The human family tree just got another -- mysterious -- branch, an African "sister species" to the heavy-browed Neanderthals who 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.

There's only way one that genetic material could have made it into modern human populations.

"Geneticists like euphemisms, but we're talking about sex," said Joshua Akey of the University of Washington in Seattle, whose lab identified the foreign 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.

"We're calling this a Neanderthal sibling species in Africa," Akey said. He added that the interbreeding likely 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.

Akey said that present-day Europeans show no evidence of the foreign DNA, meaning the mystery people were likely confined to Africa.