It’s been known for some time that our modern human ancestors interbred with other early hominin groups like the Neanderthals. But it turns out they were even more promiscuous than we thought.

New DNA research has unexpectedly revealed that modern humans (Homo sapiens) mixed, mingled and mated with another archaic human species, the Denisovans, not once but twice—in two different regions of the ancient world.

All we know about the mysterious Denisovans comes from a single set of human fossils found in a cave in the Altai Mountains of Siberia. In 2008, scientists first discovered a bone from a pinky finger in the cave, and concluded it belonged to a previously unknown ancient hominin who lived between 30,000 and 50,000 years ago. They called the species the Denisovans (pronounced “De-NEE-soh-vens”) after the cave where the fossilized finger was found.

A Neanderthal skull and some of the Mousterian tools used by the Neanderthals are shown in this display during a tour of the ‘Ancestors’ exhibit at the American Museum of Natural History 412 (Photo by Getty)

After the genome of the finger’s owner, a young girl, was published in 2010, researchers went on to discover traces of the Denisovan ancestry in two groups of modern-day humans. Some Melanesians (who live in Papua New Guinea and other Pacific islands) were found to have around 5 percent of Denisovan ancestry, while some East and South Asians have around 0.2 percent. One particular gene mutation, which the Denisovans are thought to have passed to modern Tibetans, allows them to survive at high altitudes.

Researchers assumed the Denisovan ancestry found in Asia was due to migration from Oceania, the larger region containing Melanesia. But recently, scientists from the University of Washington in Seattle stumbled on something surprising: evidence for a second, distinct instance of humans getting hot and heavy with Denisovans.

In their analysis of more than 5,600 whole-genome sequences from individuals from Europe, Asia, the Americas and Oceania, the research team looked for ancient DNA, which stands out due to the larger number of mutations that have developed over hundreds of thousands of years. When they found the ancient genetic information, they compared with Denisovan DNA and Neanderthal DNA to determine its origin.

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What they found was a distinct set of Denisovan ancestry among some modern East Asians—particularly Han Chinese, Chinese Dai and Japanese—ancestry not found in South Asians or Papuans. According to the study’s findings, published in the journal Cell this week, this Denisovan DNA is actually more closely related to the sample taken from the girl in the Siberian cave.

“Although the Papuans ended up with more Denisovan ancestry, it turns out to be less similar to the sequenced Denisovan,” Sharon Browning, a research professor of biostatistics at the University of Washington School of Public Health and senior author of the study, told New Scientist. “Our research demonstrates that there were at least two distinct populations of Denisovans living in Asia, probably somewhat geographically distant.”

(Credit Browning et al.Cell)

Browning and her colleagues assume that modern humans mixed with the Denisovans shortly after migrating out of Africa, around 50,000 years ago. While they’re not sure of the location, they believe the interbreeding occurred in at least two places: eastern Asia, and further south, in Indonesia or Australia.

While the new study confirms that modern humans interbred at least three times with ancient hominins—once with Neanderthals, and twice with the Denisovans—it also raises the possibility of even more extensive intermixing on the part of our ancient ancestors. As reported in New Scientist, one-quarter of the ancient DNA that the researchers found in living humans didn’t match up with either Denisovan or Neanderthal DNA, suggesting there may be other mystery mates out there to find.