Genetic analysis of the mandible of an anatomically modern Homo sapiens who lived in what is now Romania between 42,000 and 37,000 years ago reveals that early humans interbred with Neanderthals when they first came to Europe.

Neanderthals are thought to have disappeared in Europe around 39,000 – 41,000 years ago but they have contributed up to 3 percent of the DNA of present-day Eurasians.

“We know that before 45,000 years ago, the only humans in Europe were Neanderthals. After 35,000 years ago, the only humans in Europe were modern humans. This is a dramatic transition,” explained Dr David Reich of Howard Hughes Medical Institute and the Broad Institute of MIT and Harvard, who is a co-lead author of the paper discussing the results in the journal Nature.

There is evidence that anatomically modern humans interacted with Neanderthals during the time that they both lived in Europe: changes in tool making technology, burial rituals, and body decoration imply a cultural exchange between the groups.

But archaeologists have very few skeletons from this period. So the 40,000-year-old human jawbone that archaeologists unearthed in 2002 in the Peştera cu Oase cave in south-western Romania was an important find.

The mandible was found along with the skull of another individual. No artifacts were discovered nearby, so scientists had no cultural clues about who the individuals were or how they lived.

The physical features of the mandible were predominantly those of anatomically modern humans, but some Neanderthal traits were also apparent, and the scientists proposed that the bone might have belonged to someone descended from both groups.

Dr Reich and his colleagues from Canada, the United States, China, Germany and Romania, have now analyzed DNA from the mandible and found that on the order of 6 – 9 percent of the genome of the Oase individual is derived from Neanderthals, more than any other modern human sequenced to date.

“It’s an incredibly unexpected thing. In the last few years, we’ve documented interbreeding between Neanderthals and modern humans, but we never thought we’d be so lucky to find someone so close to that event,” Dr Reich said.

The scientists also estimated that the Oase man had a Neanderthal ancestor in the previous four to six generations.

“Three chromosomal segments of Neanderthal ancestry are over 50 centimorgans in size, indicating that this individual had a Neanderthal ancestor as recently as four to six generations back,” they said.

“The data from the jawbone imply that humans mixed with Neanderthals not just in the Middle East but in Europe as well,” said study first author Dr Qiaomei Fu from the Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences.

However, the Oase individual is not responsible for passing his Neanderthal ancestry on to present day humans.

“It may be that he was part of an early migration of modern humans to Europe that interacted closely with Neanderthals but eventually became extinct,” Dr Reich explained.

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Qiaomei Fu et al. An early modern human from Romania with a recent Neanderthal ancestor. Nature, published online June 22, 2015; doi: 10.1038/nature14558