Over the past half-billion years, there have been at least twenty mass extinctions, when the diversity of life on earth has suddenly and dramatically contracted. Five of these—the so-called Big Five—were so devastating that they are usually put in their own category. The first took place during the late Ordovician period, nearly four hundred and fifty million years ago, when life was still confined mainly to water. Geological records indicate that more than eighty per cent of marine species died out. The fifth occurred at the end of the Cretaceous period, sixty-five million years ago. The end-Cretaceous event exterminated not just the dinosaurs but seventy-five per cent of all species on earth.
The significance of mass extinctions goes beyond the sheer number of organisms involved. In contrast to ordinary, or so-called background, extinctions, which claim species that, for one reason or another, have become unfit, mass extinctions strike down the fit and the unfit at once. For example, brachiopods, which look like clams but have an entirely different anatomy, dominated the ocean floor for hundreds of millions of years. In the third of the Big Five extinctions—the end-Permian—the hugely successful brachiopods were nearly wiped out, along with trilobites, blastoids, and eurypterids. (In the end-Permian event, more than ninety per cent of marine species and seventy per cent of terrestrial species vanished; the event is sometimes referred to as “the mother of mass extinctions” or “the great dying.”)
Once a mass extinction occurs, it takes millions of years for life to recover, and when it does it generally has a new cast of characters; following the end-Cretaceous event, mammals rose up (or crept out) to replace the departed dinosaurs. In this way, mass extinctions, though missing from the original theory of evolution, have played a determining role in evolution’s course; as Richard Leakey has put it, such events “restructure the biosphere” and so “create the pattern of life.” It is now generally agreed among biologists that another mass extinction is under way. Though it’s difficult to put a precise figure on the losses, it is estimated that, if current trends continue, by the end of this century as many as half of earth’s species will be gone.
50,000 years ago in Australia:
It is difficult to say when, exactly, the current extinction event—sometimes called the sixth extinction—began. What might be thought of as its opening phase appears to have started about fifty thousand years ago. At that time, Australia was home to a fantastic assortment of enormous animals; these included a wombatlike creature the size of a hippo, a land tortoise nearly as big as a VW Beetle, and the giant short-faced kangaroo, which grew to be ten feet tall. Then all of the continent’s largest animals disappeared. Every species of marsupial weighing more than two hundred pounds—there were nineteen of them—vanished, as did three species of giant reptiles and a flightless bird with stumpy legs known as Genyornis newtoni.
This die-off roughly coincided with the arrival of the first people on the continent, probably from Southeast Asia. Australia is a big place, and there couldn’t have been very many early settlers. For a long time, the coincidence was discounted. Yet, thanks to recent work by geologists and paleontologists, a clear global pattern has emerged. About eleven thousand years ago, three-quarters of North America’s largest animals—among them mastodons, mammoths, giant beavers, short-faced bears, and sabre-toothed tigers—began to go extinct. This is right around the time the first humans are believed to have wandered onto the continent across the Bering land bridge. In relatively short order, the first humans settled South America as well. Subsequently, more than thirty species of South American “megamammals,” including elephant-size ground sloths and rhino-like creatures known as toxodons, died out.
And what goes for Australia and the Americas also goes for many other parts of the world. Humans settled Madagascar around two thousand years ago; the island subsequently lost all mammals weighing more than twenty pounds, including pygmy hippos and giant lemurs. “Substantial losses have occurred throughout near time,” Ross MacPhee, a curator at the American Museum of Natural History, in New York, and an expert on extinctions of the recent geological past, has written. “In the majority of cases, these losses occurred when, and only when, people began to expand across areas that had never before experienced their presence.” The Maori arrived in New Zealand around eight hundred years ago. They encountered eleven species of moas—huge ostrichlike creatures without wings. Within a few centuries—and possibly within a single century—all eleven moa species were gone. While these “first contact” extinctions were most pronounced among large animals, they were not confined to them. Humans discovered the Hawaiian Islands around fifteen hundred years ago; soon afterward, ninety per cent of Hawaii’s native bird species disappeared.
“We expect extinction after people arrive on an island,” David Steadman, the curator of ornithology at the Florida Museum of Natural History, has written. “Survival is the exception.”
BTW, The New Yorker has opened its archives for three months; after that, it all goes beyhind a pay wall. Slate has a list of must-read articles, which is where I found this one.