A detailed genetic analysis has settled the question of how and when the first Americans arrived in the continent.
Scientists have found that Native American populations from Canada to the southern tip of Chile arrived in at least three waves.
Most are descended entirely from a single group of migrants that crossed over through Beringia, a land bridge between Asia and America that existed during the ice ages, more than 15,000 years ago.
"For years it has been contentious whether the settlement of the Americas occurred by means of a single or multiple migrations from Siberia," says Professor Andres Ruiz-Linares of University College London.
"But our research settles this debate: Native Americans do not stem from a single migration. Our study also begins to cast light on patterns of human dispersal within the Americas."
The team studied more than 300,000 specific DNA sequence variations, Single Nucleotide Polymorphisms, in 52 Native American and 17 Siberian groups, examining theirgenetic similarities and differences.
They found that the second and third migrations have left an impact only in Arctic populations that speak Eskimo-Aleut languages, and in the Canadian Chipewyan who speak a Na-Dene language.
However, even these populations have inherited most of their genome from the first migration. Eskimo-Aleut speakers derive more than 50 percent of their DNA from these First Americans, and the Chipewyan around 90 percent.
Once in the Americas, people expanded southward, sticking to the coast with populations splitting off along the way.
"There are at least three deep lineages in Native American populations," says David Reich, professor of genetics at Harvard Medical School.
"The Asian lineage leading to First Americans is the most anciently diverged, whereas the Asian lineages that contributed some of the DNA to Eskimo-Aleut speakers and the Na-Dene-speaking Chipewyan from Canada are more closely related to present-day East Asian populations."