The only problem is one of the skull specimens, about 100,000 years old, features the inner ear structure of Neanderthals, which lived in Eurasia from 200,000 to 30,000 years ago. The inner ear is the "labyrinth" within the skull's temporal bone; it houses the mechanisms that convert sound waves into electrical signals a brain can understand and help humans keep their balance.
"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," explained study co-author Erik Trinkaus, an anthropologist at Washington University in St. Louis.
Neanderthals and other lineages of ancient humans are all grouped under the genus Homo, but various populations of modern man's relatives each had distinct characteristics.
This latest confusing discovery is further evidence that the lines separating one group (like the Neanderthals of Eurasia) from another (like the archaic humans of ancient China) remain exceptionally blurry.
"Evolution was very much mosaic," Trinkaus said. "To the extent that this might indicate population contact, it emphasizes that human populations were very dynamic on this landscape."
In the new study, published this week in the Proceedings of the National Academy of Sciences, Trinkaus and his colleagues argue that the mixing and matching of various genetic traits is proof that different groups of modern humans routinely interbred.
"What these findings say to me are that characteristics were probably more varied in ancient human populations than we think," Trinkaus added. "The idea of distinct, separate lineages in this time period in human evolution is meaningless -- it was much more of a labyrinth than that."
2014: The Year in Music [PHOTOS]