Ancient Americans came all at once – or so a baby’s DNA suggests.

Science looks at the mystery of when the first Americans arrived over the land bridge of Beringia, and have found some interesting clues in an ancient Alaskan infant’s DNA:

The genome comes from an 11,500-year-old infant found in 2013 at the site of Upward Sun River in central Alaska’s Tanana River Basin, a part of Beringia that’s still above sea level. The infant, one of two from the site, belonged to a population that likely numbered in the low thousands, who hunted Beringia’s abundant herds and gathered plants.

A team led by geneticist Eske Willerslev of the University of Copenhagen and the University of Cambridge in the United Kingdom isolated DNA from bone powder taken from the infant’s skull. The researchers sequenced the DNA repeatedly to get a virtually complete copy of the genome. They compared it to that of modern Native Americans, as well as to other ancient and living people across Eurasia and the Americas. By looking at genetic similarities and estimating how long it would take for key mutations to pop up, the researchers assembled a family tree with rough dates.

The infant’s group was most closely related to modern Native Americans—but it wasn’t a direct ancestor. Instead, it and modern Native Americans shared common ancestors who must have entered Beringia some 25,000 years ago, the researchers report today in Nature. Perhaps 21,000 years ago, those ancient settlers branched into at least two groups: one that included the infant and another that gave rise to Native Americans.

That supports the idea that Asian migrants lingered in Beringia and became genetically isolated—the so-called Beringian standstill model—says anthropologist Connie Mulligan of the University of Florida in Gainesville. “Because they have the whole nuclear genome, you can really tell a lot about when and where this migration happened,” she says.

[via Archaeological News]