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Mungo. Artifact-bearing soil layers beneath the burial could be as old as 50,000 years—the earliest evidence of modern humans far from Africa.

No physical trace of these people has been found along the 8,000 miles from Africa to Australia—all may have vanished as the sea rose after the Ice Age. But a genetic trace endures. A few indigenous groups on the Andaman Islands near Myanmar, in Malaysia, and in Papua New Guinea—as well as almost all Australian Aborigines—carry signs of an ancient mitochondrial lineage, a trail of genetic bread crumbs dropped by the early migrants.

People in the rest of Asia and Europe share different but equally ancient mtDNA and Y-chromosome lineages, marking them as descendants of the other, stalled branch of the African exodus. At first, rough terrain and the Ice Age climate blocked further progress. Europe, moreover, was a stronghold of the Neandertals, descendants of a much earlier migration of pre-modern humans out of Africa.

Finally, perhaps 40,000 years ago, modern humans advanced into the Neandertals' territory. Overlapping layers of Neandertal and early modern human artifacts at a cave in France suggest that the two kinds of humans could have met.

How these two peoples—the destined parvenu and the doomed caretaker of a continent—would have interacted is a potent mystery. Did they eye each other with wonder or in fear? Did they fight, socialize, or dismiss each other as alien beings?

All we know is that as modern humans and distinctly more sophisticated toolmaking spread into Europe, the once ubiquitous Neandertals were squeezed into ever shrinking pockets of habitation that eventually petered out completely. On current evidence, the two groups interbred rarely, if at all. Neither mtDNA from Neandertal fossils nor modern human DNA bears any trace of an ancient mingling of the bloodlines.

About the same time as modern humans pushed into Europe, some of the same group that had paused in the Middle East spread east into Central Asia. Following herds of game, skirting mountain ranges and deserts, they reached southern Siberia as early as 40,000 years ago. As populations diverged and became isolated, their genetic lineages likewise branched and rebranched. But the isolation was rarely if ever complete. "People have always met other people, found them attractive, and had children," says molecular anthropologist Theodore Schurr of the University of Pennsylvania.

Schurr's specialty is the peopling of the Americas—one of the last and most contentious chapters in the human story. The subject seems to attract fantastic theories (Native Americans are the descendants of the ancient Israelites or the lost civilization of Atlantis) as well as ones tinged with a political agenda. The "Caucasoid" features of a 9,500-year-old skull from Washington State called Kennewick Man, for instance, have been hailed as proof that the first Americans came from northern Europe.

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