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Crossing From Asia, the First Americans Rushed Into the Unknown
Three new genetic analyses lend detail, and mystery, to the migration of prehistoric humans throughout the Western Hemisphere.
By Carl Zimmer
Nearly 11,000 years ago, a man died in what is now Nevada. Wrapped in a rabbit-skin blanket and reed mats, he was buried in a place called Spirit Cave.
Now scientists have recovered and analyzed his DNA, along with that of 70 other ancient people whose remains were discovered throughout the Americas. The findings lend astonishing detail to a story once lost to prehistory: how and when humans spread across the Western Hemisphere.
The earliest known arrivals from Asia were already splitting into recognizably distinct groups, the research suggests. Some of these populations thrived, becoming the ancestors of indigenous peoples throughout the hemisphere.
But other groups died out entirely, leaving no trace save for what can be discerned in ancient DNA. Indeed, the new genetic research hints at many dramatic chapters in the peopling of the Americas that archaeology has yet to uncover.
“Now, this is the grist for archaeologists,” said Ben Potter of the University of Alaska, who was not involved in the new papers. “Holy cow, this is awesome.”
Earlier studies had indicated that people moved into the Americas at the end of the last ice age, traveling from Siberia to Alaska across a land bridge now under the Bering Sea. They spread southward, eventually reaching the tip of South America.
Until recently, geneticists could offer little insight into these vast migrations. Five years ago, just one ancient human genome had been recovered in the Western Hemisphere: that of a 4,000-year-old man discovered in Greenland.
The latest batch of analyses, published in three separate studies, marks a turnaround. In the past few years, researchers have recovered the genomes of 229 ancient people from teeth and bones discovered throughout the Americas.
Northern
branch
About 16,000 years ago, the ancestors of living indigenous Americans split into two main branches.
NORTH
AMERICA
Ancient Montana boy
Around 14,000 years ago, the southern branch split into new branches that rapidly spread into South America.
Spirit Cave
Southern
branch
Starting 9,000 years ago, a wave of people from North or Central America replaced older populations in South America.
By at least 4,200 years ago,
a group of people related to ancient Californians had become widespread in the Central Andes.
SOUTH
AMERICA
Waves of Americans
Analysis of ancient DNA suggests that a small population of people from Siberia populated the Americas. Over thousands of years, different branches expanded southward, mixing with or replacing earlier waves.
David Reich of Harvard and his colleagues found evidence for the waves shown above. Other teams have come to broadly similar conclusions, though many questions remain.
Waves of Americans
Analysis of ancient DNA suggests that a small population of people from Siberia populated the Americas. Over thousands of years, different branches expanded southward, mixing with or replacing earlier waves.
David Reich of Harvard and his colleagues found evidence for the waves shown below. Other teams have come to broadly similar conclusions, though many questions remain.
Northern
branch
Southern
branch
NORTH
AMERICA
Ancient Montana boy
Spirit Cave
About 16,000 years ago, the ancestors of living indigenous Americans split into two main branches.
About 14,000 years ago, the southern branch split into new branches that rapidly spread into South America.
Starting 9,000 years ago, a wave of people from North or Central America replaced older populations in South America.
SOUTH
AMERICA
By at least 4,200 years ago, a group of people related to ancient Californians had become widespread in the Central Andes.
One of them is a rare individual, only the second so-called Ancient Beringian whose DNA has ever been analyzed.
The first, described in January by Eske Willerslev, a geneticist at the University of Copenhagen, was an 11,500-year-old girl whose remains were found in eastern Alaska.
The second was discovered hundreds of miles away, in western Alaska, and lived 9,000 years ago, Dr. Willerslev and his colleagues reported on Thursday in the journal Science.
The Ancient Beringians separated from the ancestors of living indigenous people in the Americas about 20,000 years ago. The new findings suggest they endured for several thousand years. Then they disappeared, leaving no known genetic trace in living people.
But another wave of migrants from Siberia did not stop in Alaska. They kept moving, eventually arriving south of the ice age glaciers. Then they split into two branches.
