In 2013, researchers discovered the remains of two infants buried about 10 centimeters (4 inches) apart in a pit at the Upward Sun River site in Alaska. The two infants – a 6- to 12-week-old baby and a preterm 30-week fetus, possibly a stillborn – were buried 11,500 years ago. During the Late Pleistocene, this area was part of Beringia, the vast land bridge that once connected Asia and North America. The earliest Americans are thought to be descendants of the population that crossed over from Asia, though the genetic characterization of that source population remains unclear.
Now, researchers sequencing DNA from the two infants reveal that they had different mothers, and both of those maternal lineages are rare in northern North American populations today. The findings, published in Proceedings of the National Academy of Sciences this week, suggest that early colonists migrating from Asia took a long layover in Beringia, where they underwent a period of isolation lasting thousands of years before expanding to the rest of the continent and beyond.
After isolating DNA from the infants’ skull bones, University of Utah’s Justin Tackney and colleagues were able to obtain two whole mitochondrial genomes (or mitogenomes) from east Beringia that postdates the end of the initial colonization by just a few millennia.
Since mitochondrial DNA (mtDNA) is inherited only from mothers, the team was able to identify one infant as a member of the Native American lineage C1b and the other as a member of the B2 lineage. That makes these infants the northernmost known kin to Native American lineages found much farther south – indicating greater genetic diversity in early Beringia than in modern populations living there now.
"These infants are the earliest human remains in northern North America, and they carry distinctly Native American lineages," University of Utah’s Dennis O'Rourke says in a statement. "We see diversity that is not present in modern Native American populations of the north and we see it at a fairly early date. This is evidence there was substantial genetic variation in the Beringian population before any of them moved south."
To the right is a map showing the location of the double infant burial and of Native American groups that are part of the same B2 (blue) and C1 (orange) lineages.
The results provide evidence for a population genetics model known as the Beringian Standstill Model. That we're seeing more genetic diversity in the past at this far northern site supports Beringia as the location for the ancestral populations that gave rise to the primary wave of Native Americans, Tackney explains to IFLScience. Additionally, the B2 lineage, and to some extent the C1b lineage, fall at the root of their mtDNA trees. "We have here a snapshot at 11,500 years ago showing these two mitochondrial lineages on their way to evolving into modern Native American variants, but already evolved from the Asian roots," he adds. "The standstill (and isolation) allowed differentiation from Asia to occur."
Image in the text: Ben Potter/University of Alaska Fairbanks