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Oldest genome of a modern human points to mixed ancestry for Indians

by Meeri Kim

The Washington Post

The genetic analysis of a 24,000-year-old arm bone of an ancient Siberian boy suggests that Native Americans have a more complicated ancestry than scientists had previously realized, with some of their distant kin looking more Eurasian than East Asian.

The new study, published online Wednesday in the journal Nature, represents the oldest genome of a modern human being ever fully sequenced.

Modern-day Native Americans share anywhere from 14 to 38 percent of their DNA with the Siberian hunter-gatherers — who are not closely related to East Asians — with the bulk coming from East Asian ancestors.

Most scientists had previously thought that the first Americans came only from the East Asian populations.

“If you read about the origins of Native Americans, it will say East Asians somehow crossed the Bering Sea,” said study author and evolutionary biologist Eske Willerslev at Copenhagen University. “This is definitely not the case; it’s more complex than that.”

It isn’t known where or when the meeting of the two peoples happened, but a likely candidate could be Beringia, the region surrounding the gap between Alaska and Siberia. Although occupied by the Bering Strait and its surrounding waters in the present, the glaciers of roughly 20,000 years ago locked up much of the Earth’s water, exposing a land bridge between the two continents.

The prehistoric crossroads provided an easy way for humans, animals and plants to spread.

Originally excavated in the 1950s, the remains of the boy had been tucked away in the bowels of a museum in St. Petersburg. He was only about 3 years old when he died, buried with a variety of grave goods such as a swan figurine and ivory pendant.

When evolutionary biologist Eske Willerslev at Copenhagen University first sequenced the DNA, extracted from the boy’s upper arm bone, he swore it was a mistake. It said the boy belonged to a lineage commonly found among Europeans, but not in East Asians.

“We put the study on hold for a year because I thought it was contamination,” Willerslev said.

They tried again, this time digging deeper and looking at his Y chromosome. It and the rest of the genome told the same story: This boy had connections to present-day western Eurasians and Native Americans, but not East Asians. Eurasians live on the western edge of the Asian continent.

They also sequenced another, more recent Siberian adult whose DNA wasn’t as well preserved, with similar genetic results.

“They were members of a really cosmopolitan group that probably reflect early modern humans leaving Africa and spreading into Central Asia,” study author and Texas A&M anthropologist Kelly Graf said.

Their results support fossil evidence from early Paleo-Indian humans, such as a well-preserved skeleton known as Kennewick man found in Washington state. Dated to about 9,000 years old, he has facial features that don’t look East Asian but rather somewhat Caucasian — a mystery replicated in other skulls.

The fact that the first Americans were already mixed to begin with could answer these controversies, said Willserslev. Any Western Eurasian genetic signatures found in Native Americans today were previously attributed to post-1492 colonial mixing with Europeans.

“Maybe it has much deeper roots — from Siberia, not Europeans coming over in their boats,” said Graf.

Graf and Willserslev said that their next step is to gather DNA samples of early American populations to try to find evidence of those proto-Eurasian roots in the New World.