One group turned and headed north, following the retreating glaciers into Canada and back to Alaska. The other branch took a remarkable journey south.
The genetic data suggest that this group spread swiftly across much of North America and South America about 14,000 years ago. The expansion may have taken only centuries.
“It’s basically an explosion,” Dr. Willerslev said.
The man from Spirit Cave in Nevada belonged to this so-called southern branch of migrants. He also was closely related to a 12,700-year-old boy found on the other side of the Rocky Mountains in Montana, Dr. Willerslev also found.
But the man from Spirit Cave also turned out to have a close genetic link to 10,400-year-old skeletons found in Brazil, on the other side of the Equator.
David Reich of Harvard University and his colleagues found a similar pattern in their own research, published on Thursday in the journal Cell.
They uncovered a link between the ancient Montana boy and another group of ancient South Americans, including a 10,900-year-old skeleton in Chile. Like Dr. Willerslev’s work, the kinship suggests that migrants moved quickly from North America to South America.
“We agree that this must be a rapid radiation,” said Dr. Reich.
Starting about 9,000 years ago, both teams found, additional waves of people moved southward. Dr. Willerslev’s research suggests the new arrivals mixed with older South American populations.
Dr. Reich, on the other hand, sees evidence for two waves of migrants who completely replaced the people who had lived in South America.
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The new research also revealed instances of remarkable continuity, kinships that spanned thousands of years.
Dr. Willerslev and his colleagues compared the genome of the man from Spirit Cave to those of four sets of remains found nearby in Nevada’s Lovelock Cave, who lived as recently as 600 years ago.
All of these people were closely related, his team found, despite being separated by 10,000 years of history.
A similar bond was found in the Andes. John Lindo of Emory University and his colleagues analyzed DNA from seven people who lived at high elevations between 6,800 and 1,400 years ago.
The researchers estimate that people who lived above 7,500 feet in the mountains were separated from the lowland populations between 9,200 and 8,200 years ago. Today, the mountain people still show a strong genetic link to the ancient remains.
“This is not something that you see in most other regions of the world,” said Dr. Reich.
In 2015, Dr. Reich and his colleagues found that some living people in the Amazon carry some DNA that’s most similar to that of people who live today in Australia and New Guinea.
The researchers speculated that their ancestry included an unknown group, which the scientists called Population Y, who separately made their way into the Americas.
In their new study, Dr. Reich and his colleagues found no trace of Population Y — but Dr. Willerslev’s team succeeded in identifying their DNA in some of the 10,400-year-old skeletons in Brazil.
“The million-dollar question obviously is, how did this happen?” Dr. Willerslev said.
Perhaps another group of Asians entered the Americas long before the ancestors of the man from Spirit Cave and other early Native Americans. Maybe they interbred with people in the Amazon before disappearing altogether.
Or perhaps a few of the early members of the southern branch happened to have some odd genes that survived through the generations.
The new rush of genetic samples reflects an improving relationship between scientists and indigenous peoples. For decades, many tribes rejected requests for DNA from researchers.
The man from Spirit Cave, for example, was dug up by archaeologists in 1940 and stored in a museum. The local tribe, the Fallon Paiute-Shoshone, didn’t learn of the body till 1996. For years they fought for its repatriation.
“It’s utterly disrespectful,” said Rochanne L. Downs, a member of the tribe’s cultural committee. “If someone went into Arlington Cemetery and dug the grave of one of soldiers and took their medals, there would be outrage.”
Initially, the tribe was opposed to looking for DNA in the skeleton, because scientists would have to destroy much of it. Dr. Willerslev met with the tribe and explained that he would require only a tooth and a small piece of ear bone.
The tribe agreed to give him one shot at finding DNA in the Spirit Cave remains.
Dr. Willerslev’s results led the Bureau of Land Management to turn over the skeleton to the tribe. They buried the man from Spirit Cave at an undisclosed location last year.
Ms. Downs wouldn’t rule out similar studies in the future, but said each request would require careful consideration.
“It’s all going to be on a case-by-case basis,” she said. “The main thing is our respect for the remains.”
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