Human evolution is looking more tangled than ever. A new genetic study of nearly two thousand people from around the world suggests that some of our ancestors bred with other species of humans, such as Neanderthals, at least twice.
"The researchers suggest the interbreeding happened about 60,000 years ago in the eastern Mediterranean and, more recently, about 45,000 years ago in eastern Asia," Nature News reports from the annual meeting of the American Society of Physical Anthropologists in Albuquerque, New Mexico.
That conclusion is based on a study of over 600 genetic markers, called microsatellites, sequenced in nearly 100 different populations.
As humans began fanning out from Africa, between 50 and 100,000 years ago, these markers changed, allowing researchers to determine the relationship between different populations and to estimate when they split from one another.
If humans bred only with other humans, all these markers would create a neat phylogenetic tree, showing that human genetic diversity can be traced to a single population that existed in Africa in the last 100,000 years.
Instead, a team led by Jeffrey Long, at the University of New Mexico, found evidence that some of the markers looked far too old to have come from humans. Inbreeding with other ancient species is the likeliest explanation. "It means Neanderthals didn't completely disappear," he told Nature.
True, Neanderthals are the likeliest contenders for our ancestors' sexual partners, but they aren't the only ones.
Last month, a team at the Max Planck Institute for Evolutionary Anthropology in Leipzig Germany recovered hominin DNA in Mongolia from a 30-50,000-year-old finger bone that appears to be neither Neanderthal nor human. Its ancestors, or another yet-to-be-discovered kind of archaic hominin, could have bred with humans.
Previous studies of small parts of the Neanderthal genome have found no evidence for interbreeding.
But with a complete Neanderthal genome due to be published any day now, and more DNA from the newly-discovered hominin in the works, scientists will have the best chance yet to determine whether our species shared a bed with any others.