The Sixth Extinction: An Unnatural History

by Elizabeth Kolbert

Any event that has occurred just five times since the first animal with a backbone appeared, some five hundred million years ago, must qualify as exceedingly rare. The notion that a sixth such event would be taking place right now, more or less in front of our eyes, struck me as, to use the technical term, mind-boggling. Surely this story, too—the bigger, darker, far more consequential one—deserved telling. If Wake and Vredenburg were correct, then those of us alive today not only are witnessing one of the rarest events in life’s history, we are also causing it.

This means that amphibians have been around not just longer than mammals, say, or birds; they have been around since before there were dinosaurs.

Since the breakup of Pangaea, they’ve adapted to conditions on every continent except Antarctica. Worldwide, just over seven thousand species have been identified, and while the greatest number are found in the tropical rainforests, there are occasional amphibians, like the sandhill frog of Australia, that can live in the desert, and also amphibians, like the wood frog, that can live above the Arctic Circle.

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.

Part of what made the situation so mystifying was the geography; frogs seemed to be vanishing not only from populated and disturbed areas but also from relatively pristine places, like the Sierras and the mountains of Central America.

He found a strange microorganism on the animals’ skin, which he eventually identified as a fungus belonging to a group known as chytrids. Chytrid fungi are nearly ubiquitous; they can be found at the tops of trees and also deep underground. This particular species, though, had never been seen before; indeed, it was so unusual that an entire genus had to be created to accommodate it. It was named Batrachochytrium dendrobatidis—batrachos is Greek for “frog”—or Bd for short.

Subsequent research showed that Bd interferes with frogs’ ability to take up critical electrolytes through their skin. This causes them to suffer what is, in effect, a heart attack.

The chytrid fungus, it turns out, does not need amphibians in order to survive. This means that even after it has killed off the animals in an area, it continues to live on, doing whatever it is that chytrid fungi do. Thus, were the golden frogs at EVACC allowed to amble back into the actual hills around El Valle, they would sicken and collapse.

(Though the fungus can be destroyed by bleach, it’s obviously impossible to disinfect an entire rainforest.)

At this point, it appears to be, for all intents and purposes, unstoppable.

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. Mass extinctions are different. Instead of a background hum there’s a crash, and disappearance rates spike.

The history of life thus consists of “long periods of boredom interrupted occasionally by panic.”

“I sought a career in herpetology because I enjoy working with animals,” Joseph Mendelson, a herpetologist at Zoo Atlanta, has written. “I did not anticipate that it would come to resemble paleontology.”

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. But extinction rates among many other groups are approaching amphibian levels. 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.

There are all sorts of seemingly disparate reasons that species are disappearing. But trace the process far enough and inevitably you are led to the same culprit: “one weedy species.”

Without being loaded by someone onto a boat or a plane, it would have been impossible for a frog carrying Bd to get from Africa to Australia or from North America to Europe. This sort of intercontinental reshuffling, which nowadays we find totally unremarkable, is probably unprecedented in the three-and-a-half-billion-year history of life.

Extinction finally emerged as a concept, probably not coincidentally, in revolutionary France. It did so largely thanks to one animal, the creature now called the American mastodon, or Mammut americanum, and one man, the naturalist Jean-Léopold- Nicolas-Frédéric Cuvier, known after a dead brother simply as Georges. Cuvier is an equivocal figure in the history of science. He was far ahead of his contemporaries yet also held many of them back; he could be charming and he could be vicious; he was a visionary and, at the same time, a reactionary. By the middle of the nineteenth century, many of his ideas had been discredited. But the most recent discoveries have tended to support those very theories of his that were most thoroughly vilified, with the result that Cuvier’s essentially tragic vision of earth history has come to seem prophetic.

He made his way to New Orleans and from there shipped the tusk, the teeth, and the giant femur to France. They were presented to Louis XV, who installed them in his museum, the Cabinet du Roi. Decades later, maps of the Ohio River valley were still largely blank, except for the Endroit où on a trouvé des os d’Éléphant—the “place where the elephant bones were found.” (Today the “place where the elephant bones were found” is a state park in Kentucky known as Big Bone Lick.)

(This would disprove the theory, popular in Europe at the time, that the animals of the New World were smaller and more “degenerate” than those of the Old.)

If it could not be found in Virginia, it was roaming those parts of the continent that “remain in their aboriginal state, unexplored and undisturbed.” When, as president, he dispatched Meriwether Lewis and William Clark to the Northwest, Jefferson hoped that they would come upon live incognita roaming its forests. “Such is the economy of nature,” he wrote, “that no instance can be produced of her having permitted any one race of her animals to become extinct; of her having formed any link in her great work so weak as to be broken.”

And if there were four extinct species, Cuvier declared, there must be others. The proposal was a daring one to make given the available evidence. On the basis of a few scattered bones, Cuvier had conceived of a whole new way of looking at life. Species died out. This was not an isolated but a widespread phenomenon.

Before he’d begun his “researches,” there had been—depending upon who was doing the counting—zero or one extinct vertebrate. Thanks for the most part to his own efforts, there were now forty-nine. As Cuvier’s list grew, so, too, did his renown. Few naturalists dared to announce their findings in public until he had vetted them.

This pattern led Cuvier to another extraordinary insight about the history of life: it had a direction. Lost species whose remains could be found near the surface of earth, like mastodons and cave bears, belonged to orders of creatures still alive. Dig back farther and one found creatures, like the animal from Montmartre, that had no obvious modern counterparts. Keep digging and mammals disappeared altogether from the fossil record. Eventually one reached a world not just previous to ours, but a world previous to that, dominated by giant reptiles.

