For decades, science posited that a single population crossed the iced-over Bering Strait around 15,000 years ago and gradually colonized the Americas, from north to south.

That story is beginning to get a bit more complicated – and a bit more diverse, according to a new study in the journal Nature published today.

Genetic data showed that there were compelling similarities between a few Native American groups in Brazil and indigenous groups in Australia, New Guinea and the Andaman Islands, according to the team from Harvard Medical School.

Those similarities should not have been there. They bucked the single “founding population” model of the colonization of Americas, the Harvard team said.

“That was an unexpected and somewhat confusing result,” said David Reich, Harvard professor of genetics and senior author of the study. “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 Amazonian populations of the Surui, Karitiana, and Xavante had a genetic ancestor more closely related to Australasian natives, as noticed by Pontus Skoglund, the first author of the paper and a postdoctoral researcher with Reich.

The mysterious ancestor has been named Population Y, short for the Tupi word for ancestor, “Ypykuera,” according to the Harvard team. The ancestor doesn’t appear to have left genetic traces in other Native American groups across South, Central or North America – and doesn’t appear to exist back across the Pacific, either.

“We’re done a lot of sampling in East Asia and nobody looks like this,” Skoglund said. “It’s an unknown group that doesn’t exist anymore.”

“We spent a really long time trying to make this result go away and it just got stronger,” added Reich.

Reich and his team of researchers have also found other wrinkles in the single-population-colonization theory in recent years. In 2012, they published findings that indigenous populations of northern Canada came to the Western hemisphere in at least two subsequent waves of travel.

The vast majority of Native Americans genetic information has so far showed connection to the migration across the Bering Strait, including prehistoric remains like that of the 9,000-year-old Kennewick Man, recently proven to have Native American genetic lineage.