Modern humans inter-bred with Neanderthals after they had arrived in Europe and they lived together as recently as 40,000 years ago, says a study.

The study involved the genetic analysis of the jawbone of a human who lived in Europe about 40,000 years ago and found that he might have had a Neanderthal ancestor just a few generations back.

"In the last few years, we have documented inter-breeding between Neanderthals and modern humans, but we never thought we would be so lucky to find someone so close to that event," said one of the researchers David Reich from Harvard Medical School.

"We know that before 45,000 years ago, the only humans in Europe were Neanderthals. About 35,000 years ago, the only humans in Europe were modern humans. This is a dramatic transition."

There is archaeological evidence that 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 we have very few skeletons from this period," Reich said.

So the jawbone that archaeologists uncovered in Romania in 2002, which radiocarbon dating determined was between 37,000 and 42,000 years old, was an important discovery.

"The sample is more closely related to Neanderthals than any other modern human we have ever looked at before."

"We estimate that six to nine percent of its genome is from Neanderthals. This is an unprecedented amount. Europeans and East Asians today have more like two percent. It suggests that the individual's ancestry was recent."

As DNA is passed on from generation to generation, segments are broken up and recombined, so that the DNA inherited from any one individual becomes interspersed with the DNA of other ancestors.

Reich found segments of intact Neanderthal DNA in the fossil that were large enough to indicate that the individual had a Neanderthal ancestor just four to six generations back.

That suggests that modern humans inter-bred with Neanderthals after they had arrived in Europe.

Reich found no evidence that he is closely related to later Europeans.

"This sample, despite being in Romania, does not yet look like Europeans today," he said.

The scientists reported their results in the journal Nature.