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Odd Skull Boosts Human, Neandertal Interbreeding Theory

Brian Handwerk
for National Geographic News
August 2, 2007
 
A human skull from a Romanian bear cave is shaking up ideas about ancient sex.

The Homo sapiens skull has a distinctive feature previously found only in Neandertals, providing further evidence of interbreeding between the two species, according to a new study.

The human cranium was found during World War II mining operations in 1942, in a cave littered with Ice Age cave bear remains.

Recently the fossil was radiocarbon dated to 33,000 years ago and thoroughly examined, revealing the controversial anatomical feature.

The otherwise human skull has a groove at the base of the back of the skull, just above the neck muscle, that is ubiquitous in Neandertal specimens but has never been seen in the remains of a modern human, argues study leader Erik Trinkaus, an anthropologist at Washington University in St. Louis, Missouri.

"I was frankly quite surprised to see it when I was looking at the specimen," Trinkaus said. "My first reaction was, that shouldn't be there."

Writing in the August issue of Current Anthropology, Trinkaus and his colleagues say that the skull supports interpretations of other remains found in France, Romania, and the Czech Republic that also have "archaic" or unusual features suggesting interbreeding.

Ancient Debate

Archaeological evidence shows that humans and Neandertals (often spelled Neanderthals) both lived in Europe for several thousand years after the arrival of modern humans about 35,000 years ago. (Learn more about human migrations.)

But the relationship between the two species is hotly debated.

Many researchers believe that modern humans killed off or simply outcompeted Neandertals until the latter went extinct.

Trinkaus and others, however, have suggested that Neandertals became absorbed into Homo sapiens. (Related: "Neandertals, Modern Humans Interbred, Bone Study Suggests" [October 30, 2006].)

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."

Random Aberrations?

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.'"

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