A genetic analysis of ancient and modern humans suggests that the ancestors of Native Americans entered the North American continent from Siberia some 23,000 years ago—and that they did so in a single wave.
According to the new study, which now appears at Science, the original Americans came to North America via Beringia at the height of the last Ice Age, and they stayed in the north for potentially thousands of years before diverging into two distinct populations some 13,000 years ago. One group dispersed across North and South America, while the other stayed in North America.
Credit: Raghavan et al., Science (2015).
An international team of researchers reached these conclusions by sequencing and comparing the genomes of 31 living Native Americans, Siberians, and people from Oceania, and the genomes of 23 ancient individuals from both North and South America who lived between 6,000 to 200 years ago.
The study challenges the assumption that an earlier wave of people from East Asia arrived in North America prior to the last glacial maximum, and that the continent was populated through multiple independent waves of human settlers. DNA analysis suggests that humans entered into North America no earlier than 23,000 years ago, and not 30,000 years ago as some archaeologists and anthropologists have speculated. What’s more, the study dispels the notion that Polynesians or Europeans contributed to the genetic heritage of Native Americans.
The “single wave hypothesis” would also seem to imply that genetic differences in present-day Native American populations came about after migration, and are not the product of incoming waves of Asian populations.
“The diversification of modern Native Americans appears to have started around 13,000 years ago when the first unique Native American culture appears in the archeological record: the Clovis culture,” noted UC Berkeley professor and study co-author Rasmus Nielsen in a statement. “We can date this split so precisely in part because we previously have analyzed the 12,600-year-old remains of a boy associated with the Clovis culture.”
Fascinatingly, the genetic analysis revealed that both populations of Native Americans have a mixture of genes from East Asians and Australian Melanesians, including Papuans, Solomon Islanders, and Southeast Asian hunter-gatherers.
“It’s a surprising finding and it implies that New World populations were not completely isolated from the Old World after their initial migration,” noted study lead author Eske Willerslev from the Centre for GeoGenetics at the Natural History Museum, University of Copenhagen. “We cannot say exactly how and when this gene flow happened, but one possibility is that it came through the Aleutian Islanders living off the coast of Alaska.”
But not everyone agrees with this interpretation. In a separate study, also released this week, professor David Reich from Harvard Medical School argues that there were multiple pulses of migration into the Americas.
Credit: Pontus Skoglund, Harvard Medical School.
The BBC explains:
According to Prof Reich, the discovery of Oceanian ancestry among certain Native American groups indicates that the Americas were peopled by a more diverse set of populations than previously accepted.
“The simplest possible model never predicted an affinity between Amazonians today and Australasians,” he said.
“This suggests that there is an ancestral population that crossed into the Americas that is different from the population that gave rise to the great majority of Americans. And that was a great surprise,” he said.
Prof Reich believes that the most plausible explanation is that there was a separate migration from Australasia, possibly around 15,000 years ago. This group, he speculates, was probably more widely dispersed across North America but was eventually pushed out by other native American groups.
The Willerslev study, on the other hand, makes the case that the traces of Australasian DNA are the result of a later migration, around 8,000 years ago along the Pacific coast.
Read the entire study at Science: “Genomic evidence for the Pleistocene and recent population history of Native Americans”. And check out Reich’s study at Nature: “Genetic Evidence for Two Founding Populations of the Americas.”