The two groups saw each other as kindred spirits and, when conditions were right, they mated.
How often this happened will never be known, but paleoanthropologist Erik Trinkaus says it probably occurred more often than is generally imagined.
In his latest work, published last week in the journal Proceedings of the National Academy of Sciences, Trinkaus, of Washington University in St. Louis, analyzed prehistoric fossil remains from various parts of Europe. He concluded that a significant number have attributes associated with both Neanderthals and the modern humans who replaced them.
"Given the data we now have, it would be highly improbable to argue there is no Neanderthal contribution to the early European population that came out of Africa," Trinkaus said. "I believe there was continuous breeding between the two for some period of time.
By the time the Neanderthals were dying about 30,000 years ago, the fossil record suggests that about 10 to 20 percent of the genetic material in European humans was from Neanderthals, he says.