The infants were buried in what's now Alaska. (Ben Potter, University of Alaska Fairbanks)

We have a rough idea of how humans first came to the Americas: They came from Siberia, crossing a vast land bridge that disappeared along with the low sea levels of the Ice Age. But this land bridge was no tiny crossing. At over 600 miles wide in some places, the bridge -- known as Beringia -- may have served as a long-term home for the would-be American settlers, many scholars believe.

New genetic information may be the best evidence this theory could hope for. An analysis of two infants buried 11,500 years ago in what is now Alaska reveal mothers with vastly different genomes.

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What's that got to do with anything? Well, we know that modern Native Americans belong to one of five distinct genetic groups. Some suggest that this could be evidence of five distinct migrations across Beringia. Others believe that this diversity is evidence of what's called the Beringian standstill model -- the hypothesis that one group migrated through Beringia but took its sweet time getting from one side of the region to the other. In that case, the Siberians could have settled for thousands of years on the land bridge, forming distinct communities and eventually showing evidence of such in their genes. Later, as the Ice Age ended and the seas rose, these Beringians finally became Americans.

The two babies -- one a few weeks old and the other a preterm fetus -- are described in a study published this week in the Proceedings of the National Academy of Sciences. An analysis of their mitochondrial DNA (which is inherited from mothers) showed that the infants were from distinct genetic groups. But their DNA was still distinctly Native American, indicating that their mothers had been separate from Asian groups for quite some time.

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"You don't see any of these lineages that are distinctly Native American in Asia, even Siberia, so there had to be a period of isolation for these distinctive Native American lineages to have evolved away from their Asian ancestors," senior author and University of Utah anthropology professor Dennis O'Rourke said in a statement. "We believe that was in Beringia. 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."

The findings are intriguing for more than just the genetic evidence they provide. The researchers were surprised to find that the two infants had different mothers -- because a joint burial would suggest some relation. Because the researchers were only able to analyze mitochondrial DNA, it's possible that the infants shared a father. But it's just as possible that the scientists stumbled upon an unexpected cultural practice.

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