There's only one way the foreign DNA could have made it into modern human populations. "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.
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.
Heavyset people whose thick double brows, broad noses and flat faces set them apart from modern humans, Neanderthals disappeared 25,000 to 30,000 years ago.
Another mysterious group of extinct people known as the Denisovans - recently identified from a finger bone in Siberia - also left some DNA in modern Pacific Islanders.
And while modern humans and the newly found "archaic" Africans might be classified as distinct species, they managed to produce viable offspring. Likewise, donkeys and horses, lions and tigers, and whales and dolphins can mate and make babies.
"They had to be similar enough in appearance to anatomically modern humans that reproduction would happen," said Akey. But with no fossils in hand, it's impossible to say what these people looked like.
One thing is clear: This enigmatic group left its DNA all across Africa. The researchers found it in the forest-dwelling pygmies of central Africa and in two groups of hunter-gatherers on the other side of the continent: the Hadza and Sandawe people of Tanzania.
Starting a decade ago, a team led by Sarah Tishkoff of the University of Pennsylvania drew blood from five individuals in each of the three groups. Using the latest genetic technology, Tishkoff spent $150,000 to read, or sequence, the DNA of these 15 people.
The research was reported Wednesday in the journal Cell
"This is very cutting-edge population genetics work," said geneticist Spencer Wells, a National Geographic
"This 'whole genome' analysis the team performed is really revolutionizing our understanding of human history. It's an exciting time to be in the field, but it's difficult to interpret all the new data."
Wells said the oldest modern human skull, found in Ethiopia, dates to 195,000 years ago. For more than 150,000 years, then, humans shared the planet with cousin species.
Despite all the amorous advances, though, only one group survived: us.