The story of human evolution is no stranger to inter-species breeding – in fact, it's practically built on it. Homo sapiens (as in, us) regularly romped with Neanderthals over the course of past millennia. Our species have also enjoyed a fair amount of hanky panky with our mysterious hominin cousins, the Denisovans, too. Just to complete this prehistoric love triangle, we've also discovered that Neanderthals and Denisovans had children together.

But it looks like this “incestuous” kanoodling might not have just started here. New research suggests that the shared ancestors of Neanderthals and Denisovans also interbred with an unknown population of hominin that separated from their branch of the family tree around 2 million years ago. 

Reporting in the pre-print server bioRxiv, anthropologists from the University of Utah gathered heaps of genetic data on Neanderthals, Denisovans, modern Europeans, and modern West Africans. They then ran this data through a number of computer models to learn how the different genes spread between the numerous groups.

For starters, this revealed a number of interesting insights. It suggested that the Neanderthals and Denisovans separated early in the middle Pleistocene, perhaps within the past 731,000 years.

However, after introducing the possibility of interbreeding into the mix, something within the models didn’t quite add up. The only way they could explain the current distribution of genes was by introducing another hypothetical human ancestor. They argue that so-called “neandersovans”, the shared ancestors of Neanderthals and Denisovans, trekked out of Africa early in the middle Pleistocene, long before the expansion of modern humans expanded into Eurasia.

Here, they seem to have bred with a separate “superarchaic” population of ancient hominin that was already in Eurasia at the time. This eventually gave rise to Neanderthals and Denisovans. We might not know anything about this mysterious “superarchaic” hominin population, but the data suggests it could have consisted of between 10,000 and 46,000 individuals.

The story beyond this becomes even hazier, although we know that modern humans probably expanded out of Africa around 185,000 years ago and, as mentioned, eventually interbred with Neanderthals and Denisovans.

"The large effective size of the 'superarchaic' population hints that it comprised at least two deeply-divided subpopulations, of which one mixed with 'neandersovans' and another with Denisovans. We suggest that around 700,000 years ago, 'neandersovans' expanded from Africa into Eurasia, endured a bottleneck of population size, interbred with indigenous Eurasians, largely replaced them, and separated into eastern and western subpopulations –Denisovans and Neanderthals," the study authors write.

For the time being, this “superarchaic” population are a total mystery as there’s no known evidence of them within the fossil record. Equally, there are no known fossils of the Neanderthal-Denisovan ancestors either. As this study shows, the story of human evolution is far from complete.