Walking into a new world Stefanie Kay Candelada / EyeEm/Getty

The genomes of 15 ancient Americans, including six that are more than 10,000 years old, have been sequenced. The results reveal how people first spread through the Americas – and also throw up a major mystery.

The big picture is clear. Around 25,000 years ago during the last ice age, the ancestors of modern native Americans moved across the Beringian land bridge into what is now Alaska. They remained there for millennia because the way south was blocked by ice. Once a path opened up, groups of hunter-gatherers moved south very quickly.

“Once they are south of the ice, they are meeting with these amazing conditions, with lots of resources and no competition,” says Víctor Moreno-Mayar of the University of Copenhagen in Denmark, a member of the large international team that did the work.


North-south split

Southern native Americans split from northern ones around 16,000 years ago, the results suggest, and reached South America not long afterwards.

The genomes reveal many more details about this process. For instance, it appears some previously unknown group split away from northern native Americans at some point and then moved into South America around 8000 years ago, long after the initial migration.

But the study also adds to a big mystery: some groups in the Amazon are somewhat more closely related to the Australasians of Australia and Papua New Guinea than other native Americans are. The genomes show this “Australasian signal” is more than 10,000 years old. So where did it come from?

Working on an ancient site in Peru Mark Aldenderfer

If another group of people more closely related to Australasians crossed the Beringian land bridge at some point and moved down to the Amazon, why is there no trace of them in North America? And in the exceedingly unlikely event they somehow managed to cross the vast Pacific long before the Polynesians, how did they end up in the Amazon, on the other side of the Andes?

“The more we look at it, the more puzzled we are,” says Moreno-Mayar.

Based on shape of their skulls, it has also been claimed that many ancient humans found in the Americas cannot be the ancestors of present-day native Americans and instead belonged to a distinct group dubbed the “Paleoamericans”. “But we see again that they are most closely related to present-day native Americans,” says Moreno-Mayar.

This finding has led to the remains of one of the early humans, the 10,000-year-old Spirit Cave mummy, being returned to the Fallon Paiute-Shoshone Tribe after a long legal battle.

In 2015, Moreno-Mayar’s team showed that another supposed “Paleoamerican”, called Kennewick Man, was closely related to present-day native Americans.

Journal reference: Science, DOI: 10.1126/science.aav2621