This genetic connection caught many scientists off guard, and it remains “one of the most intriguing and poorly understood events in human history,” the researchers wrote in the new study.

To investigate the Y signal further, a team of scientists in Brazil and Spain dove into a large dataset containing the genetic data of 383 Indigenous people from different parts of South America. The team applied statistical methods to test whether any of the Native American populations had “excess” genetic similarity with a group they called the Australasians, or Indigenous peoples from Australia, Melanesia, New Guinea and the Andaman Islands in the Indian Ocean.

In other words, the team was assessing whether “a given Native American population shared significantly more genetic variants with Australasians than other Native Americans do,” Hünemeier and Araújo Castro e Silva said. South American groups that did have more genetic similarities with Australasians were interpreted by the new researchers as being descendants of the first Americans and Australasian ancestors, who coupled together at least 15000 years ago.