Despite the protests of creationists and their intellectual allies, and such efforts as the Kansas school board's vote last week to expunge evolution from the school curriculum, science has long taught that human beings are just another kind of animal, but most of the time this seems like a technicality. It's not just the obvious differences--language, civilization, technology--that set us apart. Even basic biology suggests that humanity has special status. Virtually every other type of animal comes in multiple varieties: dozens of species of monkeys, antelopes, whales and hawks walk, swim or fly the earth, to say nothing of beetles, whose hundreds of thousands of species inspired biologist J.B.S. Haldane's famous quip that God must have had "an inordinate fondness" for them. Even our closest kin, the great apes, fall into four species, divided into several subspecies.
But there's now only one species of human on the planet, and in the simplified view of evolution most of us have, that's all there has ever been. A few million years ago, most of us think, the half-ape known as Lucy appeared in Africa; eventually she begat a less apelike creature, who evolved in turn into something even more humanlike. Finally, after a few more begettings, Homo sapiens appeared. Except for that odd side branch known as the Neanderthals, the path from proto-apes to modern humans is commonly seen as a succession of new and improved species taking the place of worn-out evolutionary clunkers.
It's a satisfying, if slightly chauvinistic tale, but experts in human evolution have known for years that it is dead wrong. The evolution of a successful animal species almost always involves trial and error, false starts and failed experiments. "Humans are no exception to this," says anthropologist Ian Tattersall of the American Museum of Natural History in New York City, "no matter what we like to think."
True, we're descended from a creature that split off from the apes millions of years ago. But subsequent events were hardly a steady march from primitivism to perfection. Human evolution more nearly resembled an elimination tournament. At just about any given moment in prehistory, our family tree included several species of hominids--erect, upright-walking primates. All were competitors in an evolutionary struggle from which only one would ultimately emerge. Then came yet another flowering of species that would compete for survival. Neanderthals simply represented the most recent version of that contest. And while we'd find it bizarre to share our world with another human species, the fact that we've been alone since the Neanderthals vanished some 30,000 years ago is an evolutionary aberration.
The notion that multiple human species are the norm, not the exception, has only got stronger with a series of major scientific discoveries. Since 1994, four new species of hominid have been added to the human family tree, with the latest announced just a few months ago. These date from 800,000 years ago all the way back to 4.4 million years B.P. (before the present).