A development company controlled by osama bin laden's half brother once advanced the idea that it wanted to build a bridge that would 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 will pass hundreds of feet above the 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—may have crossed the strait in tiny boats, never to return. That is just one theory for how modern humans first left Africa. An alternative explanation to a journey over water has these sojourners moving up the coast and through the Sinai Peninsula.
Whether the exit route was over water or land, the reason these travelers 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 appear to have 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.
Scientists, of course, have gained insight into these wanderings because of the fossilized bones or spearheads laboriously uncovered and stored in collections. But ancestral hand-me-downs are often too scant to provide a complete picture of this remote history. In the past 25 years population geneticists have begun to fill in gaps in the paleoanthropological record by fashioning a genetic bread-crumb trail of the earliest migrations by modern humans.
Almost all our DNA—99.9 percent of the three billion “letters,” or nucleotides, that make up the human genome—is the same from person to person. But interwoven in that last 0.1 percent are telltale differences. A comparison among, say, East Africans and Native Americans can yield vital clues to human ancestry and to the inexorable progression of colonizations from continent to continent. Until recent years, DNA passed down only from fathers to sons or from mothers to their children has served as the equivalent of fossilized footprints for geneticists. The newest research lets scientists adjust their focus, widening the field of view beyond a few isolated stretches of DNA to inspect hundreds of thousands of nucleotides scattered throughout the whole genome.
Scanning broadly has produced global migratory maps of unprecedented resolution, some of which have been published only during recent years. The research provides an endorsement of modern human origins in Africa and shows how that continent served as a reservoir of genetic diversity that trickled out to the rest of the world. A genetic family tree that begins with the San people of Africa at its root ends with South American Indians and with Pacific Islanders on its youngest-growing branches.
The study of human genetic variation—a kind of historical Global Positioning System—goes back to World War I, when two physicians working in the Greek city of Thessaloníki found that soldiers garrisoned there had a differing incidence of a given blood group depending on their nationality. Beginning in the 1950s, Luigi Luca Cavalli-Sforza started formalizing the study of genetic differences among populations by examining distinct blood group proteins. Variations in proteins reflect differences in the genes that encode them.
Then, in 1987, Rebecca L. Cann and Allan C. Wilson, then at the University of California, Berkeley, published a groundbreaking paper based on analyzing the DNA of mitochondria, the cell's energy-producing organelles, which are passed down through the maternal line. They reported that humans from different populations all descended from a single female in Africa who lived about 200,000 years ago—a finding that immediately made headlines trumpeting the discovery of the “Mitochondrial Eve.” (Despite the Biblical allusion, this Eve was not the first woman: her lineage, though, is all that has survived.)