The Sixth Extinction: An Unnatural History

by Elizabeth Kolbert

Centuries of centuries and only in the present do things happen. —Jorge Luis Borges

and violating Rule No. 1 of rainforest safety: never grab onto something if you don’t know what it is.

Pope put it in his Essay on Man: All are but parts of one stupendous whole, Whose body nature is, and God the soul.

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.

I look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect; of this history 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; and of each page, only here and there a few lines.

Darwin’s successors inherited the “much slow extermination” problem. The uniformitarian view precluded sudden or sweeping change of any kind. But the more that was learned about the fossil record, the more difficult it was to maintain that an entire age, spanning tens of millions of years, had somehow or other gone missing.

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.

When all of these were considered together, a pattern emerged: mass extinctions seemed to take place at regular intervals of roughly twenty-six million years. Extinction, in other words, occurred in periodic bursts,

a summer evening, the café is supposed to be a pleasant place to sip Campari and contemplate the terrors of the past.

Other calculations of his show that to keep pace with the present rate of temperature change, plants and animals would have to migrate poleward by about ten metres a day, and that a molecule of CO2 generated by burning fossil fuels will, in the course of its lifetime in the atmosphere, trap a hundred thousand times more heat than was released in producing it.

As a general rule, the variety of life is most impoverished at the poles and richest at low latitudes. This pattern is referred to in the scientific literature as the “latitudinal diversity gradient,” or LDG, and it was noted already by the German naturalist Alexander von Humboldt, who was amazed by the biological splendors of the tropics, which offer “a spectacle as varied as the azure vault of the heavens.” “The verdant carpet which a luxuriant Flora spreads over the surface of the earth is not woven equally in all parts,” Humboldt wrote after returning from South America in 1804. “Organic development and abundance of vitality gradually increase from the poles towards the equator.” More than two centuries later, why this should be the case is still not known, though more than thirty theories have been advanced to explain the phenomenon.

It is now generally believed that ice ages are initiated by small changes in the earth’s orbit, caused by, among other things, the gravitational tug of Jupiter and Saturn.

These changes alter the distribution of sunlight across different latitudes at different times of year. When the amount of light hitting the far northern latitudes in summer approaches a minimum, snow begins to build up there. This initiates a feedback cycle that causes atmospheric carbon dioxide levels to drop. Temperatures fall, which leads more ice to

build up, and so on. After a while, the orbital cycle enters a new phase, and the feedback loop begins to run in reverse. The ice starts to melt, global CO2 ...

Currently, about 130 million square kilometres of land on the planet are ice-free, and this is the baseline that’s generally used for calculating human impacts. According to a recent study published by the Geological Society of America, people have “directly transformed” more than half of this land—roughly seventy million square kilometres—mostly by converting it to cropland and pasture, but also by building cities and shopping malls and reservoirs, and by logging and mining and quarrying. Of the remaining sixty million square kilometres, about three-fifths is covered by forest—as the authors put it, “natural but not necessarily virgin”—and the rest is either high mountains or tundra or desert.

Since then, many efforts have been made to refine Erwin’s estimates. Most have tended to revise the numbers downward. (Among other things, Erwin probably overstated the proportion of insects dependent on a single host plant.) Still, by all accounts, the figure remains shockingly high: recent estimates suggest there are at least two million tropical insect species and perhaps as many as seven million. By comparison, there are only about ten thousand species of birds in the entire world and only fifty-five hundred species of mammals. Thus for every species with hair and mammary glands, there are, in the tropics alone, at least three hundred with antennae and compound eyes.

“not only was baby’s crib likely made of chestnut,” a plant pathologist named George Hepting once wrote, “but chances were, so was the old man’s coffin.”)

To dispose of the toads humanely, the council instructs children to “cool them in a fridge for 12 hours” and then place them “in a freezer for another 12 hours.” A recent study of visitors to Antarctica found that in a single summer

“Helping a Species Go Extinct.” That year, in a last-ditch effort, the

There are several lines of evidence that argue in favor—or really against—humans. One of these is the event’s timing. The megafauna extinction, it’s now clear, did not take place all at once, as Lyell and Wallace believed it had. Rather, it occurred in pulses. The first pulse, about forty thousand years ago, took out Australia’s giants. A second pulse hit North America and South America some twenty-five thousand years later. Madagascar’s giant lemurs, pygmy hippos, and elephant birds survived all the way into the Middle Ages. New Zealand’s moas made it as far as the Renaissance.

It’s hard to see how such a sequence could be squared with a single climate change event. The sequence of the pulses and the sequence of human settlement, meanwhile, line up almost exactly.

