Beyond Stones & Bones

Paleoneurology promises to do what simplistic studies of ancient brains—which asked only how big they were—could not: explain our ancestors' great leaps forward. About 2.5 million years ago a new genus, Homo habilis, appeared in Africa. Discovered by the legendary Louis and Mary Leakey, habilis was the first hominid with a brain bigger than a chimp's, and was the first toolmaker: stone tools—sharp flakes of rock—appeared when habilis did. Their direct descendant, Homo erectus, took an equally momentous step: venturing beyond Africa. In the Republic of Georgia at a site called Dmanisi, scientists have unearthed 1.8 million-year-old fossils of erectus, "the first outpost we know of beyond Africa," says G. Philip Rightmire of Binghamton University. "It looks like these people got out and materialized everywhere in Eurasia," showing up as Java man and Peking man, among others. (None of the original fossils of Peking man survived World War II. Packed for shipment to the United States for safekeeping, they disappeared in transit; only casts remain.) Ancient humans didn't just walk: they reached Australia 60,000 years ago, across miles of open ocean.

Erectus shows that brain size is too crude a measure of a species' talents. At Dmanisi, the brains range from 600 to 770 cubic centimeters, comparable to the more primitive habilis. But while erectus did not distinguish themselves in brain size, brain structure is more telling. They were the first of our ancestors to have an asymmetric brain, as modern humans do; Australopithecus species do not. Asymmetry is a mark of increasing specialization and therefore complex cognitive ability. Erectus used it to, among other things, discover and tame fire. What they did not use it for is technology. Tools found with the Dmanisi fossils include cutting flakes, rock "cores" from which flakes were made and a chopper, all primitive even for their time. "The old idea that you needed a master's degree in stone tools to leave Africa is crazy," says Bernard Wood.

Although erectus spread across Eurasia between 2 million and 1 million years ago, DNA makes clear that the species was almost certainly a dead end and not our ancestor, as some scientists had argued. According to this idea, groups of erectus scattered across the Old World all accrued the same mutations and underwent the same natural selection that led to Homo sapiens. The Y chromosome begs to differ. The Y is passed intact from father to son; in that sense, it's like a last name and so can be used to trace ancestries. But like surnames that got Anglicized at Ellis Island, sometimes a Y changes, with the altered version being passed to all male descendants. Peter Underhill, a molecular anthropologist at Stanford University, tracked 160 such changes in the Y's of 1,062 men from 21 populations across the world. Applying the molecular-clock technique, he concludes that the most recent common ancestor of all men alive today lived 89,000 years ago in Africa. The first modern humans—and therefore, unlike the earlier wave of Homo erectus into Asia a million years ago, the ancestors of everyone today—departed Africa about 66,000 years ago.

These pilgrims were strikingly few. From the amount of variation in Y chromosomes today, population geneticists infer how many individuals were in this "founder" population. The best estimate: 2,000 men. Assuming an equal number of women, only 4,000 brave souls ventured forth from Africa. We are their descendants.

A curious thing about early Homo species is that they looked quite human early on. "By 600,000 years ago everyone had a big brain, and by 200,000 years ago people in Africa looked like modern humans," says archeologist Richard Klein of Stanford. "But there was no representational art, no figurines, no jewelry until 50,000 years ago. Some kind of cognitive advance was required, probably in language or working memory. But since size hardly changed, the brain change that produced behaviorally modern humans must have been in structure."

The source of such structural changes must come, like every aspect of our physiology, from genes. Combing the genome for genes that emerged just when language, art, culture and other products of higher intelligence did, researchers have found three with the right timing.