The old genomics is finally beginning to tell the history of America – and it looks messy.
An analysis of the dozens of old inhabitants of North and South America, who had lived for 11,000 years – one of the greatest comforts of old DNA from the region studied so far – suggests that the populations moved quickly and frequently. The findings were filed on November 8th Cell1 and Science2.
The studies suggest that North America was populated a lot over a few hundred years, and South America within one or two thousand years by related groups. Later migrations on and between the continents associated populations that live as far away as California and the Andes.
"These early populations are really sprouting across the continent," says David Meltzer, an archaeologist at Southern Methodist University in Dallas, Texas, who co-led Science study.
The studies also suggest that the prehistory of the Americas – the last large land mass to be solved – was just as complicated as that of other parts of the world.
"I think this series of newspapers will be remembered as the first glimpse of the real complexity of these multiple populations," says Ben Potter, an archaeologist at the University of Alaska Fairbanks. "It is awesome."
For decades, the populations of the Americas were painted in broad brushstrokes, using data from archaeological finds and DNA from modern people.
Scientists saw that groups crossed the Bering land bridge from Siberia to present-day Alaska and then steadily moved south when the last Ice Age ended. People who carried artifacts from a culture known as Clovis, including advanced projectile points, began populating the interior of North America around 13,000 years ago. For decades, researchers thought that people associated with this culture were the first inhabitants of the continents.
But the discovery of & # 39; pre-Clovis & # 39; settlements – including an almost 15,000-year-old site in the southern tip of Chile – pointed to an even earlier wave of migration to America, probably also on the Bering land bridge.
The first ex-DNA research from the region, the first of which was published in 2014, started to add detail to this photo. The genome of a baby boy buried some 12,700 years ago in Montana alongside Clovis artifacts3and taken from other old individuals4, hinted at two early populations of Native Americans.
The Montana baby, known as the Anzick boy, belonged to a population known as the Southern Native Americans and most closely related to the current indigenous people from South America. They split from North American Indians, who are genetically closer to many contemporary groups in eastern North America, about 14,600-17,500 years ago. The common ancestors of these two groups split 25,000 years ago from East Asians, and scientists established this year by sequencing the genome of 11,500-year-old human remains from Alaska.5.
But this timeline was based on a small number of old genomes from North and South America, and scientists expected further data to provide a much more detailed and complex picture of the early history of the continents, as well as later migrations in the region.
The same genes, far apart
The last two studies contain genome data from 64 old Americans, including more than a dozen copies older than 9,000 years.
They also offer the first detailed look at the ancient inhabitants of Central and South America and their early movements in the region.
In order to map these migrations, Meltzer and his colleague Eske Willerslev, a palaeogenetic at the University of Copenhagen and the University of Cambridge, UK, compared genetic data from the 12,700-year-old Anzick boy with genome sequences of 10,700-year-old remains of a cave of Nevada and 10400 year-old remains from the southeast of Brazil.
The taken were remarkably similar, despite the large distance between them, says Willerslev, pointing to a rapid population expansion of Alaska. "As soon as they come to the south of the continental ice caps, they explode and occupy the country," he says.
An independent team led by David Reich, a geneticist at Harvard Medical School in Boston, Massachusetts, also found evidence1 for a rapid expansion to South America, by analyzing 49 old genomes from Central and South Americans. These include a 9,300-year-old person from Belize, a 9,600-year-old from Southeast Brazil and 10,900-year-old relics from Chile.
Both teams documented several later human migrations to South America. For example, the Reich team discovered that the genetic signal of the earliest inhabitants, closely related to the Anzick boy, had largely disappeared from later South Americans, suggesting that by then different groups from the north would have settled.
Reich and his colleagues also found a bewildering connection between a 4200 year old man in the central Andes and old inhabitants of the Channel Islands off the coast of California. The team does not think that people migrated directly between these two regions – but instead they are connected by migrations by a population that was once more widespread.
Potter says that the main conclusions of the two articles are broadly consistent and outline a nuanced picture that becomes clearer with more data. "Complex and realistic are the two adjectives that I would use," he says.
Even with dozens of newly discovered old genomes from America, researchers probably still miss important aspects of the population history of the region, according to Reich. "There are many points that have not been filled in," he says. "I think that when these studies come to the surface, they make things more complicated than they do."
For example, the earliest migrations derived by the researchers seem to have been involved in people associated with the Clovis culture, but Meltzer wonders what has become of people associated with pre-Clovis sites. "If you go so fast through space, there is probably no one else in the way."
Another dormant mystery surrounds a discovery of 2015, made independent by both Rijks6 and Willerslev & # 39; s7 teams, that some modern inhabitants of the Amazon region seem to share genetic descent with Australasian groups that include both Papua New Guinean and Aboriginal Australians. Reich argued that this commonality refers to a previously unknown migration to America that disappeared from all but the most isolated groups in the Amazon.
But the Reich now raises this hypothesis because his team found no significant evidence of the Australasian origin in any of the old South and Central American genomes that it analyzed.
However, Willerslev has linked the genome of the 10400-year-old individual from Southeast Brazil to an Australasian lineage. The conclusion is that he is wondering if there have been migrations to America that date from before the sites behind the pre-Clovis sites. "I think we are faced with big surprises," he says.
Jennifer Raff, an anthropological geneticist at the University of Kansas in Lawrence, says that the new image of America is a revision of the earlier models, and more an elaboration. "It's not that everything we know is being overthrown, we only fill in details," she says. "We are now moving on to a much more detailed, much more accurate and richer history, which is where the field always went, and it's fun to be there now."