The inner ear of a 100,000-year- old fossilized early human skull found 35 years ago in northern China displays characteristics long thought to occur only in Neanderthals, who prospered across Europe and western Asia from roughly 30,000 and 40,000 years ago, according to a study out Monday.
The findings, published in the U.S. journal Proceedings of the National Academy of Sciences, suggested the inner ear characteristics were not unique to Neanderthals and were more geographically widespread than previously believed.
"We were completely surprised," said study co-author Erik Trinkaus, a physical anthropology professor at Washington University in St. Louis, in a statement.
The study is based on recent micro-CT scans revealing the inner ear structure within a temporal bone of the fossilized human skull found during 1970s excavations at the Xujiayao site in China's Nihewan Basin.
The skull at the center of this study, known as Xujiayao 15, was found along with an assortment of other human teeth and bone fragments, all of which seemed to have characteristics typical of an early non-Neanderthal form of late archaic humans.
"We fully expected the scan to reveal a temporal labyrinth ( inner ear) that looked much like a modern human one, but what we saw was clearly typical of a Neanderthal," Trinkaus said. "This discovery places into question whether this arrangement of the semicircular canals is truly unique to the Neanderthals."
Often well-preserved in mammal skull fossils, the semicircular canals are part of the inner ear that helps humans maintain balance when they change their spatial orientations, such as when running, bending over or turning the head from side-to-side.
In the mid-1990s, CT scans revealed that nearly all Neanderthals had a particular arrangement of the semicircular canals in the temporal labyrinth. Since then, this pattern has been widely used as a marker to set them apart from both earlier and modern humans.
Trinkaus, who has studied Neanderthal and early human fossils from around the globe, said this discovery only adds to the rich confusion of theories that attempt to explain human origins, migrations patterns and possible interbreedings.
While it's tempting to use the findings as evidence of population contact between central and western Eurasian Neanderthals and eastern archaic humans in China, broader implications of the Xujiayao discovery remain unclear, the researchers argued.
"The study of human evolution has always been messy, and these findings just make it all the messier," Trinkaus said. "It shows that human populations in the real world don't act in nice simple patterns."
"Eastern Asia and Western Europe are a long way apart, and these migration patterns took thousands of years to play out," he said. "This study shows that 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. "
His co-authors on this study are Xiu-Jie Wu, Wu Liu and Song Xing of the Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, and Isabelle Crevecoeur of PACEA, Universite de Bordeaux, France.