He and his colleagues looked at 938 individuals from 51 different populations whose DNA is in a repository in France. The group lead by Rosenberg and Andrew Singleton, of the University of Virginia, studied 485 people from the same collection.
With such diverse and abundant starting material, the researchers were able to sketch a picture of ethnicity far more detailed than previously known. For example, Africa’s surviving hunter-gatherers—two groups of pygmies and the San people of southern Africa who were formerly known as Bushmen—are closely related to each other, and quite distinct on a genetic basis from all other black Africans. The Hazara of Afghanistan and Pakistan and the Uighur of northwestern China are also close genetic relatives, despite living far apart. On the other hand, China’s dominant ethnicity is actually two genetically distinct groups, the northern and the southern Han.

The research shows that populations’ genetic footprints on the planet are deep, sharp, and not easily covered over by time. Both research teams using the French DNA collection found geographic distance from East Africa is a major determinant of genetic differences between groups. “Each group (migrating across the planet) carried only a subset of the genetic variation from its ancestral population. So there is a loss of genetic diversity with the distance from Africa,” Rosenberg said.

One of the more interesting consequences of that pattern is the subject of the third study, also published in Nature. Carlos Bustamante, of Cornell University, and his colleagues measured SNPs in 20 European Americans and 15 African Americans. They found that the average person carries at least 2,000 SNPs that change the meaning of a genetic “word.” However, in European Americans, a larger proportion of those changes were likely to be unhealthy or unfavourable.

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