An exhaustive study of DNA taken from dozens of Native American groups that span from Canada to the tip of South America is helping to settle a question that has long divided scientists: When people arrived in the Americas more than 15,000 years ago, the Harvard-led research shows, they came in successive waves, not all at once.
The analysis published Wednesday reveals that while one population of “First Americans” crossed a land bridge from Siberia during the last Ice Age, giving rise to most Native Americans, there were at least two subsequent migrations. These people mixed with the founding group later, leaving traces of their genes in the DNA of present-day populations in Alaska, Greenland, and Canada.
The new findings in the journal Nature highlight the growing importance of cutting-edge technologies that are allowing geneticists to probe the distant past, alongside archeologists, linguists, and paleoanthropologists who have relied on studies of such things as arrowheads and tools, language, skulls, and teeth.
“Geneticists, we’re sort of amateurs—we’re not steeped in the deep understanding of history the linguists and archeologists have, but we do have access to information” they don’t, said David Reich, a genetics professor at Harvard Medical School who led the study, along with a scientist at the University College London. Reich also played a leading role in the surprising discovery in 2010, based on a comparison of DNA from fossilized remains with present-day human genomes, that Neanderthals interbred with humans.
“It’s a different type of evidence—not as good at (establishing) dates, but much better about how people relate to each other,” he said. “You can’t tell from remains that are left behind who gave rise to who.”
Scientists not involved in the study said the findings, which involved the analysis of samples taken from nearly 750 present-day Native Americans and Siberians, deepens and enriches the story of migration into the Americas. Previous genetic analyses had indicated Native Americans descended from a single source population.
“The bottom line is there has been this debate: single versus multiple origins or migrations, and this comes down particularly on one side of that,” said Dennis O’Rourke, a professor of anthropology at the University of Utah who was not involved in the study. He said the finding was solid and the interpretation convincing, but that what is most fascinating is the way in which the new data is casting light on precisely how new streams of migrants mixed with existing ones.
“I doubt it’s the final word,” O’Rourke said. “For me it suggests that as the data become richer and we have a better handle on patterns of diversity, we are seeing our reconstructions of past populations become more complex as well.”
To do the work, scientists examined more than 360,000 spots in the genomes of each person where the DNA commonly varies. They used the frequency of genetic variations to construct a kind of family tree showing when groups split off from one another, and when populations might have mixed together.
The researchers found that at least two other Asian populations came to the Americas after the initial migration, though they were unable to date their arrival. Nor is it clear whether these groups would have come across the land bridge or made the voyage aided by boats later, after sea levels rose, according to Andres Ruiz-Linares, a professor of human genetics at University College London who coordinated the research. One wave of new migrants was detected in populations that speak Eskimo-Aleut languages found in Alaska, Canada, and Greeland, who still get more than half of their DNA from the First Americans. Another was detected in a Canadian Chipewyan group, who are 90 percent made up of First Americans’ DNA.
The genetic analysis was made more difficult by the fact that since 1492, Native Americans have mixed with European and African populations, so the researchers had to carefully sift out genetic variations that would have appeared due to this later mixing.
Ripan Malhi, an anthropologist at the University of Illinois at Urbana-Champaign, wrote in an e-mail that the new data add nuance to a consensus view that had emerged that there was a single source population that gave rise to Native Americans. The team’s explanation that there were multiple waves of migration that interbred with the earlier groups in parts of North America helps explain the overall similarity of DNA among all Native Americans as well as some unaccounted for differences in groups from North America, he wrote.
One drawback of the study was the lack of Native American populations from the continental United States. Ruiz-Linares, a Columbian who led the work with Reich, said that is because of the difficulty of obtaining such data, which requires obtaining proper consent and forming relationships within different countries and among specific tribes. The international team of 64 researchers who collaborated on the project are part of a network he painstakingly built over the last two decades.
“The question has always been the same—basically trying to reconstruct history from genetic data,” Ruiz-Linares said. “What has really changed dramatically is the technology, both the technology in the lab and our ability to collect a large amount of data.”
After the human genome project, for example, researchers have had access to technologies that can rapidly and cheaply measure large amounts of DNA, allowing unprecedented amounts of data to be collected, and enabling new analysis methods.
The data also show, in contrast to what scientists have seen on other continents, that there is a clear record of the way people geographically dispersed. The branching family tree that the researchers created suggests people migrated southward rapidly, hugging the west coast, and that there was relatively little mixing as groups branched off.