Skeleton from one of the earliest Americans yields its genome

The burial mound in Montana where the skeleton was found.

Texas A&M University

The peopling of the Americas via the Bering Sea land bridge is one of the more confusing events in recent history. Some of the earliest signs of human occupancy are actually in Chile. After that, the first distinct toolmaking culture, the Clovis people, appeared in the interior of North America and rapidly swept across the continent. There are also indications that a separate migration occurred down the Pacific Coast, possibly associated with people who had distinctive skeletal features, while the Inuit seem to be relatively recent arrivals.

The sudden appearance of the Clovis toolset has caused some people to suggest that the Clovis were a distinct migration by a passage between ice sheets directly into North America's interior. Others have even suggested that they arrived from Europe, brought by people who crossed the ice through Greenland (an idea that's favored by a certain Bigfoot researcher). Now, researchers have completed the genome of an individual who was buried with Clovis tools in Montana 12,500 years ago. The results suggest that the migration into North America was more unified than some thought.

Although Clovis tools are relatively common at many North American sites, they're generally not associated with skeletal remains. And there have been no distinctive skeletal features that label remains as belonging to a distinctive Clovis ethnic group. All of which makes Montana's Anzick site exceptional: it contains remains that were placed with Clovis tools, unambiguously tagging the skeleton as belonging to this group.

Completing a genome from a bone of this age is no longer big news. The DNA showed the expected signs of age-associated damage, and careful controls needed to be done to show that the contamination with modern DNA is minimal. In this case, the skeleton was male, which means it has a single X chromosome. Therefore, any DNA variation on the X would be a sign of contamination. By this and other measures, 99 percent of the DNA came from the individual in question.

The researchers used the resulting data to reconstruct the mitochondrial genome. And, already, the results brought a bit of clarity to the migrations of our ancestors. Its DNA sequence belongs to a type that now is almost exclusively distributed along the Pacific Coast, a distribution that some have suggested indicates the migration of a distinct group of humans, who arrived separately from those who settled the continents' interiors. The new result shows that the interiors and coasts were settled by the same people.

The Clovis genome also lacks many of the variants that are present in the modern populations, placing it at the base of the tree. The authors conclude that these results should serve as a caution against reading too much into the modern distribution of DNA variants.

The Y chromosome showed a similar story, placing it closer to all existing native populations than any sequences from Eurasia. The main genome is also consistent with the Clovis population being ancestral to most Native American populations.

But the results don't entirely argue for a single migration into North America. That's because the Clovis genome is more closely related to genomes from South America than it is to a few groups in Northern Canada and the Canadian Arcticincluding some groups that speak languages from the main Amerind group. That suggests that there might have been a second distinct migration along the interior of the ice sheets, one that left the northern populations of North America a bit more mixed than those of more southerly locations.

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Skeleton from one of the earliest Americans yields its genome