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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.
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.
Heavy-set 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 finger bone in Siberia -- known as the Denisovans -- also left some of their DNA in modern day 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. It's also impossible to say whether the matings were consensual or forced.
But one thing is clear: This enigmatic group left their 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.
Besides finding evidence of the now-extinct species, the team discovered a huge range of genetic diversity between the three groups. The human genome contains about three billions letters, or base pairs, of DNA. Before this study, scientists had found that some 40 million of these letters vary across human populations.
But in the 15 Africans, Tishkoff found another three million genetic variants - a huge trove of human diversity. Among this stunning variety, Tishkoff says she has pinpointed some of the genes responsible for the short stature of the pygmies, who average less than five feet in height. She also found that immune system genes and genes for taste and smell varied wildly between the three groups -- confirming Africa as the seat of the most human diversity.
The research was reported Wednesday in the journal Cell.
"This is very cutting-edge population genetics work," said geneticist Spencer Wells, a National Geographic explorer. "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.
Akey said: "As we were conquering the world, we also conquered similar human populations that were dying out."
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