A newly discovered "sister species" to the Neanderthals that once roamed Europe has left behind snippets of DNA in present-day Africans.
And there is only one way that genetic material could have made it into modern human populations, according to The Washington Post.
"Geneticists like euphemisms, but we're talking about sex," said Joshua Akey of the University of Washington in Seattle, whose lab identified the mysterious DNA in three groups of modern Africans.
The DNA showed up in populations across the continent, both in pygmies of central Africa and in Tanzania's Hadza and Sandawe hunter-gatherers, reported TG Daily. The new discovery doesn't resemble human or Neanderthal DNA, meaning it came from an unknown group.
"Fossils degrade fast in Africa so we don't have a reference genome for this ancestral lineage, but one of the things we're thinking is it could have been a sibling species to Neanderthals," said Akey.
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According to the Post, he also added that the interbreeding with humans probably occurred 20,000 to 50,000 years ago, which was long after some modern humans made their way out of Africa to colonize Asia and Europe, and around the same time Neanderthals were starting to dwindle in Europe.
This isn't the first time a species other than humans and Neanderthals has been found to contribute to the makeup of modern-day humans. TG Daily reported that in 2010, DNA analysis showed that a species known as Denisovans gave as much as 6 percent of the genome of present-day New Guineans.