Beyond Stones & Bones

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."

In fact, starting 4 million years ago half a dozen hominids belonging to the genus Australopithecus called Africa home. Best-known for the fossil named Lucy, which was discovered in 1974, Australopithecus afarensis had apelike features such as a large jaw and jutting face, and probably scrambled up trees for safety and shelter. But she also strode the grasslands erect, a hallmark of modern humans. Footprints preserved in volcanic ash 3.6 million years old are mute testimony to how one larger afarensis and a smaller companion—woman and mate, or parent and child—walked across a plain in what is now Tanzania.

What triggered this abrupt change—what set us on the road to becoming fully human—has long stumped experts. Where stones and bones were of little help, however, genes and brains have begun to speak. Last summer scientists discovered a gene called HAR1 (for human accelerated region) that is present in animals from chickens to chimps to people. It had changed in only two of its 118 chemical "letters" from 310 million years ago (when the lineages of chickens and chimps split) to 5 million years ago. But 18 letters changed in the (relative) blink of an eye since the human lineage split from chimps', Katherine Pollard of the University of California, Davis, and colleagues reported. That high rate of change is a sign of a gene whose evolution keeps conferring advantages on those who carry it, perhaps starting with Australopithecus.

The brain, more than any other organ, may have reaped those genetic advantages. HAR1 reaches a peak of activity from the seventh to ninth week of gestation in humans, apparently spurring brain growth. And it is plentiful in cells that create the six layers of neurons in the human cortex. "HAR1 is present in neurons that play a role in the geometry and layout of the cortex," says Pollard. It likely helped the cortexes of our ancestors develop the elaborate folds characteristic of a complex brain.