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Human Journey
MARCH 2006
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Video: Written in Our DNA
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The Greatest Journey (continued)
By James Shreeve
Genetic trails left by our ancestors are leading scientists back across time in an epic discovery of human migration.

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Mitochondrial Eve was soon joined by "Y chromosome Adam," an analogous father of us all, also from Africa. Increasingly refined DNA studies have confirmed this opening chapter of our story over and over: All the variously shaped and shaded people of Earth trace their ancestry to African hunter-gatherers.

Looking more closely at DNA markers in Africa, scientists may have found traces of those founders. Ancestral DNA markers turn up most often among the San people of southern Africa and the Biaka Pygmies of central Africa, as well as in some East African tribes. The San and two of the East African tribes also speak languages that feature a repertoire of unique sounds, including clicks. Perhaps these far-flung people pay witness to an expansion of our earliest ancestors within Africa, like the fading ripples from a pebble dropped in a pond.

What seems virtually certain now is that at a remarkably recent date—probably between 50,000 and 70,000 years ago—one small wavelet from Africa lapped up onto the shores of western Asia. All non-Africans share markers carried by those first emigrants, who may have numbered just a thousand people.

Some archaeologists think the migration out of Africa marked a revolution in behavior that also included more sophisticated tools, wider social networks, and the first art and body ornaments. Perhaps some kind of neurological mutation had led to spoken language and made our ancestors fully modern, setting a small band of them on course to colonize the world. But other scientists see finely wrought tools and other traces of modern behavior scattered around Africa long before those first steps outside the continent. "It's not a 'revolution' if it took 200,000 years," says Alison Brooks of George Washington University.

Whatever tools and cognitive skills the emigrants packed with them, two paths lay open into Asia. One led up the Nile Valley, across the Sinai Peninsula, and north into the Levant. But another also beckoned. Seventy thousand years ago the Earth was entering the last ice age, and sea levels were sinking as water was locked up in glaciers. At its narrowest, the mouth of the Red Sea between the Horn of Africa and Arabia would have been only a few miles wide. Using primitive watercraft, modern humans could have crossed over while barely getting their feet wet.

Once in Asia, genetic evidence suggests, the population split. One group stalled temporarily in the Middle East, while the other followed the coast around the Arabian Peninsula, India, and beyond. Each generation may have pushed just a couple of miles farther.

"The movement was probably imperceptible," says Spencer Wells, who heads the National Geographic Society's Genographic Project, a global effort to refine the picture of early migrations (see page 70). "It was less of a journey and probably more like walking a little farther down the beach to get away from the crowd."

Over the millennia, a few steps a year and a few hops by boat added up. The wanderers had reached southeastern Australia by 45,000 years ago, when a man was buried at a site called Lake

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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