Key Concepts

• Scientists trace the path of human migrations by using bones, artifacts and DNA. Ancient objects, however, are hard to find.
• DNA from contemporary humans can be compared to determine how long an indigenous population has lived in a region.
• The latest studies survey swathes of entire genomes and produce maps of human movements across much of the world. They also describe how people’s genes have adapted to changes in diet, climate and disease.

A development company controlled by Osama bin Laden’s half brother revealed last year that it wants to build a bridge that will span the Bab el Mandeb, the outlet of the Red Sea to the Indian Ocean. If this ambitious project is ever realized, the throngs of African pilgrims who traverse one of the longest bridges in the world on a journey to Mecca would pass hundreds of feet above the probable route of the most memorable journey in human history. Fifty or sixty thousand years ago a small band of Africans—a few hundred or even several thousand—crossed the strait in tiny boats, never to return.

The reason they left their homeland in eastern Africa is not completely understood. Perhaps the climate changed, or once abundant shellfish stocks vanished. But some things are fairly certain. Those first trekkers out of Africa brought with them the physical and behavioral traits—the large brains and the capacity for language—that characterize fully modern humans. From their bivouac on the Asian continent in what is now Yemen, they set out on a decamillennial journey that spanned continents and land bridges and reached all the way to Tierra del Fuego, at the bottom of South America.

   
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