A bone found by chance on the banks of a Siberian river has yielded the oldest modern human genome yet recovered, according to a new study that sheds light on when people left Africa and first interbred with Neanderthals living in Europe and Asia.

The man, who lived 45,000 years ago, was definitely related to both humans and Neanderthals, the study published in the journal Nature reports. His DNA showed that the two human groups first mated around 60,000 years ago.

Project leader Chris Stringer of the Natural History Museum in London explained to Discovery News that the Siberian man belonged to a population that was closely related to the ancestors of today's Europeans and Asians. He carried only slightly more Neanderthal DNA than they do.

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"But his genomic segments of Neanderthal ancestry are on average about three times the length of those found in genomes today," Stringer said.

This is highly informative, he continued, "as the chunks of Neanderthal DNA have been gradually broken up each generation since the time of interbreeding."

He and his team charted the rate of that change to the present, when all living non-Africans possess 2% Neanderthal in their DNA. Going backwards in time, the researchers could then see that the mating with Neanderthals took place 7,000 to 10,000 years before the Siberian man lived. This means the human/Neanderthal interbreeding happened no more than 60,000 years ago.

A simple explanation would then be that Homo sapiens first left Africa at around 60,000 years ago, but other finds dispute that. Anthropologists have found 100,000-year-old skeletons for our species in the Israeli caves of Skhul and Qafzeh.

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Putting the pieces together results in two possible scenarios:

1. People left Africa sometime around 100,000 years ago but failed to have successful, lasting settlements. A later group left Africa approximately 60,000 years ago, resulting in a successful dispersal. This group gave rise to all of today's non-Africans.

2. People left Africa 100,000 years ago and were successful. Members of their group took a while to disperse, with one wave reaching southern Asia before 75,000 years ago, eventually reaching Australia and New Guinea. Then, Stringer said, describing this scenario, "a second wave at around 60,000 years ago carried the ancestors of present day Eurasians and Native Americans out of Africa."

So which scenario is the one that really happened? The Siberian man provides a constraint on the theories, indicating that interbreeding with Neanderthals is unlikely to have happened before 60,000 years ago.

"Thus the ancestors of Australasians (people from Australia, New Zealand, the island of New Guinea and neighboring islands in the Pacific Ocean), with their similar input of Neanderthal DNA to Eurasians, must have been part of a late, rather than early, dispersal through Neanderthal territory," Stringer said.

He concluded, "While it is still possible that modern humans did traverse southern Asia before 60,000 years ago, those groups could not have made a significant contribution to the surviving modern populations outside of Africa, which contain evidence of interbreeding with Neanderthals."

This article originally published at Discovery News here