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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

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