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The molar tooth of a Denisovan. Image courtesy of David Reich et al., Nature

It’s weird to think that tens of thousands of years ago, humans were mating with different species—but they were. That’s what DNA analyses tell us. When the Neanderthal genome was sequenced in 2010, it showed that as much as 1 to 4 percent of the DNA of non-Africans might have been inherited from Neanderthals. (Given that no African populations are known to have Neanderthal DNA, the matings must have occurred as modern humans moved into Europe and Asia). Scientists also announced last year that our ancestors had mated with another extinct species, and this week, more evidence is showing how widespread that interbreeding was.

We know little about this extinct species. In fact, we don’t even have a scientific name for it; for now, the group is simply known as the Denisovans. The Denisovans were discovered after a group of scientists led by Johannes Krause, now at Tübingen University in Germany, analyzed DNA extracted from the tip of a child’s finger bone. The bone was found in 2008 in Denisova Cave in the Altai Mountains of Siberia and was dated to between 30,000 and 50,000 years ago. At that time, you’d expect to find either modern humans or Neanderthals living in Eurasia. But the finger bone’s DNA didn’t match human DNA or Neanderthal DNA. Some other kind of hominid must have also been living in the region.

A subsequent study of Denisovan DNA, in Nature, further analyzed the finger bone’s DNA and DNA from an adult molar tooth also found in Denisova Cave. Based on the physical characteristics of the tooth, it didn’t appear to be from a human or a Neanderthal, and the DNA was similar to that from the finger. David Reich of Harvard University and his colleagues furthermore compared Denisovan DNA with modern human DNA and concluded that as much as 5 percent of the DNA of people living in Melanesia could be from Denisovans—evidence of more interbreeding. Another study confirmed that Australian aborigines, Polynesians and other people of Oceania also had a Denisovan heritage. Now it appears that Southeast Asians do as well. This week Pontus Skoglunda and Mattias Jakobsson, both of Uppsala University in Sweden, reported in PNAS that Denisovan DNA may account for about 1 percent of modern Southeast Asian DNA.

The idea that our ancestors mated with other species may not be too shocking. Species today will mate with other closely related species if they come across each other in nature (or captivity). This occurs among olive baboons and hamadryas baboons that have overlapping ranges in Ethiopia. The idea probably seems surprising because it’s hard to imagine we once shared the planet with beings so similar to us. What was it like to meet other human-like individuals who weren’t quite human?

The story of our past inter-species matings is far from complete. We still don’t know who the Denisovans really were. Today, the fragment of the finger bone and the molar tooth are the sole fossils that scientists have assigned to the group. It’s impossible to say what physical features distinguished the species. But it is possible we’ve already found other Denisovan fossils. Denisovans could belong to a species whose DNA we’ve never been able to analyze, such as Homo heidelbergensis. And there are some hominid fossils in China that are hard to fit into any of the known species. If we could read their DNA, perhaps it would reveal they were Denisovans, too.