First migrants came in three distinct waves that interbred once in the New World

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About 15,000 to 18,000 years ago, the first migrant wave spilled from Asia down the Pacific coast and then pushed inland, eventually peopling the land from “the tip of South America all the way to Hudson Bay,” says Andrew Kitchen, a genetic anthropologist at the University of Iowa who was not involved in the new research. That first migrant wave contained the ancestors of all South and Central American tribes, and North Americans, too. But something different was going on in North America, an international team of researchers has discovered....

The analysis, published August 12 in the Proceedings of the National Academy of Sciences, supports the widely accepted notion of an initial coastal migration wave. A second wave of migration probably left Siberia only a couple thousand years after the first wave. Instead of trickling down the coast, the second group slipped through an ice-free corridor running from Alaska into what is now southern Canada, the team found. The second wave never made it south of the present-day United States.

The mixture of first-wave and second-wave genetic signatures in some Native Americans today indicates that the newcomers and existing populations interbred.

A third wave of migration started around 4,000 years ago in Alaska and swept mostly eastward across Canada.

Previous studies of human migration into the Americas have sometimes focused on two types of languages that emerged among the tribes: the Na-Dene language family, including Navajo, Apache and Tlingit, and non-Na-Dene languages, including Algonquin, Ojibwe and Chippewa. Scientists had thought the language groups reflected genetic separation, with the second wave being restricted to the Na-Dene language family. But Torroni and his colleagues discovered that second-wave genetic marks occurred in people who spoke languages from both groups...