The remains of two ancient skulls found in China have raised the possibility of an entirely new species or a previously undiscovered mix between species, according to a study published Friday in the journal Science. The skulls shared a mix of physical distinctions present in both Homo sapiens and Neanderthals but likely don’t belong to either group.

“The biological nature of the immediate predecessors of modern humans in eastern Eurasia has been poorly known from the human fossil record,” Erik Trinkaus, a professor in the department of anthropology at Washington University in St. Louis and one of the study’s authors, said in a statement. “The discovery of these skulls of late archaic humans, from Xuchang, substantially increases our knowledge of these people.”

The partial skulls, assembled from more than 40 bone fragments, were found by researchers during a series of excavations that took place in Lingjing, Xuchang County in Hunan Province, China from 2007 to 2014. They were estimated to be between 105,000 and 125,000 years old. Modern humans existed in Africa around 200,000 years ago before spreading to Asia, Europe and America around 60,000 years ago.

The skulls could be related to the Denisovans, a little-known group of humans who are only known from the analysis of two small bones found in Siberia. Trinkaus said it was impossible to tell just yet whether the skulls belonged to the group.

Some of the characteristics, including the low, broad braincase, were similar to those of Neanderthals.

“The features of these fossils reinforce a pattern of regional population continuity in eastern Eurasia, combined with shared long-term trends in human biology and populational connections across Eurasia,” said Trinkaus. “They reinforce the unity and dynamic nature of human evolution leading up to modern human emergence.”

A DNA test had not yet been conducted on the fragments, leaving the remains shrouded in mystery.

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