Friday, January 05, 2018

Genetics Rewrites the History of Early America—And, Maybe, the Field of Archaeology

[Smithsonian] The story of how Homo sapiens spread from Africa to the rest of the world is a tangled epic, full of false starts and dead ends. Yet perhaps nowhere is the puzzle more difficult than in the Americas, two landmasses divided from the rest of the world by two huge oceans. Zoom out, though, and you’ll see that isolation has only been imposed for the last 11,000 years; before then, a narrow land bridge called Beringia stretched between Siberia and Alaska, providing an icy highway for travelers.
This week, scientists reported explosive new findings on the genetic story of one of those ancient travelers: an infant girl named Xach’itee’aanenh T’eede Gaay by the local indigenous people, who lived for a brief time 11,500 years ago in an Alaskan community now called Upward Sun River. The infant’s genome has the power to rewrite what we know about the human journey into North America—and in doing so, points to the larger genetic revolution that is reshaping the field of archaeology.
For decades, archaeologists have hypothesized that humans entered the Americas from Asia using Beringia (the first man to suggest the existence of a land bridge was actually a 16th-century Spanish missionary named Fray Jose de Acosta). But even as more sites of occupation were discovered in Siberia and Alaska, pointing to human occupation and the movement from west to east, questions remained. When exactly did the migration happen, and how did it happen? In one wave, or many?
In January 2017, researchers at the Canadian Museum of History concluded that a horse jawbone found in the Bluefish Caves of the Yukon bore human markings from 24,000 years ago, meaning that early Americans had settled here by 22,000 BC. That would push back the date of human occupation in North America by 10,000 years. But those findings—like so many in this field—proved controversial, and haven’t been universally accepted by the archaeology community.
The new report on Xach’itee’aanenh T’eede Gaay complicates this narrative further. While she may be “just” 11,500 years old, she provides incontrovertible evidence for the timing of human migration.
Within her genome is the story of a newly discovered population of early Americans whose ultimate fate remains a mystery, as their genes are no longer visible in modern populations. “This individual represents a previously unknown population, which is also the earliest known population of Native Americans,” says Eske Willerslev, an evolutionary geneticist and one of the authors of the new study. “We can address fundamental questions such as when people came into North America because this population is related to everyone else.”
The Upward Sun River girl, buried next to an even younger infant in a ceremonial grave with red ochre on both of them, is a member of what researchers are calling the Ancient Beringians. Prior to sequencing her genome, scientists had identified two main groups of Native Americans: Northern Native Americans and Southern Native Americans, who split off sometime after entering the continent. This infant child belongs to neither of those two groups. That means that, somewhere along the way, another split must have occurred to create this unique Ancient Beringian group.
Using demographic modeling, the researchers concluded that the founding population of Native Americans began splitting from their ancestors in East Asia around 36,000 years ago. By 25,000 years ago, they had made a complete split. By 20,000 years ago, another divergence had happened, this time between the Ancient Beringians and the rest of the Native Americans. And within the next 3,000 to 6,000 years, the Native Americans further divided into Northern and Southern groups.
All this, from the ancient DNA of one long-dead child. Read More