stop; it continued to move east.

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.

define mass extinctions as

events that eliminate a “significant proportion of the world’s biota in a geologically insignificant amount of time.

Today, amphibians enjoy the dubious distinction of being the world’s most endangered class of animals;

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.

“Life on earth has often been disturbed by terrible events,” he wrote. “Living organisms without number have been the victims of these catastrophes.”

“The thread of operations is broken,” Cuvier wrote. “Nature has changed course, and none of the agents she employs today would have been sufficient to produce her former works.”

he produced the first stratigraphic map of the Paris basin

he saw signs of cataclysmic change.

The shifts from one environment to the other—from marine to terrestrial, or, at some points, from marine to freshwater—had, Cuvier decided, “not been slow at all”; rather, they had been brought abou...

Cuvier’s essay was pointedly secular. He cited the Bible as one of many old (and not entirely reliable) works, alongside the Hindu Vedas and the Shujing.

The empirical grounds of Cuvier’s theory have, by now, largely been disproved.

The stratigraphy of the Paris basin reflects not sudden “irruptions” of water but rather gradual changes in sea level and the effects of plate tectonics.

Life on

earth has been disturbed by “terrible events,” and “organisms without number” have been their victims.

the American mastodon vanished around thirteen thousand years ago. Its demise was part of a wave of disappearances that has come to be known as the megafauna extinction.

the crisis Cuvier discerned just beyond the edge of recorded history was us.

every feature of the landscape was the result of very gradual processes operating over countless millennia—processes like sedimentation, erosion, and vulcanism, which were all still readily observable. For generations of geology students, Lyell’s thesis would be summed up as “The present is the key to the past.”

Darwin saw that the key to understanding coral reefs was the interplay between biology and geology.

Lyell’s adamant opposition to transmutation, as it was known in London, is almost as puzzling as Cuvier’s.

With his theory of natural selection,

Darwin recognized that just as the features of the inorganic world—deltas, river valleys, mountain chains—were brought into being by gradual change, the organic world similarly was subject to constant flux.

From Darwin’s premises, an important prediction followed. If extinction was driven by natural selection and only by natural selection, the two processes had to proceed at roughly the same rate. If anything, extinction had to occur more gradually.

It stood to reason that extinction should have been that much more difficult to witness. And yet it wasn’t.

during the years Darwin spent holed up at Down House, developing his ideas about evolution, the very last individuals of one of Europe’s most celebrated species, the great auk, disappeared. What’s more, the event was painstakingly chronicled by British ornithologists. Here Darwin’s theory was directly contradicted by the facts, with potentially profound implications.

As best as can be determined, great auks lived much as penguins do. In fact, great auks were the original “penguins.”

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.)

Newton argued for a ban on hunting during breeding season, and his lobbying resulted in one of the first laws aimed at what today would be called wildlife protection: the Act for the Preservation of Sea Birds.

Either there had to be a separate category for human-caused extinction, in which case people really did deserve their “special status” as a creature outside of nature, or space in the natural order had to be made for cataclysm, in which case, Cuvier—distressingly—was right.

THE Alvarezes’ paper, “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction,” was published in June 1980.

No single mechanism explains all the mass extinctions in the record, and yet changes in ocean chemistry seem to be a pretty good predictor. Ocean acidification played a role in at least two of the Big Five extinctions (the end-Permian and the end-Triassic) and quite possibly it was a major factor in a third (the end-Cretaceous).

Warming today is taking place

at least ten times faster than it did at the end of the last glaciation, and at the end of all those glaciations that preceded it. To keep up, organisms will have to migrate, or otherwise adapt, at least ten times more quickly.

How many species overall will be capable of moving fast enough remains an open question, though, as Silman pointed out to me, in the coming decades we are probably going to learn the answer, whether we want to or not.

If warming were held to a minimum, the team estimated that between 22 and 31 percent of the species would be “committed to extinction” by 2050. If warming were to reach what was at that point considered a likely maximum—a figure that now looks too low—by the middle of this century, between 38 and 52 percent of the species would be fated to disappear.

Climate change alone “is unlikely to generate a mass extinction as large as one of the Big Five,” he wrote. However, there’s a “high likelihood that climate change on its own could generate a level of extinction on par with, or exceeding, the slightly ‘lesser’ extinction events” of the past. “The potential impacts,” he concluded, “support the notion that we have recently entered the Anthropocene.”