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.

There's only one way 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, whose lab identified the foreign DNA in three groups of modern Africans.

These genetic leftovers do not resemble DNA from any modern humans. The foreign DNA also does not resemble Neanderthal DNA, which shows up in the DNA of some modern 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 about 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.

The find offers more evidence that for thousands of years, modern-looking humans shared the Earth with evolutionary cousins that later died out. And whenever the groups met, they did what came naturally: They bred.

The once controversial idea that humans mated with other species is now widely accepted among scientists. In fact, hominid hanky-panky seems to have occurred wherever humans met others who looked kind of like them.

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

Link:
Sex with early mystery species of humans seen in DNA