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.