These differences suggest that "modern human evolution did not stop when people we call moderns appeared," he said. "There have been significant changes in human anatomy since the time we have the first modern humans."
Moreover, Trinkaus said, the unusual features suggest intermixing between modern humans and Neandertals.
The archaic features had been lost in early modern humans in Africa, he said.
Therefore, the features' reappearance in the Romanian skull either requires a reversal of evolution once modern humans reached Europe or some degree of intermixing with Neandertals, according to the study.
"It's much more likely the latter that happened," Trinkaus said.
Eric Delson is an anthropologist at Lehman College and the American Museum of Natural History, both in New York.
The Romanian skull is definitely a modern human's with some unusual characteristics, he agreed—but the interpretation is questionable.
"It's not very clear that the features they've located indicate interaction with Neandertals as opposed to a holdover from the more archaic past or local population differences," he said.
According to Delson, analysis of the skull with a more statistically advanced technique that looks at the entire skull shape instead of individual features might help resolve some of these issues.
Richard Potts, director of the Human Origins program at the Smithsonian Institution in Washington, D.C, said the skull shows "there is continuing evolution for the modern human skull even after humans got to Europe and other places in the world."
But, he said, as the authors point out, the skull lacks uniquely Neandertal traits and thus does not prove interbreeding. In fact, he said, similar archaic traits are found in modern human remains found in a cave in China.
Nevertheless, he added, evidence for interbreeding between modern humans and Neandertals would not be a surprise.
"Evolution has involved sloppiness of boundaries between lineages," he said. "But if [interbreeding] occurred, it is extremely rare. Otherwise, we'd find much more of a mixing."
Trinkaus has long argued that Neandertals interbred with modern humans as they spread across Europe.
The Romanian skull supports the notion that the two "got it together," he said. "For some reason, many people find that very difficult to accept."
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