Scientists have long suspected that modern humans and our ancient Neanderthal cousins interbred, and now DNA evidence shows that it did occur, and dates when it occurred, researchers say.
Sequencing of DNA in a thighbone of one of the earliest modern humans, discovered in 2008 in Siberia, shows interbreeding occurred sometime between 50,000 and 60,000 years ago, they say.
The genetic evidence remains in the leg bone of a man living in Siberia around 45,000 years ago, just when modern humans were poised to expand into Europe and Asian after emerging from Africa.
"The amazing thing is that we have a good genome of a 45,000 year old person who was close to the ancestor of all present-day humans outside Africa," says researcher Svante Paabo of the Max Planck Institute in Leipzig, Germany.
The DNA evidence suggests modern humans and Neanderthals interbred during an era dubbed the Initial Upper Paleolithic, a period that saw a significant expansion of modern human culture, the researchers say.
Some earlier estimates had put possible interbreeding much earlier, when our direct human ancestors were less modern both anatomically and in behavior.
The DNA evidence says otherwise, researchers say.
"This new paper definitively says it was modern humans with modern human behavior that interbred with Neanderthals," says David Reich of Harvard University, a co-author of the study published in Nature.
The Initial Upper Paleolithic is when complex bone and stone tools began to appear in Eurasia, accompanied by other evidence of modern human behaviors such as body ornamentation with animal teeth and pierced shells, use of pigments and even development of musical instruments, research team member Tom Highham of Oxford University says.
The DNA of the Siberian human bone shows evidence of interbreeding between modern humans and our cousin species Neanderthals who evolved outside of Africa, the researchers say.
The amount of Neanderthal DNA present suggests interbreeding between Neanderthals and the ancestors of the Siberian man occurred within only a few dozen generations of when he lived, they say.
The thighbone represents the oldest modern human remains ever found outside of Africa and the Middle East.
A small imprint of Neanderthal DNA, about 2 percent, exists in all humans today except in Africans, evidence of interbreeding between the two species only after the "Out of Africa" scenario of human development, researchers say.
The Siberian man appears to have lived at a point that was a crossroads for human evolution in terms of both time and geography, scientists say.
"This does seem to mark a watershed where modern humans were pushing the boundaries further and further in their dispersal out of Africa," says Chris Stringer of the Natural History Museum in London.