Since the 1930s, it’s been a generally accepted theory that indigenous Americans are descendants of Siberians who came to the New World by crossing a land bridge into Alaska around 15,000 years ago.
But, the details of that migration remain a source of contention. Did the Asians who trekked across the Bering Strait arrive in one or several waves? Were some of them isolated from the rest, settling on the land bridge until it submerged beneath the water of melting glaciers?
Two new studies—relying on genetic data from living individuals and ancient skeletons—offer possible answers, albeit with different interpretations.
The first research paper, published this week in Nature, suggests that there were two founding populations. The investigating scientists, led by Harvard University geneticist David Reich, discovered that present-day Amazonian peoples in South America can trace at least part of their ancestry to indigenous Australasian populations in New Guinea, Australia, and the Andaman Islands.
Native Americans in Central and North America lack these genetic signatures, leading Reich and his team to conclude that the Amazonian peoples are descended from a distinct, second immigrant population—although they are uncertain as to precisely how and when they reached South America.
“The new genetic data is surprising, but importantly the authors don’t make the claim that indigenous Australasian travelled to South America by boat,” University of Adelaide ancient DNA expert Jeremy Austin told an Australian news site. “Instead, it seems that a small group of Australasians may have travelled around the Pacific Rim, leaving no genetic trace, finally settling in and surviving only in the Amazon Basin.”
The second research paper, published in Science, suggests its own simple solution to this mystery: there was no second migration to the New World. The study, conducted under the supervision of Eske Willerslev, an expert on ancient genetics at the University of Copenhagen, likewise found traces of Australasian ancestry among Amazonians. However, the scientists propose that those genes were already among the population of Siberians from which all Native Americans descended.
Willerslev’s research team found evidence indicating that genetic differences among Native Americans emerged after they had already arrived in the New World.
They believe this diversification began some 13,000 years ago, when the prehistoric immigrants split into two separate branches—a literal fork in the road, when, according to the study, warming temperatures opened “habitable routes along the coastal and interior corridors into unglaciated North America." The Native Americans might have gone their separate ways along these routes, dividing and isolating the populations, which ultimately spread across the continent.
Follow Mark Strauss on Twitter.