Over 40,000 years ago, two other hominins roamed around Eurasia with the ancestors of modern humans: Neanderthals and a mysterious species known as the Denisovans. By the sound of things, a strange love triangle existed between this promiscuous bunch of hominins.

A fleck of bone discovered in a remote cave in Siberia suggests Neanderthals and Denisovans had children together, as reported today in the journal Nature. Researchers have long argued that these two species must have interbred, but no physical evidence had been found to back it up – until now.

In 2012, scientists unearthed a small bone fragment of a hominin at the Denisova Cave in Siberia. Recent genetic analysis of the ancient remains has revealed that it once belonged to a teenage female, no younger than 13 years old, that had a Neanderthal mother and a Denisovan father. 

“We knew from previous studies that Neanderthals and Denisovans must have occasionally had children together," study author Viviane Slon, a researcher at the Max Planck Institute for Evolutionary Anthropology (MPI-EVA), said in a statement. "But I never thought we would be so lucky as to find an actual offspring of the two groups."

Making things even more intriguing, the Neanderthal mother appears to be genetically closer to the Neanderthals that lived in western Europe than to the Altai Neanderthals that are known to have lived earlier in the Denisova Cave. The genome analysis also showed that the Denisovan father had at least one Neanderthal ancestor in his family tree. This suggests that the newly discovered Neanderthal-Denisovan "love child" was not a one-off.

"So from this single genome, we are able to detect multiple instances of interactions between Neanderthals and Denisovans," explains study co-author Benjamin Vernot of the MPI-EVA.

Svante Pääbo, director of the Department of Evolutionary Genetics at the MPI-EVA, added: "Neanderthals and Denisovans may not have had many opportunities to meet. But when they did, they must have mated frequently – much more so than we previously thought."

Taking a few steps back, you might be wondering about Denisovans themselves. Unfortunately, even scientists know relatively little about them. The Denisova Cave, where this discovery was made, is the only place where physical remains of Denisovans have ever been discovered. However, researchers do know that Denisovans, the closest extinct relatives of currently living humans, split from Homo sapiens around 744,000 years ago.

It’s also known that early humans mated with Denisovans. Remarkably, the genetic legacy of Denisovans is believed to be what allows Tibetans to thrive in high altitudes, as well as influences the immune systems and allergies of certain people. 

Of course, it’s also widely known that early humans had offspring with Neanderthals. In fact, most people of European or Asian descent have approximately 2 percent Neanderthal DNA.

The story of ancient hominins and early humans is a murky one, and as this research shows, it remains a tale filled with mystery and intrigue.