How Life Ends and Begins Again

Darwin didn’t think all that much of extinctions. He certainly knew about the work of naturalists like Cuvier, who argued that catastrophes had punctuated life’s history, each one clearing the world for a new set of creatures to take its place. But Darwin, so impressed by Lyell’s vision of gradual change, thought that the idea was hopelessly old‑fashioned. “The old notion that all the inhabitants of the Earth having been swept away by catastrophes at successive periods is generally given up,” he claimed in Origin of Species.

For Darwin, extinction was simply the exit that losers took out of the evolutionary arena. They did not leave in great stampedes; they only trickled away, as individual species were gradually outcompeted to oblivion. The fossil record might suggest that many species had gone extinct at once, but with countless fossils still undiscovered, it could well be misleading. In the future, as paleontologists dug up more fossils, Darwin was sure that these seeming catastrophes would dissolve into a smooth, gentle continuum of extinctions.

Paleontologists since Darwin’s time have indeed found many more fossils, as he would have hoped, and they’ve even been able to pin down their ages with great precision. Yet all this new information has shown that Darwin was wrong about extinctions. Catastrophic waves of extinctions are a reality. They have ripped through the fabric of life, destroying as many as 90 percent of all species on Earth in a geological instant. The suspects behind these mass extinctions are many, including volcanoes, asteroids, and sudden changes to the oceans and the atmosphere. All of these culprits seem to have put life under worldwide stress; once that stress passes a certain threshold, entire ecosystems collapse like a house of cards. And once mass extinctions strike, it takes millions of years for life to recover its former diversity. In the wake of mass extinctions life can change for good. They can wipe out old dominant forms and let new ones take their place. In fact, we may owe our own success to such shifts of fortune.

It also looks as if we are now entering another period of mass extinctions. But for the first time in the history of the planet, a single species–ourselves–is an agent of destruction. The overture of this extinction began thousands of years ago as humans arrived in Australia and other continents for the first time and hunted down the biggest native animals. But in the past few centuries the tempo of extinctions has been accelerating as humans have come to dominate the planet, destroying tropical forests and introducing alien invaders that are outcompeting native life. In the coming century, humans may even raise the temperature of the planet, putting more stress on species that are already on the brink of extinction. According to some estimates, more than half of the world’s species will disappear in the next 100 years.

Mass extinctions remain one of evolution’s great mysteries, and from the hills of northern Italy to the deserts of South Africa paleontologists are struggling to understand their role in the history of life. Their work is not merely academic. It may be able to show the direction in which humanity is steering the course of evolution. And that knowledge makes this period of mass extinctions different from past ones in another way: not only is this extinction pulse being caused by a single species, but by a species that is capable of understanding and controlling its own fate.

The Great Curve

The rough outlines of the history of extinctions became clear by the 1840s. As geologists surveyed formations, they often found that a given fossil species was limited to a particular layer of rock. Hundreds of miles away, the same fossils appeared in another span of rock layers. The geologists began to tie the rocks of the world into a single stratigraphy: a unified grand history of life. In the 1840s an English naturalist named John Phillips recognized that the fossils documented three great eras: the Paleozoic, the Mesozoic, and the Cenozoic (“ancient life,” “middle life,” and “early life,” respectively). According to Phillips, these three eras were divided by great extinctions. He drew his argument on a piece of paper. The diversity of life rose from nothing at the beginning of the Paleozoic, dipping and rising from time to time before plunging at the end of the era. As the Mesozoic era began, life took another steep climb and then plummeted again at the boundary with the Cenozoic. The dominant life‑forms of each era dwindled during the extinctions, a new menagerie appearing in their wake.

The curve that Phillips drew was correct but crude, like the outline of a distant‑mountain range shrouded in fog. Now, more than 150 years later, much of the fog has lifted. Geologists have unified the world’s exposed rocks into a single record. They have found cliffs and outcrops where they can touch the place where the rocks of one era give way to the rocks of the next. They have looked at the atomic clocks ticking away inside those cliffs and outcrops and can put precise dates on their ages. They’ve loaded computers with vast databases of the planet’s fossils. And, remarkably, Phillips’s curve has survived relatively intact.

The latest version of the curve is the work of a number of paleontologists, but most importantly of the late John Sepkoski of the University of Chicago. Sepkoski spent decades tallying the duration of ocean‑dwelling species, which leave the best fossil record. The curve starts about 600 million years ago at the beginning of the Cambrian, when for the first time there are enough fossils to get a reliable picture of extinctions, and runs up to today. Along the vertical axis is the number of genera of marine animals that existed at any point.

Over much of the past 600 million years, life experienced a steady, low level of extinctions. These “background” extinctions correspond to the gradual disappearance of species that Darwin proposed. Most species live between 1 and 10 million years, and new species come into existence at roughly the same rate as older species disappear. In normal times the diversity of life is like a field full of fireflies, each flash representing a species. At any moment some fireflies are starting their flashes, while others are winking out. Yet the overall number of flashes holds roughly steady.

Now imagine that half of the fireflies in a field wink off at once. The field darkens, and the missing fireflies don’t start glowing again for an hour. Something like this has happened a few times since the Cambrian period. Background extinctions have suddenly turned into mass extinctions. They have struck the ocean and land every few dozen million years, with five catastrophes standing out in particular. In each of these five mass extinctions, more than half of all species disappeared.

The wrath of these extinctions is beyond anything Darwin imagined. Just as new forms of life may come into being in ways Darwin never could have conceived, the ways they have vanished would have surprised him too. Extinction is the dark counterpart to evolution’s creativity. Conditions on this planet can change so suddenly that natural selection cannot help a species adapt and survive. A pulse of extinctions can even reroute the history of life onto a new set of tracks.