A single population of prehistoric Siberians crossed the Bering Strait into Alaska and subsequently fanned out to populate North and South America, according to a new genetic analysis of present-day indigenous Americans. The study also hints that early Americans reached Central and South America by migrating down the Pacific coast by land or sea and only later spread into the interior of South America.
     "We have good evidence that a single migration [from Siberia] contributed a large fraction of the ancestry of the Americas," says population geneticist Noah Rosenberg of the University of Michigan in Ann Arbor, who led the large international study team. The finding draws on the largest database of Native American genetics ever compiled. The data include DNA from nearly 500 people belonging to 29 groups scattered across Canada, Mexico, Central America, and South America. The researchers also studied samples from 14 Tundra Nentsi individuals living in eastern Siberia.
     The team examined 678 genetic markers in the human genome and found that one of the markers ties every Native American group to the Tundra Nentsi. The marker, moreover, is found nowhere else in the world. "It's extremely difficult to explain this kind of pattern unless all of the Native American populations have a large degree of shared ancestry," says Rosenberg. In addition, the Canadian groups share more genes with the Siberians than do the groups in Central and South America, Rosenberg and his team report online in the November PLoS Genetics.
     Tracing further migration through the Americas, the team then correlated genetic variations among different tribes with each group's location as measured along inland or coastal routes. The genetic data suggest that most migration to Central and South America followed the coast. "That's the easy way south," says Vance Holliday, an archaeologist at the University of Arizona in Tucson. He cautions, however, that the groups that populated the South American interior would have had to surmount the formidable Andes Mountains. Despite the migration findings, Holliday and Dillehay both say that southward migration along interior routes should still be considered.
     While the study points to an eastern Siberian origin for most of the genes that spread across the Americas, it can't rule out small genetic contributions from other groups, says Kari Britt Schroeder of the University of California, Davis. In 2001, scientists unearthed 8,000- to 11,000-year-old skulls in Brazil that strikingly resemble today's Australian aborigines. The find fueled speculation that several waves of immigrants from different parts of Asia reached the Americas. "Even if Native Americans share a lot of ancestry from a single origin, there still could be contributions from other groups," says Schroeder.

Sources: EurekAlert! (26 November 2007), Science News Online (1 December 2007)