Researchers have long wondered how people settled the Americas, particularly the path they took to the new territory and the timing of their expansion. Until recently, archaeologists studying these questions were limited mostly to digging up skeletons and artifacts.
But now scientists have begun extracting DNA from human bones, and the findings are providing new glimpses at the history of the first Americans. On Monday, researchers at the University of Alaska and elsewhere published an important addition to the growing genetic archive.
In the Proceedings of the National Academy of Sciences, the researchers reported that they had recovered DNA from two skeletons of children who lived in Alaska 11,500 years ago. The genetic material is not only among the oldest ever found in the Americas, but also the first ancient DNA discovered in Beringia, the region around the Bering Strait where many researchers believe Asians first settled before spreading through North and South America.
The archaeological site, near Upward Sun River, was discovered in 2010. Excavations there have revealed that between 13,200 and 8,000 years ago, people visited during the summer, catching salmon and hares. They built tentlike structures where they made fires and slept.
In 2011, archaeologists discovered cremated bones on a hearth at the site. Research revealed that the bones belonged to a 3-year-old child. Below the hearth, the team discovered a burial pit containing the skeletons of two other children.
One of the buried children was an infant who died a few months after birth; the other was likely a late-term fetus. After the baby and the fetus died, their bodies were carefully laid atop a bed of red ocher, surrounded by antlers fashioned into hunting darts.
“These things we hardly ever find — it’s a very rare window into the worldview of these people,” said Ben A. Potter, an archaeologist at the University of Alaska Fairbanks who has led the research at Upward Sun River.
Clearly, the children had been ceremonially buried. But why the two bodies ended up in the same grave is impossible to know. With the consent of Native American tribes that live around the site, Dr. Potter and his colleagues drilled small pieces of bone from the two skeletons and sent them to geneticists at the University of Utah.
The vast majority of our DNA lies in the nucleus of the cell. But the cell’s energy factories, called mitochondria, also carry small bits of their own DNA inherited solely from our mothers.
Because each cell can contain hundreds of mitochondria, it is easier to reconstruct their DNA than that of the cellular nuclei. The Utah geneticists focused their search on mitochondrial DNA in the Upward Sun River bones.
They succeeded in recovering mitochondrial DNA from both bone samples. But to their surprise, the genes were markedly different. The infant and the fetus did not share the same mother or even maternal grandmother.
The researchers can only speculate how an infant and a fetus from different mothers ended up in the same grave. They might have had the same father, or they might have belonged to different families who suffered terrible losses at the same time.
But the significance of the DNA found at Upward Sun River extends far beyond the story of two children. It sheds light on how people first moved into the Americas.
In 2007, Ripan Malhi of the University of Illinois and his colleagues proposed a model for this migration, known as theBeringian Standstill. Early Siberians expanded east into Beringia about 25,000 years ago, they proposed, and stayed there for about 10,000 years.
Humans were able to thrive in Beringia because even at the height of the last ice age, the region was not covered by glaciers. It was mainly tundra and shrub land, with scattered stands of trees.
Humans could not expand eastward into the rest of the Americas because they were blocked by glaciers. About 15,000 years ago, as the glaciers retreated, the standstill came to an end.
“It opened up new ecological zones that were being colonized by plants and animals,” said Dennis O’Rourke of the University of Utah, a co-author of the new study.
According to the standstill hypothesis, the ancestors of Native Americans would have built up a large degree of genetic diversity in the thousands of years they were confined to Beringia — more so than if these populations had migrated directly from Siberia to the Americas.
The fact that two children who died at roughly the same time in the same community shared so few genes is consistent with the idea that the population was prevented from moving into the Americas for thousands of years, Dr. O’Rourke said.
But the two children died after the glaciers melted, he noted, and their settlement “may well be a remnant of that original Beringian group. It may give us a snapshot of that earlier time.”
Dr. Malhi, who was not involved in the new study, thought the new DNA was too recent to provide proof of the idea that humans were trapped in Beringia for thousands of years. “It’s valuable information, but it’s a little bit late to be extremely informative to let us know if the Beringian Standstill hypothesis holds,” he said.
More conclusive findings would be possible if scientists found older DNA from people who lived during the Beringian Standstill. Archaeologists are now looking for skeletons from that age, but Dr. Malhi is not holding his breath.
Many of the sites where people lived may now be impossible to reach, because sea levels rose at the end of the ice age.
”There are archaeologists up there looking for such sites,” Dr. Malhi said. “But I think it’s probably unlikely, largely because a lot of Beringia is now under water.”
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