The exact process by which humanity introduced itself to the Americas has always been controversial. While there's general agreement on the most important migration—across the Bering land bridge at the end of the last ice age—there's a lot of arguing over the details. Now, two new papers clarify some of the bigger picture but also introduce a new wrinkle: there's DNA from the distant Pacific floating around in the genomes of Native Americans. And the two groups disagree about how it got there.

Prior to the advent of cheap DNA sequencing, there were all sorts of ideas regarding the peopling of the Americas. While migration across the Bering land bridge was the consensus opinion, it wasn't clear how many migrations took place—or even who, exactly, did the migrating. Because of the physical diversity of modern Native American populations, people have suggested additional migrations across the Pacific or even from Europe or Africa.

The Clovis people, who had what appears to have been the continents' first distinctive technology and culture, caused further confusion. Some argued that these represented the first humans in the Americas, while others argued they represented a distinct migration into a sparsely populated hemisphere. Language groups created their own confusion.

Bit by bit, however, DNA data has been bringing clarity to these questions. For example, it has shown that the Clovis people, DNA-wise, weren't distinct from other Native American populations. Neither was Kennewick Man, who had facial features that some interpreted as similar to different Pacific populations. And, as more Native American populations are studied, they consistently show a shared genetic heritage, largely related to East Asian populations (though with a liberal dose of ancient Siberian, as well).

But these studies have left open the possibility that other populations have made a small contribution to specific groups within the Americas. And so two studies have attempted to nail down the history of Native Americans by adding lots of additional genetic data. One has completed 31 new human genomes and got partial coverage of another 23 derived from bones obtained from archeological sites. The second scanned for DNA variation in 63 individuals from populations native to South America.

One big wave

The first analysis indicated that all Native American populations became isolated from everyone else in Asia about 23,000 years ago. That's roughly the same time as the Last Glacial Maximum, the final time the ice sheets reached their full extent prior to the onset of melting—certainly a period where it would be easy for populations to become isolated.

Within this population, two groups separated out shortly after the time that migration into the Americas seems to have started, 13,000 years ago. One went on to populate the majority of the continent. The second group, called Athabascans, is primarily found in Alaska and northwest Canada. Athabascans appear to have a more complex genetic history, having sporadic exchanges with Siberia until the Bering land bridge went under water 12,000 years ago and also interbreeding with the Inuit.

(The Inuit, while most closely related to Native Americans, are a genetically distinctive group. They arrived in North America later and largely remained separate from Native American populations. Some of them subsequently migrated back to northern Siberia.)

So, on the whole, the data from this paper comes down strongly for a single, major migration into the Americas, followed by a rapid split into two populations. Afterward, minor contributions were made by other genetic groups. Mostly, these genetic groups—the Inuit and Siberians—make geographic sense.

How'd that get here?

The Athabascans and Aleutian islanders also have a rather unexpected contribution from Australo-Melanesians, the natives of Australia, New Guinea, and the Andaman Islands. That, this study found, was absent in populations farther south.

Not so, says the study that focused on South American groups. Here, a strong signal from Australo-Melanesians was present in a number of Amazonian tribes; weaker affinities are scattered through South and Central America. At the same time, there are other groups in this region with no affinity to Australo-Melanesians.

How to explain this? The first paper takes the view that it's a product of a later addition to the already established population in North America, probably brought in by a group that was largely East Asian but had interbred with Australo-Melanesians. Whatever this group was, it appears to have vanished from Siberia and East Asia.

The second paper, however, argues that the Australo-Melanesian DNA couldn't have gotten to the Amazon undisturbed if it were just randomly being spread through interbreeding. Instead, a distinct population must have taken it there. Because the population is still largely Native American on the DNA level, but contains some DNA distantly related to Australo-Melanesians, its authors argue that this population originated in Asia and came to the Americas via a second migration.

Neither of these papers is suggesting that Australo-Melanesians made it to the Americas on their own, much less were waiting here for the arrival of Native Americans. So this doesn't do anything to overturn the general picture of the peopling of these continents. But it's not at all clear how to reconcile the two results, given the current data. It may be that we'll have to wait in order to get more.

But that's sort of missing the big question raised by this finding. DNA from the Americas won't help us understand how a population in Siberia somehow carried DNA from Australo-Melanesians. Our ancestors clearly did not sit still for long.

Science, 2015. DOI: 10.1126/science.aab3884

Nature, 2015. DOI: 10.1038/nature14895 (About DOIs).