Scientists believe that humans crossed the land bridge over the Bering Strait and entered the Americas for the first time between 23,000 and 30,000 years ago. Beyond that, the details of this migration are uncertain, as is exactly when and how these early arrivals dispersed across North and South America. A recent study based on human remains found at an ancient fishing camp in Alaska sheds new light on these questions, and suggests ancient North American society was more diverse than previously thought.

Archaeologists discovered the Upward Sun River site, located east of Fairbanks, Alaska, in 2010. Subsequent excavations turned up evidence of human dwellings and artifacts indicating that people camped at the site during the summer between 13,200 and 8,000 years ago. In addition to some of the earliest known salmon fishing in Alaska, the ancient camp also saw tragedy: In 2011, a team of archaeologists from the University of Alaska and elsewhere found the cremated bones of a 3-year-old child atop a hearth. Then, in 2013, the team discovered a burial pit underneath the hearth containing the skeletons of two infants.

The two children—one a baby between 6 and 12 weeks old, the other a stillborn or preterm 30-week-old fetus—died of uncertain causes some 11,500 years ago, and were buried together in ceremonial fashion. Laid on a bed of red ocher (a type of clay often associated with burials), they were surrounded by decorated hunting darts made from antlers. After researchers got the okay from local Native American tribes, they removed small pieces of bone from each skeleton and sent them to geneticists at the University of Utah. The resulting study was published this week in the journal Proceedings of the National Academy of Sciences.

ice age
Researchers work on excavation at the Upward Sun River site in Alaska. (Credit: Ben Potter/UAF)

The geneticists were able to remove and analyze mitochondrial DNA—genetic information extracted from the energy-producing structures within cells—from both infants. Because this genetic material is inherited only from the mother, it gives scientists a way to trace maternal lineage. The scientists found that the two infants did not have the same mother, or even the same maternal grandmother. In fact, the babies came from two separate genetic lineages. According to the study’s authors, both lineages are distinctly Native American, not found in Asia, and both are “rare to absent” from modern northern populations.

It’s exceedingly rare that scientists recover mitochondrial DNA from ancient human remains. The Upward Sun River camp is one of only eight sites in North America dating back at least 8,000 years old that have yielded such valuable genetic data. In addition, the DNA results proved surprising in several ways. Human remains found at ancient sites tend to come from single families, and the fact that the two infants were buried in the same grave suggests that they shared a special bond. The researchers speculated that the two may have had the same father, for example, or they could have been the victims of some common hardship that struck the group.

In addition, the presence of two different genetic lineages in the same ancient grave suggests that the population of Ice-Age Alaska was more diverse than previously believed. It also appears to support the idea that migrants across the Bering Strait remained in the local region (Beringia) for some 10,000 years before moving southwards—a theory known as the Beringian Standstill hypothesis. According to this version of events, humans were able to thrive in Beringia even at the height of the Ice Age, as the region remained tundra with sparse vegetation. Glaciers blocked access to the rest of the Americas, however, until around 15,000 years ago, when their melting put an end to the standstill.