To see the apocalypse firsthand, start at McKittrick Creek in Texas and hike up switchbacks, through juniper trees, along the Permian Reef trail. About 1,200 feet up are huge blocks of limestone formed from the discarded shells of an impressive variety of sea creatures—rice-like foraminifera, elaborate brachiopods that filtered seawater for bits of dead flesh, hundreds of varieties of snails. Hardly noticeable among this cornucopia of forms was Claraia, a dull scallop-like clam that wasn’t at all charismatic.
A couple hundred miles away, in Green River, Utah, the sediment corresponds to a few million years later in time. Here the incredible variety of forms is gone, replaced with a post-apocalyptic sameness. One particular kind of clam is abundant. Claraia had inherited the Earth.
What could have caused this crisis? It’s an open question. Figuring out what happened 251 million years ago—at the end of the Permian Period, when reptiles had come into their own on the giant continent of Pangea—and in what sequence, and with what cause and effect, is exceedingly difficult. The fossil record paints with a thick brush. But it seems that volcanoes in what are now the Siberian steppes were spewing lava, which was in turn vaporizing vast deposits of coal. Carbon concentrations went through the roof—much as they are doing now from human industrial activity. A surge in atmospheric carbon is a prime suspect for the Permian extinction, the biggest die-off in the 4-billion-year history of life on Earth. What does this bode for us?
Mass extinctions have happened five or six times (depending on how you count). It’s a kind of wiping the slate clean and starting anew. The death of the dinosaurs and the rise of mammals came from such an event 66 million years ago, when a meteorite fell in what is now the Yucatan. Many scientists believe that there may now be a new mass extinction event under way, caused by Homo sapiens.
Ever since humans fanned out from Africa and began to take over the world, many species have disappeared. The mammals that inhabited the Pleistocene until about 11,700 years ago—the wooly mammoths and the saber-toothed tigers—died most likely at the hands of human hunters. The passenger pigeon, which once numbered in the billions, is gone.* The rhino’s days appear to be numbered, as do the bluefin tuna’s, and so forth. The current rate of species loss, by some estimates, is 200 a day, but nobody knows with any precision. Whether this trajectory takes us to mass extinction is not something science can answer definitively at the moment.
If a mass extinction is happening, climate change would not have had much time to factor into it. Most of the species loss has so far has had little to do with pumping carbon into the atmosphere. Humans as a species have ravaged the Earth in many other ways. Fishing the waters with factory trawlers, clearing forests for wood and palm oil plantations, carrying strange flora and fauna in the bilge of ships from port to port—all these things, and more, have contributed.
In the near future, many potential triggers could lead to a cataclysm. The 20th century gave us nuclear bombs and weaponized smallpox. The 21st will surely deliver a greater variety of bioweapons. The prospect of a natural killer like the influenza virus adapting to a globalized world of 7 billion people is worrisome. The machines we have built our civilization upon—computers, software, networks—contain the seeds of destruction for the simple fact that we have come to depend on them, and they are vulnerable to manipulation. We are always figuring out new ways of bringing apocalypse on our heads. Even climate, which we tend to think of as a slowly unfolding crisis, could conceivably bite us sooner than we think. Some researchers think that weather patterns such as the ones that bring monsoons to India and sustain glaciers in Antarctica could behave like dynamical systems, prone to sudden, unpredictable, and dangerous changes.
It’s possible—perhaps likely—that any of these factors, or several acting at the same time, could cause a plunge in the human population in this century or the next. United Nations estimates have the world population, now 7 billion, rising to 10 billion by the end of the century and then leveling off. When we consider such estimates, we tend to make a questionable assumption: that the human population will behave like no other—that, after rising with breakneck speed, it will assume a steady state precisely at its peak. Ecologists will tell you that that is not usually the way it goes. Yeast cells that rapidly fill up their culture dish generally die off suddenly and in great numbers.
But extinction? That is a high bar. As bad as viruses or a revolt of the machines could be, surely some hardy remnant would survive and carry on. For Homo sapiens to go extinct—for every last man, woman, and child on the planet die, once and for all—it seems that something fundamental would have to give. The foundation of life on the planet is its geochemistry—its atmospheres, oceans, the elements that comprise them, the ground beneath them, and the relationship of this vast system to the sun. This is the stage upon which life plays out.
One of the interesting things about past mass extinctions is that they seem to happen over many millions of years. The exception, of course, is the one that doomed the dinosaurs, which basically occurred during one bad weekend with an asteroid. But some paleobiologists have recently whittled the Permian extinction down to a few tens of thousands of years, give or take. That puts it squarely on a human timescale. If we are indeed in the midst of a new mass extinction, one started by our own hand, when did the clock start ticking? Just under 200,000 years ago, when modern humans left Africa? Ten thousand years ago, when we started agriculture? And how much time do we have left?
Correction, Feb. 26, 2012: This article originally referred to the passenger pigeon as the carrier pigeon. (Return to the corrected sentence.)