A human skull has been recognized as bearing inner-ear bones that seem to belong to a Neanderthal, leading researchers to question the history of human evolution.

Around 35 years ago, an ancient human skull was found in northern China, lying among remains of late archaic humans. Dubbed Xujiayao 15, it was dated to 100,000 years in the past. Recent re-examination of the artifact revealed the presence of an inner-ear mechanism that was previously believed to exist only in Neanderthals.

Early theories of the spread of humans around the globe showed simple migrations or people throughout time. Now, those patterns of movements are believed to be much more complex than early models stated. This new discovery could make the theory even more convoluted.

"It suggests, instead, that the later phases of human evolution were more of a labyrinth of biology and peoples than simple lines on maps would suggest," Erik Trinkaus, physical anthropology professor at Washington University in St. Louis, said.

Evidence of Neanderthal bone structures in a human skull would suggest interbreeding between the humanoids and direct ancestors of modern humans. Around four percent of human DNA can be traced back to Neanderthal ancestors.

During the 1990's, early CT scans identified a particular structure of semicircular canals in Neanderthals that assisted them in maintaining balance. This particular arrangement became one of the defining features of Neanderthal skulls, differentiating them from modern humans.

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

As humans and humanoids moved around the prehistoric landscape, their paths may have crossed several times, in locations around the globe. This new study provides evidence that single pieces of bone should not be used as evidence of a simple journey.

The first true Neanderthals evolved around 250,000 years ago in Europe and Asia. Their brain capacity had an average volume of around 97 cubic inches. Modern humans average just over 85 cubic inches. Despite their larger brains, Neanderthals were not appreciably more intelligent than ancient humans, due to their greater body size. Researchers are in disagreement when the group disappeared, but the last Neanderthal likely walked the Earth around 30,000 years in the past.

Investigation of the inner ear bones in the human skull from 1,000 centuries ago will be published in an upcoming edition of the Proceedings of the National Academy of Sciences.