Neandertals, Modern Humans Interbred, Bone Study Suggests

James Owen
for National Geographic News
October 30, 2006
Trace your family tree all the way back to Stone Age Europe, and you may find Neandertals among your ancestors.

A new study suggests that modern humans and Neandertals (often spelled Neanderthals) interbred fairly regularly and even mingled physical features as Homo sapiens spread across Europe some 35,000 years ago.

The findings, based on ancient human bones from a cave in Romania, add to the long-running debate as to why Neandertals, a heavy-browed, thickset species of human, eventually went extinct. (Related: "Neandertals' Last Stand Was in Gibraltar, Study Suggests" [September 13, 2006].)

Some scientists argue that Neandertals were slaughtered or out-competed by ancestors of modern humans once they reached Europe after first emerging in Africa.

But the new research, reported today in the Proceedings of the National Academy of Sciences, supports the idea of a more intimate relationship, with Neandertals becoming absorbed into the human race through interbreeding.

Researchers from Romania and the U.S. dated fossil bones found at Petera Muierii ("Cave of the Old Woman") to around 30,000 years ago, the period when Neandertals and modern humans overlapped (map of Romania).

While the remains are largely typical of modern humans, they also show some distinctly Neandertal traits, says team member Erik Trinkaus, an anthropologist at Washington University in St. Louis, Missouri.

These telltale skeletal features include the shape of the lower jaw and the back of the skull, Trinkaus says.

Mixed Features

"[These features] are extremely unlikely to have come from earlier modern humans and very likely have come from Neandertals," Trinkaus said.

The Neandertal-type jaw features are not related to eating but are "anatomical peculiarities," he said.

The skull characteristic noted in the study is related to braincase development, he adds, and is found in around half of humans known from the same time period.

"You then find it in decreasing frequencies as you go towards modern Europeans," Trinkaus said.

The team also says that the shoulder joint of the cave-dwelling humans appears quite primitive and likely would have made throwing spears and other projectiles difficult.

"The shoulder joint suggests they were not doing a whole lot of vigorous throwing, which requires an extreme rotation of the shoulder joint," Trinkaus said.

This feature, he adds, is seen in "Neandertals and earlier humans going back millions of years. It suggests [human] behavior and anatomy is more mosaic in that time period."

The team argues that Neandertals, rather being replaced, became mixed in with modern humans.

"The only way I can explain the anatomy of these fossils and the fossils from a number of other sites across Europe is that there was a fair amount of interbreeding," Trinkaus said.

Studies of the remains of other ancient Europeans, including the 24,500-year-old skeleton of a child found in Portugal, likewise suggest Neandertals contributed to modern human makeup.

Dominant Species

With closely related animals that can interbreed, it's not unusual for the dominant species to absorb the other, Trinkaus says.

Extinct indigenous peoples such as the Tasmanians (from Australia) and the Tierra del Fuegians (from South America)—both of whom disappeared in the last thousand years—nevertheless have living descendants as they intermixed with settlers, he points out.

However, previous DNA studies of Neandertal remains suggest the hunter-gatherers left little or no impression on our genetic makeup.

These studies include recent research based on DNA extracted from fossilized Neandertal bones, which indicates the last common ancestor of modern humans and Neandertals lived some 400,000 years ago. (Related news story: "Neandertal Gene Study Reveals Early Split With Humans" [October 26, 2006].)

But Fred Spoor, professor of evolutionary anatomy at University College London, says such findings don't prove that mixing between the two human species didn't occur.

"Neandertals and modern were undoubtedly capable of interbreeding, and their offspring may well have been viable," he said.

Over the past decade most researchers have come to view the disappearance of the Neandertals as being more complex than the species either being absorbed or replaced by modern humans, he adds.

"The question is not whether every Neandertal who came in view of a modern human was immediately killed off," Spoor said, but the extent to which Neandertals interbred with modern humans and ended up in their "final mix."

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