Archaeological evidence shows that people arrived first in Australia, about fifty thousand years ago. Only much later did they reach the Americas, and only many thousands of years after t...

In a similar vein, Jared Diamond has observed: “Personally, I can’t fathom why Australia’s giants should have survived innumerable droughts in their tens of millions of years of Australian history, and then have chosen to drop dead almost simultaneously (at least on a time scale of millions of years) precisely and just coincidentally when the first humans arrived.”

How could small bands of technologically primitive people have wiped out so many large, strong, and in some cases fierce animals over an area the size of Australia or North America?

The Neanderthals lived in Europe for at least a hundred thousand years. For the most part, this was a time of cold, and for stretches, it was intensely cold, with ice sheets covering Scandinavia. It is believed, though it’s not known for certain, that, to protect themselves, the Neanderthals built shelters and fashioned some sort of clothing. Then, roughly thirty thousand years ago, the Neanderthals vanished.

Modern humans arrived in Europe around forty thousand years ago, and again and again, the archaeological record shows, as soon as they made their way to a region where Neanderthals were living, the Neanderthals in that region disappeared. Perhaps the Neanderthals were actively pursued, or perhaps they were just outcompeted. Either way, their decline fits the familiar pattern, with one important (and unsettling) difference. Before humans finally did in the Neanderthals, they had sex with them. As a result of this interaction, most people alive today are slightly—up to four percent—Neanderthal.

Five hundred thousand years ago, the dinosaurs had been dead for about sixty-five million years, so the whole Jurassic Park fantasy is, sadly, just that. On the other hand, five hundred thousand years ago modern humans did not yet exist.

For the past twenty-five years or so, the study of human evolution has been dominated by the theory known in the popular press as “Out of Africa” and in academic circles as the “recent single-origin” or “replacement” hypothesis. This theory holds that all modern humans are descended from a small population that lived in Africa roughly two hundred thousand years ago. Around a hundred and twenty thousand years ago, a subset of that population migrated into the Middle East, and from there, further subsets eventually pushed northwest in Europe, east into Asia, and all the way east to Australia. As they moved north and east, modern humans encountered Neanderthals and other so-called archaic humans, who already inhabited those regions. The modern humans “replaced” the archaic humans, which is a nice way of saying they drove them to extinction. This model of migration and

“replacement” implies that the relationship between Neanderthals and humans should be the same for all people alive today, regardless of where they come from.

never have seen two chimps carry something heavy together. They don’t have this kind of collaborative project.”

(During the last glaciation, when sea levels were so much lower than they are now, there was no English Channel to contend with.)

How many people must have sailed out and vanished on the Pacific before you found Easter Island? I mean, it’s ridiculous. And why do you do that? Is it for the glory? For immortality? For curiosity? And now we go to Mars. We never stop.”

condors

What this history reveals, in its ups and its downs, is that life is extremely resilient but not infinitely so.

continuous process”: George Gaylord Simpson, Why and How: Some Problems and Methods in Historical Biology (Oxford: Pergamon Press, 1980), 35. “codswallop”: Quoted in Browne, “Dinosaur Experts Resist Meteor Extinction Idea.” In 1984, grains of shocked quartz: B. F. Bohor et al., “Mineralogic Evidence for an Impact Event at the Cretaceous-Tertiary Boundary,” Science 224 (1984): 867-69. In a drawing that accompanies: Neil Landman et al., “Mode of Life and Habitat of Scaphitid Ammonites,” Geobios 54 (2012): 87-98. “Basically, if you were a triceratops”: Personal communication, Steve D’Hondt, Jan. 5, 2012. Birds were also hard-hit: Nicholas R. Longrich, T. Tokaryk, and D. J. Field, “Mass Extinction of Birds at the Cretaceous-Paleogene (K-Pg) Boundary,” Proceedings of the National Academy of Sciences 108 (2011): 15253-257. The same goes for lizards: Nicholas R. Longrich, Bhart-Anjan S. Bhullar, and Jacques A. Gauthier, “Mass Extinction of Lizards and Snakes at the Cretaceous-Paleogene Boundary,” Proceedings of the National Academy of Sciences 109 (2012): 21396-401. Mammals’ ranks, too, were devastated: Kenneth Rose, The Beginning of the Age of Mammals (Baltimore: Johns Hopkins University Press, 2006), 2. “the rules of the survival game”: Paul D. Taylor, Extinctions in the History of Life (Cambridge: Cambridge University Press, 2004), 2.