In a rare and high-profile scientific dispute, two recent genetic studies have reached opposing conclusions on the origins of Native Americans.
The disagreement is between a team of 101 scientists led by the University of California, Berkeley, publishing in Science, and a team of nine scientists led by Harvard Medical School, publishing in Nature.
The Berkeley team concludes that ancestral Native Americans entered from Siberia – across the land bridge that existed in the Bering Straits during the last Ice Age – no earlier than 23,000 years ago in a single wave, and then split into two genetically distinct populations around 13,000 years ago during the melting of the glaciers at the end of the last Ice Age. The Harvard team concludes that there were two, separate, founding populations of the Americas.
Both papers were based on a comparison of genome patterns of many living groups as well as of ancient skeletons. Both conclusions centre on the same new finding – that some remote populations living in the Brazilian Amazon share up to two percent of their ancestry with the indigenous peoples of Australia and New Guinea. But the researchers disagree on the source of that ancestry.
'It's incredibly surprising,' said Professor David Reich, a geneticist at Harvard Medical School and senior author of the Nature paper. 'There's a strong working model in archaeology and genetics, of which I have been a proponent, that most Native Americans today extend from a single pulse of expansion south of the ice sheets – and that's wrong. We missed something very important in the original data.'
The team analysed publicly available genetic information from 21 Native American populations from Central and South America. They also analysed DNA from nine additional Brazilian populations to ensure the link they saw wasn't an experimental artifact. The researchers then compared these genomes to those from around 200 non-American populations. The Australasian link persisted. The scientists concluded that this was evidence of a second ancestral group, perhaps as old as the first Americans – which they call Population Y – and therefore that the single migration theory doesn't fit.
Yet their rivals at Berkeley disagree with this interpretation. They argue that the Australasian–Amazonian genetic link comes from a group of people carrying those genes to the Americas much later – around 10,000 years ago.
When they saw the Australasian signals in the Amazon, the Berkeley team wanted to see how far back those genes went. They extracted more DNA from the ancient remains of Native American people whose skulls are physically similar to modern indigenous Australasians. But the DNA of those ancient remains didn't bear any similarity to current Australasians – it was like the DNA of modern Native Americans.
'What does that tell us? It tells us this signal of Melanesian and Australian DNA in Brazil, somewhere in isolated populations, it's probably not something that's very old,' said Professor Rasmus Nielsen from the Department of Integrative Biology at Berkeley and co-author of the paper in Science.
Furthermore, he offers up an alternative explanation for the link. The researchers found traces of the Australasian signals in people from the Aleutian Islands, off the Alaskan south coast. 'If you're sailing from Siberia to America, you follow this chain of islands,' said Professor Nielsen, as reported in The Verge, suggesting that the Australian ancestors used boats to get to the Americas, after the melting of the Bering land bridge.
Unless the skeleton of an ancestral Population Y member turns up, the debate around whether Population Y was in America from the start or sailed over later looks set to continue.