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The DNA of two babies buried 11,500 years ago, analyzed at the University of Utah, proves that Native Americans descend from people who crossed the Bering land bridge before settling in North and South America.

A paper on the research was published online Monday by the journal Proceedings of the National Academy of Sciences.

The babies, found together in the Upward Sun River site in what is now central Alaska, had different mothers, the U. researchers found. One was a 6- to 12-week-old baby; the other was a pre-term fetus. The discovery of the infant burials was reported in the same journal last November.

They are the earliest human remains in northern North America, and they have genes that are distinctly Native American.

That likely will come as no surprise to the scientific community.

"It nails it shut that without question, the earliest Native Americans came from the Bering land bridge," said Ted Goebel, an archaeologist at Texas A&M University.

For those who continue to believe there were even earlier people in the Americas or that some Native Americans descend from Europe or elsewhere, "Those people are going to continue to believe what they want to believe no matter how strong the evidence is," Goebel said.

Dennis O'Rourke, a U. anthropological geneticist and senior author on the paper, said the research also bolsters the Beringian standstill theory, which some geneticists have been advancing for 15 years or so.

Until now, there was scant archaeological evidence for the theory, which goes like this: Given the genetics of modern Native Americans, their ancestors must have spent a long enough time isolated from other populations for their DNA to differentiate from their Asian roots and for uniquely American lineages to develop.

The standstill theory suggests that the people migrated from Siberia 20,000 or 30,000 years ago, just before or in the middle of the last Ice Age. They remained there for maybe 10,000 years, moving into North and South America perhaps 15,000 years ago.

"What they genetically looked like, we had no idea," said Justin Tackney, a doctoral student in anthropology at the U. and the study's first author.

Now, they do. The babies' DNA is from lineages that show up in numerous Native Americans in North and South America.

"Here is a case of ancient DNA coming in and helping to inform archeology," Tackney said.

O'Rourke notes that the DNA lineages found in the babies are not found in Asia, or even in Siberia, so there had to have been a period of isolation.

Goebel, at Texas A&M, said he's not persuaded that the new DNA evidence proves the standstill theory.

Rather, Goebel believes the archaeological evidence — and some genetic testing on ancient Siberians and from the Clovis site in Montana — indicate the migrants came from Asia after the last Ice Age, perhaps 17,000 years ago. They may have stood still, but he believes it was more likely for a few thousand years.

Scientists use the name Beringia to describe the vast area that linked Siberia and North America during the last Ice Age. It's now mostly submerged, which is one reason there's little archaeological evidence.

The people who lived at Upward Sun River were probably a remnant of those who moved rapidly into the Americas about 15,000 years ago.

Even during the Ice Age, there must have been places that people and animals could survive, Tackney said. "It might not have been the most awful place to stay."

For whatever reason, those pockets of Native Americans — Beringians — who remained in the far north died out. The Eskimos and other modern natives of the Arctic region descended from later migrants from Asia.

The Upward Sun River ancient campsite was discovered in 2006 in the Tanana River valley, about 50 miles southeast of Fairbanks.

University of Alaska Fairbanks anthropologist Ben Potter led a team that in 2010 discovered remains of a cremated 3-year-old child near the hearth of a residential structure at Upward Sun River. They couldn't get DNA from the charred remains, however.

Potter and his colleagues found the remains of the two infants beneath the toddler's cremated remains in 2013. The babies were buried with what look like hunting darts made from elk antlers and stones, according to National Geographic's story on the discovery.

Potter, a co-author on the new study, asked O'Rourke to analyze the babies' mitochondrial DNA, the genes inherited from the mother.

The researchers identified the infant dubbed USR1 as belonging to Native American lineage C1b. Lineage C1 (most remains aren't identified to the subgroup C1b level) is found most often among the Pima and Hualapai Indians of Arizona, the Delta Yuman of California, and six other tribes, including the Ignaciano in Bolivia, the extinct Tainos in Puerto Rico and a group represented by 700-year-old bones at Norris Farms in Illinois, according to a U. news release.

The second infant, USR2, is part of a more common native lineage known as B2. That lineage is found most often in 37 tribes throughout the Americas, including the Yakama, Wishram, Northern Paiute-Shoshoni, Navajo, Hualapai (which also carries C1 genes), Zuni and Jemez in North America and the Quecha and Aymara in Peru. The B2 lineage also was common among the U.S. Southwest's ancient Fremont and Anasazi, the news release said.

Although the DNA tests concluded the babies did not have the same mother, they may have had the same father or be related in other ways, O'Rourke said. Another lab is studying other DNA from the infants' remains. "There is much more to be learned," he said,

The U. has one of a handful of labs in the country that's dedicated to ancient DNA.

Twitter: @KristenMoulton