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A 100,000-year-old human skull found 35 years ago in Northern China has an inner-ear formation that was long thought only to occur in Neandertals.

“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,” says Erik Trinkaus, a physical anthropology professor at Washington University in St. Louis and coauthor of a new study published in the Proceedings of the National Academy of Sciences.

“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.”

The study is based on recent micro-CT scans revealing the interior configuration of a temporal bone in a fossilized human skull found during 1970s excavations at the Xujiayao site in China’s Nihewan Basin.

Trinkaus is among the first scientists to offer compelling evidence for interbreeding and gene transfer between Neandertals and modern human ancestors.

A complete surprise

“We were completely surprised,” Trinkaus says. “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 Neandertal. This discovery places into question whether this arrangement of the semicircular canals is truly unique to the Neandertals.”

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Often well-preserved in mammal skull fossils, the semicircular canals are remnants of a fluid-filled sensing system 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.

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 considered enough to securely identify fossilized skull fragments as being from a Neandertal.

This pattern is present in almost all of the known Neandertal labyrinths. It has been widely used as a marker to set them apart from both earlier and modern humans.

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-Neandertal form of late archaic humans.

The discovery only adds to the rich confusion of theories that attempt to explain human origins, migrations patterns, and possible interbreedings, Trinkaus says.

Evolution is messy

While it’s tempting to use the finding of a Neandertal-shaped labyrinth in an otherwise distinctly “non-Neandertal” sample as evidence of population contact (gene flow) between central and western Eurasian Neandertals and eastern archaic humans in China, researchers argue that broader implications of the Xujiayao discovery remain unclear.

“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.

“Eastern Asia and Western Europe are a long way apart, and these migration patterns took thousands of years to play out,” he says. “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.”

Coauthors are from the Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, and Université de Bordeaux.

Source: Washington University in St. Louis