New research has found even more evidence in support of the amazingly complex relationships between our ancient ancestors and those of other archaic human species over the past hundreds of thousands of years. Using new analytical techniques to estimate the percentage of ancient hominin DNA now found in living people, Dr Ryan Bohlender has found evidence for multiple interbreeding events within and outside of Africa. His findings were presented at the American Society of Human Genetics in Vancouver, B.C.

This latest research backs up earlier studies that also found that the relationship between these ancient species is a lot more muddled than many thought. While initially there may have been a clean and simple split as the branches of our evolutionary tree divided, over time there were a multitude of crossovers as our ancestors mated with at least two, but probably many more distinct human species that emerged over the last 500,000 years or so.

It is now common knowledge that modern humans once had relations with our Neanderthal cousins, resulting in the fact that everyone outside of Africa has a little bit of Neanderthal still residing in their DNA. But this revelation only occurred within the last decade, as DNA analysis techniques became more routine and advanced. This latest study has added more to the story of our interactions, finding that contrary to what earlier research found, it seems that the level of mixing with Neanderthals is not significantly different between Europeans and East Asians.

But that wasn’t the only cross-species sex our ancestors got up to. Known only from a few fragments of bones, the enigmatic Denisovans evolved from the same branch that gave rise to the Neanderthals, and are thought to contribute to between 3 and 5 percent of the genomes of aboriginal Australians. That is literally all we know about the Denisovans.

But even more mysteriously, it seems that there is genetic evidence for another species that may have lived in Southeast Asia around the same time as the Denisovans and us.

While no physical fossils have ever been found, genetic analysis of Pacific Islanders seems to show that a distinct species of ancient human, thought to be another sister group to the Neanderthals and Denisovans, may have been breeding with our ancestors as they moved into Southeast Asia. Who these may have been, nobody knows. Some suggest that the environment in this part of the world means that fossils are unlikely to survive, while others argue that as we know so little about the genetic diversity of the Denisovans, this mysterious DNA could still be from them.

Yet this is not the first time that researchers have discovered what they think is evidence for an ancient species of human hidden in our genes. Analysis of people’s genetics in Africa show that something similar may have occurred there too, with suggestions of another cross-species romance. But there is still a hole in African genetics.

“Africans have been underrepresented in genetics research – they’re not as well studied as European and Asian populations, yet they are more diverse genetically than any other group,” Dr Bohlender said.

With the rich evolutionary history of hominins within Africa, this will likely yield even more interbreeding events, and it is highly likely there are other yet to be revealed instances from other parts of the planet. As ever, things are never quite as straightforward as they first seem.