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One of the fossils, called Apidima 1, turned out to be part of the back of a skull. The other, Apidima 2, consisted of 66 fragments from an individual’s face.

An early study of Apidima 2 suggested the fragments were about 160,000 years old, and so it seemed likely that Apidima 1 had fossilized around the same time.

That age made the two fossils much older than the earliest known evidence of our species in Europe. It seemed more likely that the skulls belonged to Neanderthals, who arrived in Europe about 400,000 years ago.

The Museum of Anthropology at the University of Athens invited Dr. Harvati, an expert on the shapes of human skull fossils, to take a closer look. She and her colleagues performed CT scans on the remains and then analyzed them on a computer.

When the researchers virtually reassembled the face of Apidima 2, they realized they were looking at a Neanderthal. But when the team analyzed the back of Apidima 1’s skull, they knew that they were dealing with something different.

In Neanderthals and other extinct human relatives, the back of the skull bulges outward. “It looks like when you put your hair up in a bun,” Dr. Harvati said.

But in our own species, there is no bulge . Compared with our extinct cousins, the back of the modern human skull is distinctively round.