CUVIER’S ideas about this history of life—that it was long, mutable, and full of fantastic creatures that no longer existed—would seem to have made him a natural advocate for evolution. But Cuvier opposed the concept of evolution, or transformisme as it was known in Paris at the time, and he tried—generally, it seems, successfully—to humiliate any colleagues who advanced the theory.

Having dismissed transformisme, Cuvier was left with a gaping hole. He had no account of how new organisms could appear, nor any explanation of how the world could have come to be populated by different groups of animals at different times. This doesn’t seem to have bothered him. His interest, after all, was not in the origin of species but in their demise.

Cuvier’s ideas about a globe wracked periodically by cataclysm proved very nearly as influential as his original discoveries.

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. This sort of ecumenicalism was unacceptable to the Anglican clergy who made up the faculty at institutions like Oxford, and when the essay was translated into English, it was construed by Buckland and others as offering proof of Noah’s flood.

At the same time, some of Cuvier’s most wild-sounding claims have turned out to be surprisingly accurate. Life on earth has been disturbed by “terrible events,” and “organisms without number” have been their victims. Such events cannot be explained by the forces, or “agents,” at work in the present. Nature does, on occasion, “change course,” and at such moments, it is as if the “thread of operations” has been broken.

In fact, 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. This wave coincided with the spread of modern humans and, increasingly, is understood to have been a result of it. In this sense, the crisis Cuvier discerned just beyond the edge of recorded history was us.

Later, when subsequent generations of sailors met similar-colored flightless birds in the Southern Hemisphere, they used the same name, which led to much confusion, since auks and penguins belong to entirely different families.

Much to the auk’s misfortune, a volcanic eruption destroyed the Geirfuglasker in 1830. This left the birds one solitary refuge, a speck of an island known as Eldey. By this point, the great auk was facing a new threat: its own rarity. Skins and eggs were avidly sought by gentlemen, like Count Raben, who wanted to fill out their collections. It was in the service of such enthusiasts that the very last known pair of auks was killed on Eldey in 1844.

THE last people to see great auks alive were around a dozen Icelanders who made the trip to Eldey by rowboat. They set out one evening in June 1844,

He concluded that the auk was gone—“for all practical purposes therefore we may speak of it as a thing of the past”—and he developed what one biographer referred to as a “peculiar attraction” to “extinct and disappearing faunas.” Newton realized that the birds that bred along Britain’s long coast were also in danger; he noted that they were being gunned down for sport in great numbers.

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.

Darwin assumes that his readers are familiar with such “events” and already habituated to them. He himself seems to find nothing remarkable or troubling about this. But human-caused extinction is of course troubling for many reasons, some of which have to do with Darwin’s own theory, and it’s puzzling that a writer as shrewd and self-critical as Darwin shouldn’t have noticed this.

At the heart of Darwin’s theory, as one of his biographers has put it, is “the denial of humanity’s special status.”

These animals had obviously not been done in by a rival species gradually evolving some competitive advantage. They had all been killed off by the same species, and all quite suddenly—in the case of the great auk and the Charles Island tortoise over the course of Darwin’s own lifetime. 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.

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. 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.”

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? At such moments, what Paul Taylor, a paleontologist at London’s Natural History Museum, calls “the rules of the survival game” abruptly change. Traits that for many millions of years were advantageous all of a sudden become lethal (though it may be difficult, millions of years after the fact, to identify just what those traits were).

Until the end of the eighteenth century, the very category of extinction didn’t exist.

At the end of the Ordovician, some 444 million years ago, the oceans emptied out. Something like eighty-five percent of marine species died off. For a long time, the event was regarded as one of those pseudo-catastrophes that just went to show how little the fossil record could be trusted. Today, it’s seen as the first of the Big Five extinctions, and it’s thought to have taken place in two brief, intensely deadly pulses.

As in Tolstoy, every extinction event appears to be unhappy—and fatally so—in its own way. It may, in fact, be the very freakishness of the events that renders them so deadly; all of a sudden, organisms find themselves facing conditions for which they are, evolutionarily, completely unprepared.

(A recent study of pollen and animal remains on Easter Island concluded that it wasn’t humans who deforested the landscape; rather, it was the rats that came along for the ride and then bred unchecked. The native palms couldn’t produce seeds fast enough to keep up with their appetites.)

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.”

Human activity has transformed between a third and a half of the land surface of the planet. • Most of the world’s major rivers have been dammed or diverted. • Fertilizer plants produce more nitrogen than is fixed naturally by all terrestrial ecosystems. • Fisheries remove more than a third of the primary production of the oceans’ coastal waters. • Humans use more than half of the world’s readily accessible fresh water runoff.

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.

Like many small places, Castello Aragonese is a product of very large forces, in this case the northward drift of Africa, which every year brings Tripoli an inch or so closer to Rome. Along a complicated set of folds, the African plate is pressing into Eurasia, the way a sheet of metal might be forced into a furnace.

Thanks to all this extra CO2, the pH of the oceans’ surface waters has already dropped, from an average of around 8.2 to an average of around 8.1. Like the Richter scale, the pH scale is logarithmic, so even such a small numerical difference represents a very large real-world change. A decline of .1 means that the oceans are now thirty percent more acidic than they were in 1800.

“Because it’s so important, we humans put a lot of energy into making sure that the pH of our blood is constant,” Hall-Spencer says, raising his voice to be heard over the noise of the running water. “But some of these lower organisms, they don’t have the physiology to do that. They’ve just got to tolerate what’s happening outside, and so they get pushed beyond their limits.”

It is common and abundant from west Africa to Wales. In the pH 7.8 zone, which corresponds to the seas of the not-too-distant future, Balanus perforatus was gone. Mytilus galloprovincialis, a blue-black mussel native to the Mediterranean, is so adaptable that it’s established itself in many parts of the world as an invasive. It, too, was missing.