New genetic evidence suggests people first settled the Americas in not one, but three waves of migration from Siberia, fleshing out the mysterious prehistory of the New World, scientists reported Wednesday in Nature.

In the most comprehensive analysis so far on the subject, an international research team led by geneticist David Reich at Harvard Medical School compared thousands of genetic variations among 52 contemporary Native American populations to 17 Siberian groups, seeking traces of their common ancestry hundreds of...