Both papers suggest contradictory interpretations of a new perplexing find: some remote populations living in the South American Amazon share a strong genetic connection with the indigenous peoples of Australasia and Melanesia — the regions of Oceania containing Australia, New Zealand, New Guinea, and its surrounding islands. Approximately 2 percent of today's Amazonians come from this Australasian lineage.
Contradictory interpretations of a new perplexing find
That link is surprising; it's not present anywhere else in the Americas, and it seems to upend some previously held theories about where the first Native Americans came from. Many researchers believe that the Americas were populated by a single migration of people from East Asia crossing into America over the Bering Strait. In ancient times, the strait's waters were much lower than they are now, exposing a land bridge that connected Alaska and Siberia.
But in light of these new findings, the researchers had a hard time explaining how the Australasian-Amazon link fit into that model. If the Americas were populated by a single migration, then all Native Americans should really only share traits with current East Asian and Siberian populations. "This link between the Amazon populations and the Australasian populations — this is very surprising to us and to many scientists," says Pontus Skoglund, first author of the Nature paper and a postdoctoral researcher at Harvard University. "We spent over a year trying to refine our analysis, and exclude that there are no other explanations."
Skoglund and his research team argue that the Australasian DNA signals in the Amazon mean the single migration theory doesn't work. In fact, they propose that there were at least two additional waves of migrations after the first initial migration of East Asians across the Bering Strait, or perhaps there was a long stream of migrations tens of thousands of years ago. "There were multiple pulses of people going into the Americas, and these pulses may have been genetically quite similar, but there were at least such differences where we are able to detect this contrast today," says Skoglund.
Thousands of years ago, the waters in the Bering Strait were low enough to expose a land bridge that people could walk across. (NASA)
Yet researchers at the University of California-Berkeley say that interpretation isn't quite right. They also observed the Australasian-Amazon link during their research, but they argue that the DNA signals don't eliminate the single migration theory. Instead, further tests from the Science team indicate that Population Y — the group of people containing the Australasian link — came to the Americas much later, around 10,000 years ago. "What we found was that the divergence time between people that landed in East Asia and Native Americans happened about 23,000 years ago," says co-author Rasmus Nielsen, from the department of integrative biology at UC Berkeley.
Both papers do agree in some ways
Nielsen and his team write in Science that when they observed the Australasian signals in the Amazon, they decided to do some more testing. So the researchers extracted additional DNA from the ancient remains of Native American people in Central and South America whose skulls are physically similar to the indigenous Melanesians and Australians of today. They wanted to see how far back this link could be observed. But the DNA of those ancient remains didn't have any affinity to current Australasians; it was just 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," says Nielsen.
Instead, the Science team offers up another explanation for the link. The researchers also found a few traces of Australasian signals of people living in the Aleutian Islands — a small chain of islands off the southern coast of Alaska. "If you’re sailing from Siberia to America, you follow this chain of islands," says Nielsen. The team theorizes that a much smaller group of people that traveled through the Aleutian Islands brought the Australasian link to America — sometime around 10,000 years ago.
The type of terrain traveled by Native Americans when migrating to the Americas
Both papers do agree in some ways; they argue that an additional source population is responsible for this strange Australasian signal in the Amazon. The question is when.
"Neither paper is making an argument that Australians came over and peopled America," says Cecil Lewis, an associate professor of anthropological genetics at the University of Oklahoma, who was not involved in either study. "They're arguing that in the old world and in Siberia and East Asia, there was potentially more than one source population that constituted the gene pool that ended up in the Americas."
There’s no way to know for sure until all possible theories have been explored
The Nature team thinks that Population Y came over sometime in ancient times, while the Science team thinks Population Y came over much later. Lewis says both ideas hold merit, and that both research teams are extremely skilled at analyzing genetic data. But he says there's room for questioning both interpretations. Additionally there are even more theories that could explain the Australasian link, says Lewis, who brings up the possibility of genetic drift.
"Genetic drift means when populations reproduce over and over again, we expect their genetic makeup to change slightly from random chance," says Lewis. "When populations are small in size, those random fluctuations are quite pronounced. So one possible explanation is what we’re seeing in the Amazon jungle could be attributable to chance effects."
There’s no way to know for sure until all possible theories have been explored and all but one have been completely rejected. That means more DNA of ancient remains must be collected to paint the best possible picture of Native American origins.