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Ancient Americans migrated in a single wave from Siberia

Human migration Two new DNA studies shed light on the migration of ancient people into the Americas, including 'surprising' links to present day Australo-Melanesians.

The first study, published in Science, reveals that Native American ancestors reached the New World in a single, initial migration from Siberia at most 23,000 years ago, only later differentiating into today's distinct groups.

The study settles a long-standing debate about when the ancestors of Native Americans crossed the Bering land bridge.

An international team, led by researchers at the Centre for GeoGenetics at the University of Copenhagen, sequenced the genomes of 31 living Native Americans, Siberians and people from around the Pacific Ocean, and the genomes of 23 ancient individuals from North and South America, spanning a time between 200 and 6,000 years ago.

They found the ancestral pool split into two main branches about 13,000 years ago, coinciding with glacier melt and the opening of routes into the North American interior.

These became the groups which anthropologists refer to as Amerindians (American Indians) and Athabascans (a native Alaskan people).

Previous research had suggested that Amerindian and Athabascan ancestors had crossed the strait independently.

"Our study presents the most comprehensive picture of the genetic prehistory of the Americas to date," says Maanasa Raghavan, one of the study's lead authors from the Centre for GeoGenetics.

"We show that all Native Americans, including the major sub-groups of Amerindians and Athabascans, descend from the same migration wave into the Americas."

This was distinct from later waves which gave rise to the Paleo-Eskimo and Inuit populations, she adds.

Given that the earliest evidence for the presence of humans in the Americas dates to 15,000 years ago, the first ancestors may have remained in Beringia for about 8000 years before their final push into the New World, the team say.

This is much shorter than the tens of thousands of years of isolation theorised by some earlier research.

But diversification into the distinct tribes we know today, happened only after arrival in the Americas, not before.

Indigenous Australian connection

The team also found a later gene flow into some Native Americans from groups related to present-day East Asians and Australo-Melanesians.

"It's a surprising finding and it implies that New World population were not completely isolated from the Old World after their initial migration. We cannot say exactly how and when this gene flow happened, but one possibility is that it came through the Aleutian Islanders living off the coast of Alaska," says Professor Eske Willerslev also from Centre for GeoGenetics at the University of Copenhagen.

A second study, published in Nature, reveals that some Amazonians descend from forefathers more closely related to the Indigenous peoples of Australia, New Guinea and the Andaman Islands than present-day fellow Native Americans.

The US and Brazilian researchers analysed genome-wide data from 30 Native American populations of Central and South America, and from 197 non-American populations sampled worldwide.

"Present-day groups in South America have a small but distinct genetic link to Australasians," says co-author Pontus Skoglund of Harvard Medical School,

This may explain a long-standing riddle: why, if Native Americans came from Eurasia, do some early American skeletons share traits with present-day Australasians?

But how and when this forefather came to the Americas remains "an open question," say the researchers.

Related: Ancient genome offers clues to human waves