The pit contained three sets of human bones, as well as animal skeletons and broken pottery. Workers at a private company found the pit in 2013, during the construction of a highway near the town of Osijek in eastern Croatia.

The skulls were intact. Two of them were also misshapen.

“A friend, the owner of this company, he called and told me, ‘Oh Mario, I think we have an alien here,’” recalled Mario Novak, an archaeologist at the Institute for Anthropological Research in Croatia. “Of course, it was a joke.”

Rather, the misshapen skulls — one had a flattened top, the other resembled an elongated egg — bore signs of artificial cranial deformation.

Practiced for some 5,500 years by cultures around the world, artificial cranial deformation involves binding the growing heads of infants and children with bandages, planks, boards or bricks. As the skull grows under constant pressure, it becomes misshapen, resulting in oblong heads and other unusual shapes. Anthropologists have documented this practice on every inhabited continent, but it isn’t common. Today it continues in only a few remote tribes.