Washington, D.C. ? A skull found in a cave in Romania includes features of both modern humans and Neanderthals, possibly suggesting that the two may have interbred thousands of years ago.

Neanderthals were replaced by early modern humans. Researchers have debated whether the two groups mixed, though most doubt it. The last evidence for Neanderthals dates from at least 24,000 years ago.

The skull bearing older and modern characteristics is discussed in a paper by Erik Trinkaus of Washington University in St. Louis. The report appears in today’s issue of Proceedings of the National Academy of Sciences.

Radiocarbon dating indicates the skull is at least 35,000 years old.

The researchers said the skull had the same proportions as a modern human head and lacked the large brow ridge commonly associated with Neanderthals. However, there also were features that are unusual in modern humans, such as frontal flattening, a fairly large bone behind the ear and exceptionally large upper molars.