EVOLUTION, THE MERRY PRANKSTER

Beyond Stones & Bones (Newsweek, March 13th, 2007)

Now the contentious part. In 2001, a team digging in Chad unearthed what it claimed was the oldest fossil of an ancestor of humans but not chimps. If so, it must have lived after the two lineages split. Trouble was, Sahelanthropus tchadensis (nicknamed Toumai, the local word for "child") lived close to 7 million years ago. The genetic data, pointing to a human-chimp split at least 1 million years later, suggest that Toumai is not the ur-hominid—the first creature ancestral only to human and not our chimp cousins—after all.

If Toumai is not our ancestor, what is he doing with such a humanlike face and teeth, which look like those of species 5 million years his junior? "A 7 million-year-old hominid should be just starting to look like a hominid, not have a trait you see so much later in the fossil record," says paleoanthropologist Bernard Wood of George Washington University. Even if he is not our ancestor, Toumai is valuable because he undermines the "begat" model of human evolution—that Toumai begat Australopithecus who begat Homo habilis who begat Homo erectus who begat Homo sapiens. That model assumes that each biological innovation, whether bipedality or a large brain or any other, evolved only once and stuck.

Instead, evolution played Mr. Potato Head, putting different combinations of features on ancient hominids then letting them vanish until a later species evolved them. "Similar traits evolved more than once, which means you can't use them as gold-plated evidence that one fossil is descended from another or that having an advanced trait means a fossil was a direct ancestor of modern humans," says Wood. "Lots of branches in the human family tree don't make it to the surface."