In 2013, excavators working at Upper Sun River (USR), an archaeological site in central Alaska’s Tanana River Basin, made a surprising and tragic discovery. At the bottom of a circular hearth in what had once been an early Native American hut, the team found the skeletons of two infants. One had died shortly after birth, and the other, a late-term fetus, had apparently been stillborn. Radiocarbon dating and other evidence showed that they were laid to rest together, about 11,500 years ago.
This week, the researchers report another surprise: Ancient DNA from the infants shows that they came from two mothers who belonged to genetically distinct population groups. The findings provide important new support for the hypothesis that the first Americans spent thousands of years living in the far north before finally sweeping into North and South America as the Ice Age began winding down about 15,000 years ago.
Recent research has established a few fairly sure facts about the peopling of the Americas. The first Americans originated from Siberia, and migrated into the vast Bering land bridge between Asia and North America, a region also known as Beringia, sometime after 30,000 years ago. About 15,000 years ago, as rising seas submerged the land bridge and a warming trend began to melt the glaciers covering North America, people swept rapidly into both North and South America. These were the ancestors of today’s Native Americans.
Less sure, however, is what happened in Beringia, most of which is now submerged. Did multiple waves of migrants enter from Asia at different times, and then later journey on to the Americas? This is plausible because today’s Native Americans belong to five major genetic groups (A, B, C, D, and X), which could reflect several different migrations. But some researchers favor an alternative idea, called the Beringian standstill model, which holds that there was only one major wave into Beringia, but that people stayed there so long that they differentiated into distinguishable genetic groups, which then entered the Americas.
The new findings bear directly on that question, because the two infants represent the only known multiple burial from the early Ice Age period, and are also the earliest human remains from the northern reaches of North America. Multiple lines of evidence show that the infants were buried at the same time, says Ben Potter, an archaeologist at the University of Alaska, Fairbanks, and the leader of the excavations at USR. The infants were directly adjacent to each other in a single burial pit, and both were covered with an undisturbed layer of ochre—probably part of a burial ritual ceremony. Protrusions on the infants’ lower jaws and notches on their pelvis bones suggest that they might be females.
A team led by Justin Tackney, a biological anthropologist at the University of Utah, Salt Lake City, succeeded in isolating DNA from the mitochondria—tiny powerhouses of living cells that carry their own DNA—remaining in the infants’ bones. The researchers were able to sequence the entire mitochondrial genomes of the two infants; because mitochondrial DNA is inherited only from the mother, this gives insights into their maternal ancestry but not their paternal lineage.
The genetic results, reported online today in the Proceedings of the National Academy of 24小时娱乐在线s, offered a real surprise: The infants belonged to two different mitochondrial DNA subgroups, C1b and B2, even though they were buried at the same time and place. That means that they were the children of two mothers from two distinguishable genetic subgroups. Both subgroups are common in many modern Native American populations but nonexistent in Asia, suggesting that they arose during the long occupation of Beringia when populations were isolated there and had a chance to become genetically diverse. Indeed, just such a finding is one of the predictions of the Beringian standstill model.
As further support, the team estimated when the subgroups first appeared, coming up with about 12,800 years ago for C1b and 12,000 years ago for B2—further evidence that these genetic markers arose in Beringia and did not represent different waves of migration from Asia. “The people at [USR] existed only a few thousand years after the initial expansion into the Americas occurred, so they might represent a residual Beringian group,” Tackney says. Because most of central Beringia is now underwater and unlikely to reveal its secrets to archaeologists, Tackney adds, “this is the closest we might ever get to seeing what the Beringians were like genetically.”
The new work strongly supports the Beringian standstill model and a single wave of migration to the Americas, according to one of its advocates, geneticist Connie Mulligan of the University of Florida in Gainesville. “They settled in Beringia for thousands of years,” Mulligan says of the first Americans. During that time, “genetic variants specific to the New World evolved. When the ice sheets began melting about 15,000 years ago, they crossed into the New World as the first settlers.”
Ripan Malhi, a geneticist at the University of Illinois, Urbana-Champaign, agrees. The genetic diversity between the two USR infants is what “one would expect” from a population that lived for a long time in Beringia. Moreover, Malhi adds, “finding two infants in the same burial that do not share the same mother is somewhat surprising, and this may provide information into the social systems of this community.”
One possibility is that the two infants had the same father, even though their mothers could not have been closely related. To find out more about their family relations, the team needs to sequence nuclear DNA from the skeletons, which the researchers say they plan to do in the future.?