Tens of thousands of years ago, the places that have since been named Russia and Alaska were not separated by water, but connected by a continuous bridge of land. People walked across that land, heading eastward from Asia. For a time, their journey was blocked by two gigantic ice sheets that smothered most of what is now Canada. But once the ice started melting, those early pioneers—the ancestors of today’s Native Americans—spread southward.

Sometime between 14,600 and 17,500 years ago, they split into two main lineages: a northern group and a southern one. The northern group gave rise to the Algonquian-, Na-Dené-, Salishan-, and Tsimshian-speaking peoples of Canada, and to the Ancient One—a famous 8,500-year-old skeleton found in Kennewick, Washington. The southern group included the ancestors of modern Central and South Americans, as well as Anzick-1—a 12,600 year old infant skeleton from the widespread Clovis culture.

This narrative comes from archaeology, linguistics, and most recently, genetics. By studying and comparing the DNA of the Ancient One, Anzick-1, and two infants from Upward Sun River in Alaska, scientists have started to piece together the movements—and existence—of ancient peoples. “There have been a lot of interesting ancient DNA findings in the Americas, but always based on one or two genomes,” says Christiana “Freddi” Scheib, from the University of Tartu. “I wanted to see if we could fill out this picture by getting as many ancient genomes as we could.”

Scheib and her colleagues ultimately analyzed DNA from the remains of 91 people, who lived in California’s Channel Islands and southwestern Ontario, between 200 and 4,800 years ago. And their study both confirms and complicates the existing story of how the Americas were peopled.

The team found evidence of two distinct lines of Native American ancestry, which separated after the Americas were first peopled. These lineages, known as ANC-A and ANC-B, roughly correspond to the southern and northern groups that had been previously identified. ANC-A is the southern branch, and includes Anzick-1 and the Clovis culture. ANC-B is the northern branch, and includes the ancient Ontarians and modern Algonquians. But the split between these groups was neither neat nor absolute. “We kept getting really weird answers and it took a while to figure out what they meant,” Scheib says.

They meant that a few thousand years after the two lineages had separated, they also rejoined and mingled. For that reason, the ancient Californians, and modern people from Central and South America, have genes from both groups. Even the Chilote and Huilliche peoples, who live in the south of Chile, can trace 70 percent of their ancestry to the “northern” ANC-B lineage. “These two populations split early and were isolated for a few thousand years,” says Scheib. “Then, they came back together while growing through South America.”

The two groups could have come together in North America before jointly entering the southern continent. Either of them could have entered South America first, before the other followed and repeatedly mingled with them. Something far more complicated could have happened, with both groups repeatedly mingling in North America, and repeatedly venturing into the southern continent. All of these possibilities could have happened, and for now, we have no way of telling which is right.

“This is an important step in dismantling our outdated, oversimplified models for Native American population history,” says Jennifer Raff from the University of Kansas. Deborah Bolnick from the University of Texas at Austin agrees: “There was not a simple division between the ancestors of North and South Americans, and the ancestral people whom geneticists have called the “First Americans” were probably not a single, uniform group either,” she adds.

They both suspect other complications are coming. “I think that most people have this idea that a single, simple wave [of people entered the Americas from Beringia],” Raff says, “but I suspect it was more like driblets of small groups moving down—and up—the coast, meeting up with each other to trade and marry. I expect that the more ancient genomes we get from North America, the more complex it’s going to become. It’s a very exciting time to be working in this field.”

But it’s also a time for introspection. The technology that allows us to wrest DNA from millennia-old bones has advanced rapidly, but the ethical and cultural dimensions to that work have not. Indigenous groups have had a long and troubled history with genetic science. The Ancient One’s genome, for example, was analyzed against the wishes of Columbia Plateau tribes, who wanted his remains repatriated and reburied. Barack Obama granted that wish in 2016, after a 20-year delay and a bitter dispute. But then, just last year, scientists published a study of remains from Chaco Canyon without consulting any Native peoples.

Many scientists now argue that it is unethical and exploitative to study ancient DNA without consulting with indigenous groups, and ideally before any work is done. “Ancestral remains should be regarded not as ‘artifacts’ but as human relatives who deserve respect,” wrote one group, Raff and Bolnick included, in a recent opinion piece.

Scheib realized this too late. She had come to evolutionary genetics from a career as a comedy writer, and being new in the field, she only caught wind of the ethical debate when she had already analyzed most of her remains. All of them had come from museums, and been classified as “culturally unaffiliated” with any particular tribe. “Today, it’s really not acceptable to get permission just from a museum,” she says. “You need to talk to the communities too, whether the remains are culturally affiliated or not.”

To find out how to proceed, Scheib and her supervisor Toomas Kivisild asked Ripan Malhi for advice. Malhi is an anthropologist at the University of Illinois at Urbana-Champaign who approaches indigenous communities before doing any work on ancient remains and establishes a formal memorandum of understanding with them. The tribes are involved in every step of his research from its very conception. “When Freddi and Toomas first got in touch, I was like: You can’t publish that,” he says. “And I thought that was the end of that.”

It wasn’t. In 2016, Scheib started contacting every federally recognized tribe that was potentially connected to the remains she had studied. She explained that she had already done the work, and presented her results. She asked them about their comments and concerns, and about questions they would want to answer in future work. In cases where she couldn’t get express permission from a tribe, she left the data gleaned from the respective samples out of her paper. She shared a draft of that paper with the tribes, and invited them to appear as co-authors. Several agreed. So did Malhi, who was impressed by Scheib’s efforts. “I felt disappointed that I hadn’t known better to do it a different way in the first place,” Scheib says. “But you can’t go back and change time.”

Scheib isn’t planning to do any more research on Native American ancient DNA, unless the tribes she talked to ask her to do something specific. Even then, she’s not sure if she’s the right person to do the work. “I put this to the communities: If you have any young people who are interested in genetics and interested in continuing this work, tell me, and I will find a way of getting them to our lab,” she says.