A new discovery offers the first direct evidence of interbreeding between ancient human relatives.

Richard (Bert) Roberts, Vladimir Ulianov and Maxim Kozlikin (clockwise from top) in the East Chamber of Denisova Cave, Russia. Sergei Zelensky/IAET SB RAS

We’ve never had a paternity case like this before. Scientists have discovered the first known ancient human relative whose parents came from two different parts of the human family tree. The mother, a Neanderthal. The father, a Denisovan—a mysterious hominin we still don’t know much about (today, modern humans are the only surviving type of hominin).

Their child was a 13-year-old girl who lived about 90,000 years ago. Scientists discovered her mixed parentage by analyzing DNA in her finger bone. The discovery adds to a growing body of evidence that different groups of ancient hominins interbred with each other—not just once, but many times.

This bone fragment (“Denisova 11″) was found in 2012 at Denisova Cave in Russia and represents the daughter of a Neandertal mother and a Denisovan father. Thomas Higham/University of Oxford

For a long time, scientists thought of groups like Homo sapiens, or modern humans, and Homo neanderthalensis, or Neanderthals, as separate species who couldn’t produce viable offspring. But since it’s now clear these groups did successfully mate with each other, some scientists have suggested ancient hominins like Neanderthals and Denisovans are actually subpopulations of modern humans.

Most of the evidence for this interbreeding comes from DNA tests showing that modern humans have small percentages of Neanderthal or Denisovan DNA, depending on their ancestry. The 13-year-old Neanderthal-Denisovan helps us piece this story together.

“The cool thing about this is, this is extremely direct evidence,” Svante Pääbo, a molecular geneticist who led the new research, told The Washington Post. “We’ve almost caught them in the act, so to speak.”

Drawing of a Neandertal mother and a Denisovan father with their child, a girl, at Denisova Cave in Russia. Petra Korlević

Pääbo and his colleagues published their research in the journal Nature. In 2010, Pääbo was also part of the team that discovered Denisovan DNA for the first time in Siberia’s Denisova cave. So far, that cave remains the only place that scientists have found Denisovan DNA. The remains of the 13-year-old girl of mixed ancestry are from that cave as well.

“This paper and other papers are showing the model of having isolated populations is not quite accurate,” said Emilia Huerta-Sanchez, a population geneticist at Brown University who was not involved in the new research, to The Washington Post.

In addition to swapping DNA, it’s possible that Neanderthals and humans exchanged knowledge and technology. Other recent research in Nature suggests that Neanderthals knew how to create fire 50,000 years ago.

“These other groups that coexisted with us…are part of our story,” Huerta-Sanchez says.