Our ancestors sure were a promiscuous lot. But they didn’t just limit themselves to sleeping with our own species, as ancient humans are known to have mated with other species of human. New evidence now shows that when our ancestors got to Asia, they simply couldn't resist the ancient Denisovans who were already living there, and mated with them in at least two separate events.

We have only been able to discover the existence of the Denisovans due to the advancement in genetic sequencing seen over the last decade or so. This makes the hominins one of the most intriguing species of humans discovered in recent times, and possibly ever.

It all started when archaeologists uncovered a scattering of ancient human remains in a remote cave in the Altai Mountains, dating to roughly 40,000 years ago. The unassuming fossil of a tiny finger bone and a few teeth were nothing special, but then a team of researchers decided to see if they could extract any DNA from them. The results rocked the world of anthropology.

They showed that the bones once belonged to an entirely new species of hominin, distinct from Neanderthals and modern humans who were also known to be living in the region at the same time. Considering they didn’t have a full skeleton to go on (in fact, we still don’t) this was an extraordinary conclusion. It was made more incredible when the genome of the Denisovans was compared to modern humans living today. 

It turned out that modern Papuans inherited about 5 percent of their genome from Denisovans, and when researchers started searching further afield, the ancient hominin's genes began cropping up throughout Asia. The relatively high amount of Denisovan DNA found in Papuans led researchers to assume that as their ancestors made their way through Asia, they mated with the ancient species before getting to Papua New Guinea. Following this, some people migrated back to East Asia, bringing the Denisovan genes with them.

“But in this new work with East Asians, we find a second set of Denisovan ancestry that we do not find in the South Asians and Papuans,” explained Sharon Browning, the paper's senior author. “This Denisovan ancestry in East Asians seems to be something they acquired themselves.”

The suggestion, therefore, is that there were two independent mating events. One between the ancestors of Papuans and Denisovans, and the other between the ancestors of all other East Asians and Denisovans. The former likely happened as humans migrated along a southern route, and the latter as humans went north. What is more, the evidence seems to suggest that those on the mainland are more closely related to the extinct hominin than those from Papua New Guinea. The findings are published in the journal Cell.

“When we compared pieces of DNA from the Papuans against the Denisovan genome, many sequences were similar enough to declare a match, but some of the DNA sequences in the East Asians, notably Han Chinese, Chinese Dai, and Japanese, were a much closer match with the Denisovan,” said Browning. 

The team now want to turn their attention to Africa, where they are sure more "ghost" human species exist in the genome of people still alive today, even if the environmental conditions mean that any fossil remains are unlikely to have survived.