In a re-examination of a 100,000-year-old early human skull, researchers have found the surprising presence of an inner-ear formation long thought to occur only in Neanderthals.
Theories proclaiming inter-breeding between Neanderthals and other archaic human species have gained more weight with this finding.
This discovery only adds to the rich confusion of theories that attempt to explain human origins, migrations patterns and possible inter-breeding, said Erik Trinkaus, a physical anthropology professor at Washington University in St. Louis.
"Later phases of human evolution were more of a labyrinth of biology and peoples than simple lines on maps would suggest," Trinkaus added.
The semicircular canals are remnants of a fluid-filled sensing system that help humans maintain balance when they change their spatial orientations, such as when running, bending over or turning the head from side-to-side.
"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. This discovery places into question whether this arrangement of the semicircular canals is truly unique to the Neanderthals," Trinkaus added.
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The study is based on recent micro-CT scans revealing the interior configuration of a temporal bone in a fossilised human skull found during 1970s excavations at the Xujiayao site in China's Nihewan Basin.
Since the mid-1990s, when early CT-scan research confirmed its existence, the presence of a particular arrangement of the semicircular canals in the temporal labyrinth has been widely used as a marker to set Neanderthals apart from both earlier and modern humans.
The skull at the centre 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.
The study is forthcoming in the journal Proceedings of the National Academy of Sciences.