Published online 8 January 2009 | Nature | doi:10.1038/news.2009.7

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Earliest Americans took two paths

Genetic analysis suggests there were at least two migrations into the Americas.

Native AmericanDid Native Americans take different paths into the Americas?Steve Bly / Alamy

The ancestors of Native Americans took at least two different paths into the Americas, a new genetic analysis suggests. Although both groups travelled across the Bering land bridge, which connected Asia and North America during the last ice age, the migrants later took divergent paths: one along the Pacific coast, and the other following a route that lead them east of the Rocky Mountains.

The new results counter the notion, recently growing in popularity and support, that the first Americans derived from a single founder population that moved along the deglaciated Pacific coast.

For example, a recent analysis of genetic markers in 422 Native Americans from across the Americas concluded that there was probably only a single major migration into the region1. And another study found that all Native Americans share a unique genetic marker that is not present in other lineages, suggesting that Native Americans derived from a single, distinct founder population2.

Geneticist Antonio Torroni, of the University of Pavia in Italy, says he also subscribed to that hypothesis. But then he and his collaborators analysed full genome sequences from mitochondria — energy-producing structures in the cell that carry their own, maternally-inherited genetic information. "We started to get data at a higher resolution," he says, "and then we began to see things that were not visible before."

The rare lineages

Torroni and his colleagues looked at 69 mitochondrial genome sequences, most of which came from members of two rare Native American lineages, called D4h3 and X2a. They compared these sequences with published sequences from other Native American populations and, based on the accumulation of changes in the DNA sequences, concluded that most had existed as distinct groups since about 15,000–17,000 years ago, close to the expected arrival date of the first Americans.

The researchers also looked at where members of the two genetic lines are found. D4h3 members are found along the Pacific coast — a 10,300-year-old skeleton found in Alaska has also been identified as part of this group. But X2a is found only in northern North America, and no samples were obtained south of the United States. Most X2a samples were found near the Great Lakes.

That distribution, says Torroni, suggests that there were two separate migrations into the Americas: a large migration along the Pacific coast and an inland route along a corridor formed between two ice sheets. The results are published this week in Current Biology3.

The finding could change our theories of how culture, technology and language developed in the New World, he says. "Our data raise the possibility that, at the time of entry, the first Americans might have spoken languages already differentiated, and possibly belonging to more than one language family," he says. "This is a very heated issue among linguists."

Divided by chance

The work makes an important contribution to the field, says Brian Kemp, a molecular anthropologist at Washington State University in Pullman. "This is work I dreamed of doing," he says. But although Kemp acknowledges that there might have been multiple migrations from a single population, he remains unconvinced that the earliest Americans derived from separate populations with unique genetics and cultures.

Instead, the distribution patterns we see today may have been shaped by chance. "All indications suggest that a relatively small group that occupied two entirely empty continents expanded very rapidly," he says. "The chance that we would have lost lineages in the first few years is, I think, relatively high." And the results do not explain the presence of D4h3 in an ancient burial ground found in Illinois, he notes. "It's the one outlier, but it still needs an explanation," he says.

Meanwhile, Torroni hopes that future analysis will allow researchers to dissect additional American lineages and to study their geographic distribution as well. "We expect that some of these sub-branches will have distributions paralleling those that we have observed in the current study," he says, "while others may reveal still unknown migratory events." 

  • References

    1. Wang, S. et al. PLoS Genet. 3, e185 (2007). | Article | PubMed | ChemPort |
    2. Schroeder, K. B. et al. Biol. Lett. 3, 218–223 (2007).
    3. Perego, U. A. et al. Curr. Biol. doi:10.1016/j.cub.2008.11.058 (2008).
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