Ancestors of people living in what is today West Africa may have reproduced with a species of ancient humans unknown to scientists, new research suggests.

Scientists know Europeans mated with Neanderthals and people in Oceania with Denisovans, but a new study published Wednesday in the peer-reviewed journal Science Advances found that genetic variation within West African populations is best explained by the presence of a new ancient human species altogether.

With difficulties in obtaining a full fossil records and ancient DNA, scientists’ understanding of the genetic diversity within West African populations has been poor. To get a fuller picture, researchers at University of California, Los Angeles compared 405 genomes of West Africans with Neanderthal and Denisovan genomes.

Sriram Sankararaman, one of the study’s authors, told NPR that the researchers used statistical modeling to figure out which parts of the DNA they were analyzing did not come from modern humans, then compare those to the two ancient hominin species. What they found is the presence of DNA from “an archaic ghost population” in modern West African populations’ genetic ancestry.

“We don’t have a clear identity for this archaic group,” Sankararaman told NPR. “That’s why we use the term ‘ghost.’ It doesn’t seem to be particularly closely related to the groups from which we have genome sequences from.”

Sankararaman and co-author Arun Durvasula found this introgression, or sharing of genetic information between two species, between the “ghost population” and ancestors of West Africans may have occurred within the last 124,000 years. The “ghost population” likely split from humans and Neanderthals into a new species between 360,000 to 1.02 million years ago, the study says.

The study also says the breeding may have occurred over an extended period of time, rather than all at once.

“It’s very likely that the true picture is much more complicated,” Sankararaman told the Guardian.

John Hawks, an anthropologist at the University of Wisconsin-Madison, told the newspaper that studies like this one, “Open a window showing us that there is much more than we thought to learn about our ancestors.”

“Actually knowing who those ancestors were, how they interacted, and where they existed is going to take fieldwork to find their fossil and archaeological remains,” he told the Guardian. Hawks was not involved in the study.

This interbreeding may also have a great impact on the genetic makeup of modern populations: Anywhere from 2% to 19% of their genetic ancestry could be derived from the “ghost population.”

However, whether that affects modern day people will require further research, Sankararaman says.

“Are they just randomly floating in our genomes? Do they have any kind of adaptive benefits? Do they have deleterious consequences?” he told NPR.

Follow USA TODAY’s Ryan Miller on Twitter: @RyanW_Miller