Peopling the Americas

An ancient skull from Lagoa Santa, Brazil

Natural History Museum of Denmark

Genetic studies suggest the first humans arrived in the Western Hemisphere, across what is now the Bering Strait, roughly 25,000-23,000 years ago after splitting from Siberians and East Asians. Over the next several millennia, these early humans migrated further south and diverged into northern and southern populations that are the ancestors of present-day indigenous peoples of North and South America. Some researchers have argued, however, that unrelated groups of Australasians and so-called “Paleoamericans”—identified by their distinct skull features—migrated to the Americas even earlier than did the Ancient Beringians. A recently published analysis of ancient human genomes has uncovered how and when early humans spread across North and South America and contributed to various indigenous ancestries.

An international team of fifty-four researchers, led by Eske Willerslev of the Natural History Museum of Denmark at the University of Copenhagen, analyzed the genomes of fifteen humans, whose remains ranged in age from 500 to more than 10,000 years and ranged in geography from Alaska to Patagonia. By comparing the genomes of these remains (obtained in collaboration with several indigenous groups) to those of modern humans and ancient humans from other parts of the world, the researchers were able to determine relationships between, and to estimate the timing of, early human migrations throughout the Americas. They concluded that groups spread rapidly and in multiple migration pulses, with some later mixing. Ancient humans spread across North America potentially on the scale of centuries, and into eastern South America perhaps just one or two thousand years later.

Additionally, the study found an Australasian influence, or genetic admixture, around 10,400 years ago among some of the peoples of South America, but no signs of it in North America—perplexing, given that the indigenous peoples of North and South America diverged from the same group. This finding could mean that “an earlier group possessing the [genetic trait] had disappeared or that a later-arriving group passed through North America without leaving any genetic trace,” observed the researchers.

Finally, in a blow to the “Paleoamerican” hypothesis, all fifteen genomes showed these far-ranging ancient humans were most closely related to modern indigenous peoples, despite their differing skull shape. According to Willerslev, “Looking at the bumps and shapes of a head does not help you understand the true genetic ancestry of a population—we have proved that you can have people who look very different but are closely related.” (Science)