Recent DNA research has cast doubt on this theory, suggesting that although humans and Neandertals share some 99.5 percent of their genetic blueprint, their last common ancestor lived some 400,000 years ago. (Related: "Neandertal Gene Study Reveals Early Split With Humans" [October 26, 2006].)
According to Trinkaus, however, the fossil record yields a pretty clear picture of the early humans who first moved out of Africa to populate the rest of the world.
Neandertal features like the skull groove were either not present among those populations or were so rare that they've not yet been found, he said.
"So when we find them in early modern humans in places like Europe, it's a probability statement—either they were very rare in ancestral humans but popped up in these humans or they were something acquired through some small level of admixing with Neandertals," he added.
"We have enough of them now that with each trait the probability of it being just something we haven't seen yet in the early Africans becomes less and less."
Some interbreeding shouldn't be surprising, he continued, because of the sparse populations of humans and Neandertals in ancient Europe.
"As for sex in the Pleistocene [Ice Age] ... I expect they had it," he said. "Neither humans nor Neandertals had a lot of mate choice and, well, that's what happens in the real world. People do what people do."
But the story of the skull is not so straightforward.
Eric Delson, a paleoanthropologist at Lehman College in the Bronx, New York, cautions that the groove feature might not be exactly the same as those found in Neandertals.
It may simply be a type of unusual feature expected in a variable human population like the group that colonized Europe, he said.
"If you look at a thousand modern humans, you can often find one or two that have a bump here, or a groove or depression there," he said.
"That doesn't make them Neandertals or prove that there was a Neandertal in their ancestry some 30,000 years ago."
Delson, however, did not rule out the theory of human-Neandertal interbreeding.
"[Individuals] may cross species boundaries in mating, and [in this case] we are defining the two different species by morphology—these are [all] people. People might see each other, for whatever reason, as potential mates.
"But the genetic evidence is not in favor of hybridization, and this fossil does not convince me, nor do the several from Central Europe. I am still waiting for a 'smoking gun,' or perhaps in this case 'a bleeding hand axe.'"
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES