The human family tree just got another mysterious branch, an African sister species to the heavy-browed Neanderthals that once roamed Europe.

While no fossilized bones have been found from these enigmatic people, they did leave a calling card in present-day Africans: snippets of foreign DNA.

Theres only one way that genetic material could have made it into modern human populations.

Geneticists like euphemisms, but were talking about sex, said Joshua Akey of the University of Washington in Seattle, whose lab identified the mystery DNA in three groups of modern Africans.

These genetic leftovers do not resemble DNA from any modern-day humans. The foreign DNA also does not resemble Neanderthal DNA, which shows up in the DNA of some modern-day Europeans, Akey said. That means the newly identified DNA came from an unknown group.

Were calling this a Neanderthal sibling species in Africa, Akey said. He added that the interbreeding probably occurred 20,000 to 50,000 years ago, long after some modern humans had walked out of Africa to colonize Asia and Europe, and around the same time Neanderthals were waning in Europe.

The find offers more evidence that for thousands of years, modern-looking humans shared the Earth with evolutionary cousins who later died out. And whenever the groups met, whether in Africa or Europe, they did what came naturally they bred. In fact, hominid hanky-panky seems to have occurred wherever humans met others who looked kind of like them a controversial idea until recently.

In 2010, researchers from the Max Planck Institute for Evolutionary Anthropology in Germany announced finding Neanderthal DNA in the genomes of modern Europeans.

Barrel-chested people whose thick double brows, broad noses and flat faces set them apart from modern humans, Neanderthals disappeared around 25,000 to 30,000 years ago.

Another mysterious group of extinct people recently identified from a 30,000-year-old finger bone in Siberia known as the Denisovans also left some of their DNA in modern-day Pacific Islanders.

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Neanderthal-type species once roamed Africa, DNA shows