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A Siberian skeleton nearly 10,000 years ago revealed a DNA revealing a kinship with the living Amerindians, scientists said Wednesday.
The discovery, published in the journal Nature, provides a new important clue to the migrations that brought the first peoples of the Americas.
"With regard to the settlement of the Americas, we found near the missing link," said Eske Willerslev, a geneticist at the University of Copenhagen and co-author of the new paper. "This is not the direct ancestor, but it is extremely close."
Decades of research by archeologists and linguists suggest that the first settlers to the American continent at the end of the last Ice Age 14,500 years ago. Most experts believe that this route was a land bridge that connected Alaska and Siberia to the Bering Sea.
But Siberia is a vast country that has sheltered many cultures over the course of thousands of years. The researchers turned to DNA in the hope of specifying which were the ancestors of the Amerindians.
The first studies were inconclusive: the Amerindians did not seem to have many genetic links with any living group of Siberians. Mr. Willerslev suspected that the DNA of ancient Siberians could help solve this puzzle.
Around the world, he and his colleagues have discovered that people who live in a place have few genetic links with those who lived there thousands of years ago.
The history of Siberia is amazingly deep. After evolving in Africa, humans began to spread to other continents around 70,000 years ago. About 45,000 years ago, humans reached the northern border of Siberia, where they hunted mammoths and other big game.
Vladimir V. Pitulko, an archaeologist at the Russian Academy of Sciences, and his colleagues provided Dr. Willerslev with two baby milk teeth from a site in Siberia called Yana. His team extracted DNA from both teeth, which turned out to be from two boys.
The teeth are 31 600 years old, making DNA the oldest human genetic material recovered from Siberia.
When Dr. Willerslev and his colleagues compared genetic variants of Yana's DNA with living and old people, they discovered that Siberian boys belonged to a previously unknown population. Scientists call them the ancient Northern Siberians.
Most of their ancestors date back to the first migration of Africa, particularly from people who might have spread to Europe.
Thousands of years before Yana boys' lives, the ancient Siberians from the North were meeting people closer to East Asia. People from both populations crossed paths and Yana boys thus inherited a mix of both ancestors.
To their surprise, however, geneticists did not find any living person with significant ancestry in northern Siberia.
"The first inhabitants of northeastern Siberia are people we did not know and who are not Native American ancestors," said Dr. Willerslev.
What happened to the ancient North Siberians? An index emerged of a skull fragment that Dr. Pitulko and his colleagues provided Dr. Willerslev. These remains, dating back to 9,800 years, were discovered on a site near Yana called Kolyma.
Dr. Willerslev's team also found the DNA in the skull of Kolyma. A small fraction of the ancestry of this individual originated in ancient Northern Siberia. But most came from a new population. Mr. Willerslev and his colleagues call them the ancient Paleo-Siberians.
The DNA of ancient Paleo-Siberians is remarkably similar to that of Native Americans. Willerslev estimates that Native Americans can allocate about two-thirds of their ancestors to these previously unknown people.
One of the reasons why the ancient Paleo-Siberians were unknown until now is that they were largely replaced by a third population of people of different origins. East Asia. This group only arrived in Siberia in the last 10,000 years – and they are the ancestors of most living Siberians.
"It may have been cold and windy, but it was really very resource-rich, like big mammals, and people wanted to go," Dr. Willerslev said.
The Kolyma individual lived long after the origin of the Native American branch. Willerslev estimates that the ancestors of Native Americans and ancient Paleosiberians separated 24,000 years ago.
The story gets complicated: shortly after this split, the ancestors of the Amerindians met another population with genetic links to Europe. All living Amerindians carry a mixture of genes from these two groups.
The new study does not make it possible to determine exactly where Amerindians emerged from the meeting of these two peoples. The ice age was at its peak 24,000 years ago and different populations of Siberia and surrounding areas could be withdrawn to refuges where game still survived.
Anne Stone, an anthropologist geneticist from Arizona State University who was not involved in the new study, hypothesized that the Native American population could have emerged in one of these refuges on the land bridge which connected Siberia to Alaska about 34,000 to 11,000 years ago.
But testing this idea will be difficult, she warned. "I think it's going to be very slow," she said. "Finding human remains of this age is really disheartening."
The task is even more difficult with the fact that melting glaciers drowned the land bridge at the end of the ice age, thus overwhelming all human remains that may contain more DNA.
Yet the disappearance of the land bridge has not prevented the movement of people between continents. Later, waves of people crossed the Bering Sea.
Teasing aside this traffic is proving difficult for scientists – and has led to debates about how migrations have shaped the origins of living Amerindians.
In his research on ancient DNA, Dr. Willerslev's team has highlighted the fact that a second wave of ancient Paleo-Siberians reached Alaska between 9,000 and 6,000 years ago. They made contact with the Amerindians and met each other.
Mr. Willerslev argues that some living Amerindians inherited this extra-ancient Paleo-Siberian ancestry. These peoples, including the tribes of Alaska, Canada, and Southwest, all speak a language family called NaDene.
But in a separate study, a team led by Stephan Schiffels of the Max Planck Institute for the Science of Human History in Germany came to a different conclusion.
The analysis of this team, also published Wednesday in Nature, traces the ancestry of NaDene speakers to an enigmatic people called the Paleo-Eskimos.
Archaeologists have known Palaeo-Eskimos for years thanks to their distinctive tools and artifacts. They first emerged in the Arctic suburbs of Siberia and Canada about 5,000 years ago, before spreading to Greenland.
But the signs of these people disappear about 1,000 years ago.
In 2010, Willerslev and his colleagues sequenced the genome of a 4,000-year-old Paleo-Eskimo from Greenland. They discovered that he had no genetic connection with the Inuit living in Greenland today.
To conduct a new study on paleo-Eskimos, Dr. Schiffels and his colleagues obtained permission from the tribes of Alaska and Canada to obtain new DNA samples from living people and ancient skeletons.
Their analysis indicates that after arriving in Alaska about 5,000 years ago, the Paleo-Eskimos split into three groups. "It's a complicated sequence of mixtures and movements," said Dr. Schiffels.
A group has spread along the empty Arctic coast until it reaches Greenland. A second group settled in Alaska, where they met people whose ancestors were from Siberia about 10,000 years ago.
They crossed paths and NaDene speakers carry this mixed ancestry today.
Dr. Schiffels and his colleagues argue that the third group encountered another group of Native Americans on the Alaskan coast and crossed with them. These people are the ancestors of Inuit and Aleuts.
But some of them also crossed the Bering Strait in Siberia. And from there, about 1,000 years ago, another wave of people returned to North America, where they spread in the Arctic and replaced the original Paleo-Eskimos of the Greenland.
Dr. Schiffels was not surprised that he and Dr. Willerslev came to different conclusions about these complex migrations.
"In the coming weeks, I think our team will analyze their data, and their team will analyze our data," said Dr. Schiffels. "I do not know if there will be a big eureka moment then."
Willerslev hoped that the new research would spur more ancient DNA research in Siberia and Alaska.
The migration that brought the ancestors of Amerindians living in the Americas may not have been the first. According to Dr. Willerslev, it is possible that the ancient North Siberians arrived in Alaska or Canada thousands of years ago.
"This begs the question: should we go further for older sites?" Said Dr. Willerslev. "And now, we know what to look for."
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