Scientists have found new genetic evidence that they say may answer the longstanding question of whether modern humans and Neanderthals interbred when they co-existed thousands of years ago. The answer is: probably yes, though not often.

In research being published online this week by the Proceedings of the National Academy of Sciences, the scientists reported that matings between Neanderthals and modern humans presumably accounted for the presence of a variant of the gene that regulates brain size.

Bruce T. Lahn of the University of Chicago, the report’s senior author, said the findings demonstrated that such interbreeding with relative species, those on the brink of extinction, contributed to the evolutionary success of modern humans.