Americas saw three waves of ancient settlers

WHO were the first Americans? The most comprehensive analysis of DNA from Native American populations to date supports the controversial theory that the Americas were populated in three initial waves rather than just one.

The first settlers are thought to have set foot in North America some 15,000 years ago, crossing a land bridge between Siberia and Alaska that existed at the time.

Linguists originally suggested that Native Americans could be divided into three groups based on their languages, and argued that these may represent three waves of migration. Although differences in dental morphology between people belonging to the language groups supported the idea, not all linguists agreed about lumping so many diverse languages together.

Analysis of mitochondrial and Y-chromosome DNA suggested instead that migration occurred in a single wave.

To get a clearer picture of the settlement patterns, David Reich of Harvard Medical School in Boston and his colleagues compared DNA from 52 Native American populations across Canada, Greenland and Central and Southern America, focusing on variations called single-nucleotide polymorphisms (SNPs) in protein-coding and non-coding regions across the genome. They also examined DNA from populations in Siberia, where Native Americans are thought to have originated. Unlike mitochondrial or Y-chromosome sequencing, which trace the history of a single male or female ancestor, SNP analysis paints a broader picture of ancestry.

"What's striking is that populations from the northern parts of Canada to the southern parts of South America are consistent with descending from a single stream of migration from Asia," says Reich. These migrants were probably the first Americans.

But that's not the whole story. The SNP analysis also shows there were two further waves of Asian migration, whose populations interbred with the original settlers. Speakers of Eskimo-Aleut languages, found in the Aleutian Islands and Greenland, inherit almost half their genes from the second wave, while the Na-Dene-speaking Chipewyan in central Canada inherit around a tenth of their genes from a third wave - although all groups can claim to have "first American" DNA (Nature, DOI: 10.1038/nature11258).

This picture fits with the original classifications made by the linguists. "It doesn't mean that most Native American languages are related to each other, it just means that the populations that speak those languages descend from a single ancestral group," says Reich.

The study also sheds light on how the first Americans dispersed through the continent; Reich found that populations along the coast showed far more genetic diversity than those living in the interior of the continent. This suggests there was a coastal migration route that required some kind of seafaring facility, says Silvia Gonzalez of Liverpool John Moores University in the UK, who investigates early human footprints in the Americas.

What the new study can't tell us, however, is when these migrations occurred. To do this we would need to compare DNA from present populations with genetic material from ancient human remains - a political hot potato in some regions, including the US (see "The legal fight over prehistory").

The legal fight over prehistory

Working out how and when America was colonised is made harder by a lack of genetic data from the US. The absence is the result of tensions between native groups and the scientists who want to study them.

The study by David Reich of Harvard Medical School in Boston and colleagues (see main text) relied on existing genetic samples from Native American groups. Reich sought to confirm that the person each sample came from had given full consent. US samples were poorly documented, so he left them out. "We were trying to be extra-specially careful," he says.

Previous researchers were less rigorous. In 2010, Arizona State University agreed to pay $700,000 to a Native American tribe after samples collected to study diabetes were used to probe the tribe's history.

Fossils can be just as contentious. When a skeleton was found in Kennewick, Washington, in 1996, it quickly became the focus of a battle between scientists and Native Americans. Kennewick Man was only released to scientists in 2005, after the tribe failed to prove that the skeleton was one of their ancestors.

History is now repeating itself. Two Californian skeletons are the focus of a legal battle between the Kumeyaay tribe and three anthropologists.

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