Remains of a Neanderthal woman who lived around 100,000 years ago in the Altai Mountains of Siberia reveal that human and Neanderthals mated much earlier than previously thought.

One or more of her relatives were actually humans, a new study shows.

It has been known that Neanderthals contributed DNA to modern humans, so people today of European and Asian descent retain Neanderthal DNA in their genomes, but the Neanderthal woman offers the first evidence that gene flow from interbreeding went from modern humans into Neanderthals as well.

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The study, published in the journal Nature, "is also the first to provide genetic evidence of modern humans outside Africa as early as 100,000 years ago," Sergi Castellano, who co-led the study and is a researcher at the Max Planck Institute for Evolutionary Anthropology, told Discovery News.

Given the now closely intertwined histories of Neanderthals and Homo sapiens, Castellano added that "it is better to refer to Neanderthals and modern humans as two different human groups, one archaic and one modern, and not different species."

Earlier research determined that Neanderthals and modern humans met and mated outside of Africa sometime between 47,000–65,000 years ago. The Siberian woman shows that such interbreeding could have happened as early as 120,000 years ago, since it is now believed modern humans and Neanderthals were both then present in the region around the Persian Gulf and in the area where the following countries are now located: Cyprus, Egypt, Israel, Lebanon, Syria and Turkey.

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Castellano, Martin Kuhlwilm and colleagues analyzed the Siberian woman's DNA and identified portions of her genome that match sequences taken from people who are now living in Africa. The researchers also analyzed the remains of a Denisovan individual and the remains of two Neanderthals that were found in European caves (one from Croatia, and the other from Spain). No modern human DNA was detected in those three other individuals.

The scientists therefore think that a population of Neanderthals likely migrated from Europe through the Near East, where mating with modern humans occurred. They then continued to travel up into the Altai Mountains, where the climate was much milder 125,000 years ago than it is now.

Castellano and his team further believe that the modern humans who were in the Near East at this early time eventually died out and did not contribute to the present day Homo sapiens genome.

Nevertheless, Castellano said, "Some Asians and Oceanians (living today) have more Neanderthal DNA because a second pulse of admixture might have happened with Neanderthals."

A cave in the Altai Mountains of Siberia where a Neanderthal with human genes was found.Bence Viola

The evidence throws a wrench in the widely held view that modern humans first left Africa about 60,000 years ago. Instead, it seems that one or more groups of modern humans were leaving Africa tens of thousands of years earlier, and apparently at least some were mating with Neanderthals along the way.

Montgomery Slatkin is a professor in the Department of Integrative Biology at the University of California at Berkeley. He told Discovery News, "I think the results (of the new study) are important because they show how complex the relationships between the ancestors of modern humans and Neanderthals were. It will be interesting to find out whether the early interbreeding between Neanderthals and humans affects only the Neanderthals found in Asia, or all Neanderthals."

Chris Stringer, a research leader in Human Origins at the Natural History Museum, London, wonders how the interbreeding happened. He speculated that "the possibilities range from relatively peaceful exchanges of partners, to one group raiding another and stealing females -- which happens in chimps and some modern hunter-gatherers -- through to adopting abandoned or orphaned babies."

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"Eventually, geneticists should be able to show if the transfer of DNA in either direction was mainly via males, females or about in equal proportion, but it will need a lot more data before that becomes possible," he added.

Stringer also wonders if all of the modern human dispersals out of Africa before 60,000 years ago failed, such that all of those individuals died out and made no contribution to the present Homo sapiens genome.

"If both the early modern human groups and the early intermixed Neanderthals went completely extinct, then this was still ultimately a failed expansion," he said. "But the increasing resolving power of genetic studies means that the search is now on for further traces of these mysterious early moderns, and their Neanderthal relatives in Asia."