The Sixth Extinction: An Unnatural History

by Elizabeth Kolbert

“Are We in the Midst of the Sixth Mass Extinction? A View from the World of Amphibians.” The authors, David Wake, of the University of California-Berkeley, and Vance Vredenburg, of San Francisco State,

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.”

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.

Then how does (Jefferson) account for the 5 mass extinctions that preceded? Or the 6th we are currently in — the Anthropocene — which came after Jefferson’s time? Of course, Jefferson died in July 1826. In 1859 Charles Darwin’s book “On the Origin of Species” was published; espousing a controversial theory about the origins of all species, including Man, that would set the world spinning in a whole other direction concerning science, religion and theology, and critical thinking about either.

“In science, sometimes it’s better to be lucky than smart,”

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?

The reason this book is being written by a hairy biped, rather than a scaly one, has more to do with dinosaurian misfortune than with any particular mammalian virtue.

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

(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.

The International Commission on Stratigraphy, or ICS, is the group responsible for maintaining the official timetable of earth’s history.

the Anthropocene Working Group,

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.

One of the defining features of the Anthropocene is that the world is changing in ways that compel species to move, and another is that it’s changing in ways that create barriers—roads, clear-cuts, cities—that prevent them from doing so.

If, on the other hand, people were to blame—and it seems increasingly likely that they were—then the import is almost more disturbing. It would mean that the current extinction event began all the way back in the middle of the last ice age. It would mean that man was a killer—to use the term of art an “overkiller”—pretty much right from the start.

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.

“When the chronology of extinction is critically set against the chronology of human migrations,” Paul Martin of the University of Arizona wrote in “Prehistoric Overkill,” his seminal paper on the subject, “man’s arrival emerges as the only reasonable answer” to the megafauna’s disappearance.

Though it might be nice to imagine there once was a time when man lived in harmony with nature, it’s not clear that he ever really did.

Neanderthals were extremely similar to modern humans; probably they were our very closest relatives. And yet clearly they were not humans. Somewhere in our DNA must lie the key mutation (or, more probably, mutations) that set us apart—the mutations that make us the sort of creature that could wipe out its nearest relative, then dig up its bones and reassemble its genome.

The same stretch of chromosome 5 from the human, Neanderthal, and chimp genomes.

With the capacity to represent the world in signs and symbols comes the capacity to change it, which, as it happens, is also the capacity to destroy it. A tiny set of genetic variations divides us from the Neanderthals, but that has made all the difference.

DOES it have to end this way? Does the last best hope for the world’s most magnificent creatures—or, for that matter, its least magnificent ones—really lie in pools of liquid nitrogen? Having been alerted to the ways in which we’re imperiling other species, can’t we take action to protect them? Isn’t the whole point of trying to peer into the future so that, seeing dangers ahead, we can change course to avoid them?

Fair questions, all.

Alfred Newton

John Muir

Congress in 1974 passed the Endangered Species Act.

World Wildlife Fund, the National Wildlife Federation, Defenders of Wildlife, the Wildlife Conservation Society, the African Wildlife Foundation, the Nature Conservancy, and Conservation International.

What I’ve been trying to do is trace an extinction event—call it the Holocene extinction, or the Anthropocene extinction, or, if you prefer the sound of it, the Sixth Extinction—and to place this event in the broader context of life’s history. That history is neither strictly uniformitarian nor catastrophist; rather, it is a hybrid of the two. What this history reveals, in its ups and its downs, is that life is extremely resilient but not infinitely so. There have been very long uneventful stretches and very, very occasionally “revolutions on the surface of the earth.”

To argue that the current extinction event could be averted if people just cared more and were willing to make more sacrifices is not wrong, exactly; still, it misses the point. It doesn’t much matter whether people care or don’t care. What matters is that people change the world.

Stanford ecologist Paul Ehrlich: IN PUSHING OTHER SPECIES TO EXTINCTION, HUMANITY IS BUSY SAWING OFF THE LIMB ON WHICH IT PERCHES.

Right now, in the amazing moment that to us counts as the present, we are deciding, without quite meaning to, which evolutionary pathways will remain open and which will forever be closed. No other creature has ever managed this, and it will, unfortunately, be our most enduring legacy. The Sixth Extinction will continue to determine the course of life long after everything people have written and painted and built has been ground into dust and giant rats have—or have not—inherited the earth.