People arrived in North and South America in three distinct waves instead of one, with most modern Native Americans descending from a single batch of migrants who slipped into Alaska before traveling south into the rest of the hemisphere, according to a new analysis of more than 364,000 DNA sequences from 69 Native American and Siberian groups.
"This is the most comprehensive survey of genetic diversity in Native Americans so far," the authors wrote. "Our analyses show that the great majority of Native American populations — from Canada to the southern tip of Chile — derive their ancestry from a homogeneous 'First American' ancestral population, presumably the one that crossed the Bering Strait more than 15,000 years ago."
Following that first group of settlers were two populations intimately familiar to Alaskans — the ancestors of modern Athabascans, Tlingits, Eyaks and Haidas who speak Na-Dene languages (the study focused on the Chipewyan people of the Northwest Territories rather than any modern Alaskan group) and the ancestors of those who speak Aleut and Eskimo languages, the authors said.
But even these Arctic peoples — still carrying evidence of their East Asian ancestry in their genes — inherited most of their genetic heritage from the very first arrivals.
"Eskimo-Aleut speakers derive more than 50 percent of their DNA from First Americans, and the Chipewyan around 90 percent," according to this story about the research. "This reflects the fact that these two later streams of Asian migration mixed with the First Americans they encountered after they arrived in North America."
Single or multiple migrations
The study — published this week in the journal of Nature with 64 named authors from four continents — adds yet another wrinkle in the long-running anthropological controversy over the origins of the First Americans: who they were, how they traveled and when they arrived.
"For years it has been contentious whether the peopling of the Americas occurred by means of a single or multiple migrations from Siberia," said study coordinator Andres Ruiz-Linares, professor of human genetics at the University College London, in this story posted by Harvard.
"But our research settles this debate: Native Americans do not stem from a single migration. Our study also begins to cast light on patterns of human dispersal within the Americas."
For most of the 20th Century, scientists believed that humans walked across the thousand-mile-wide Bering Land Bridge exposed by lower sea levels. They then spread out into North America through openings in the continental ice sometime before 12,000 years ago in a single epic migration. These First Americans were originally thought to be big-game hunters using Clovis spear points and the direct ancestors of all Native populations in the New World.
But this single-migration theory has been undermined repeatedly over the past few decades, with scientists uncovering archaeological sites that contradict the timing or yield artifacts that complicate the cultural picture. People were probably living at the tip of South America, for instance, before an overland corridor opened through Alaska and Canada. One recent study suggests that people would have found ice-free routes along the coast thousands of years before continental glaciers retreated.
Many waves of newcomers
"From Alaska to Brazil and southern Chile, artifacts and skeletons are forcing archaeologists to abandon Clovis orthodoxy and come to terms with a more complex picture of earliest American settlement," wrote John Noble Wilford in an older New York Times story with excellent background about the issue. "People may have arrived thousands to tens of thousands of years sooner, in many waves of migration and by a number of routes."
This new study appears to prove the case:
"There are at least three deep lineages in Native American populations," said co-author David Reich, professor of genetics at Harvard Medical School, in this story. "The Asian lineage leading to First Americans is the most anciently diverged, whereas the Asian lineages that contributed some of the DNA of Eskimo–Aleut speakers and the Na-Dene-speaking Chipewyan from Canada are more closely related to present-day East Asian populations."
In what bolsters the notion that First Americans might have been beachcombers and paddlers, the research team concluded that these ancient migrants probably traveled south from Alaska along the West Coast, with populations periodically splitting off to immigrate inland. Once the people settled in the far reaches of the two continents, there apparently was little additional gene flow among the populations, especially involving Native inhabitants of South America.
The Alaskan exception
But one "striking exception" to this "simple dispersal" involved ancient Alaskans.
"The Naukan and coastal Chukchi from northeastern Siberia carry 'First American' DNA," the authors explained here. "Thus, Eskimo-Aleut speakers migrated back to Asia, bringing Native American genes."
Another exception occurred in Central America, where Chibchan-speakers have ancestry from both North and South America, the authors said.
Sorting out the immensely complex genetic picture was difficult. Native American populations have been intermixing with European and African immigrants for at least 500 years.
"We developed a method to peel back this mixture to learn about the relationships among Native Americans before Europeans and Africans arrived," Reich said here, "allowing us to study the history of many more Native American populations than we could have done otherwise."
Contact Doug O'Harra at doug(at)alaskadispatch.com