A unique inner-ear configuration once though exclusive to our ancient human cousins the Neanderthals has been found in a 100,000-year old human skeleton.

The inner ear arrangement was revealed by recent CT scans of a fossil human skull discovered in China in the 1970s, scientists say.

"We were completely surprised," says study co-author Erik Trinkhaus of Washington University in St. Louis. "We fully expected the scan to reveal a temporal labyrinth that looked much like a modern human one, but what we saw was clearly typical of a Neanderthal."

Trinkaus, an authority on early human evolution, says it may be evidence of interbreeding and gene transfer between Neanderthals and our modern human ancestors.

The semi-circular canals of the inner ear are a fluid-filled structure that allows humans to maintain their balance, and a distinctive configuration of that structure had previously been considered a way to differentiate modern human remains of those of Neanderthals.

The new discovery throws that into disarray, Trinkhaus says.

"The discovery places into question a whole suite of scenarios of later Pleistocene human population dispersals and interconnections based on tracing isolated anatomical or genetic features in fragmentary fossils," he says.

The skull subjected to the CT scan, dubbed Xujiayao 15 after the site where is was discovered, was unearthed along with a number of human teeth plus bone fragments displaying characteristics normally found in early non-Neanderthal forms of archaic but modern humans, the researchers say.

"The study of human evolution has always been messy, and these findings just make it all the messier," Trinkaus says. "It shows that human populations in the real world don't act in nice simple patterns."

Trinkhaus and his Chinese research collaborators say that despite the temptation to do so, using the discovery of a Neanderthal-configured labyrinth system in an otherwise definitely "non-Neanderthal" skull as strong evidence of gene flow or population contact between Eurasian Neanderthals and archaic humans in China should be approached cautiously.

Migration patterns between widely separated Western Europe and Eastern Asia evolved over thousands of years, they say, and relying on a single anatomical feature to make far-reaching assumptions about when, where and how hominid species moved from one locale to another is risky.

"You can't rely on one anatomical feature or one piece of DNA as the basis for sweeping assumptions about the migrations of hominid species from one place to another," Trinkaus says.

"What these findings say to me are that characteristics were probably more varied in ancient human populations than we think."

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