How and when the first people arrived in the Americas has long been a contentious debate, and despite loads of new data from the burgeoning field of genomics, two separate studies published Tuesday show the debate is far from over.
Both studies, one published in the journal Science and the other in Nature, sequenced DNA information to try to pin down the ancestry of Native Americans. They reach different and somewhat contradictory conclusions. The Science article describes a single pulse of migration from Asia, whereas the Nature article depicts a genetic history that's more complicated.
While there's a general consensus that people first arrived in the Western Hemisphere from Asia, via a land bridge connecting Siberia and Alaska, other remnants of their genetic code remain a mystery.
At the heart of the debate is how closely related Native American populations are to people from Australia and its neighboring Melanesian islands. While it's well-known among scientists that the populations share a bit of their genes, what's at issue is how and when they got all mixed up.
One theory, called the Paleoamerican model, suggests the American continents were settled by two waves of migrants. The first brought a group of people from Asia in the last millennium of the Pleistocene (which ended around 11,000 years ago), but they split off before migrating across the Bering Strait, giving rise to both the first Americans — called the Paleoamericans — and the Australo-Melanesians. The second migration took place when East Asians swept the Americas, replacing the Paleoamericans and making them look more like Asians than native Australasian people.
The Science article challenges the Paleoamerican model, arguing that there was only one wave of migration across the Bering Strait and suggesting the apparent relation to Australasian people happened relatively recently.
Authors of the Nature article suggest there was a mystery population with Australasian ancestry that was introduced to the region very early on, consistent with the Paleoamerican model. They didn't speculate on how or when that happened, but they dismissed any suggestion that only one wave of migration could account for the amount of genetic diversity in Native American DNA.
The discrepancy highlights how contentious the debate is on the first Americans. It also shows that, even in a heavily data-driven field such as genetics, there's still plenty of room for opinion.
"It's often like that in science," said Rasmus Nielsen, an author of the Science study and a professor at the University of California at Berkeley. "There are things that are for sure and other things that are up for interpretation."
Nielsen's team suggests that the relationship between Native Americans and Australasians could be the result of Alaskan populations that are genetically closer to Australasians, such as the Aleutian Islanders. The other research team quickly dismissed that theory.
"I think that's very weak — it's very, very speculative," said David Reich, an author of the Nature study and a geneticist at Harvard Medical School, adding that the sample size leading to that conclusion in the Science article was too small and could lead to bias.
"It's nothing about the sample; it's all in the interpretation," Nielsen retorted.
But that's just how science goes sometimes. It makes sense that the two studies were published around the same time, given the recent explosion in the field of genomics. The timing is a bit strange, though, because authors from both teams said they weren't aware of the other's work while doing their research. The Science paper would have been published Thursday, but because of the similarities between the two studies, Science moved up the publication date to match Nature's.
"Technology moves in leaps," Nielsen said. "We go to a lot of meetings together, so people may be getting inspired at the same time."
There are a few things in the genome data that just about everyone agrees on, and they're mostly related to the timing. Data from the Science article reinforces evidence on when the first Americans crossed over the land bridge to Alaska — no earlier than 23,000 years ago (not the Inuit, though; they came later). The data also found that the amount of time that the population spent in the land bridge area could not have been more than 8,000 years.
The data also suggests that after they entered America, about 13,000 years ago, they split into two genetic branches, one that is dispersed across the Americas and one that is just in North America.