My bus tour through the Andes of southern Peru took an unexpected stop. We were in the cold, dry highlands, less than 100 miles from Arequipa, when the tour guide insisted that my fellow travelers and I get off the bus “to take a small hike.” We walked through a small farm with some rocky ruins of indeterminate age. But then the guide pointed to a big rock positioned over a hole and told us to look inside.

There were a number of skulls in the hole, and they didn’t look quite right. The crown was too dome-shaped, taller and more cylindrical than usual, it seemed. The guide said these skulls were made to look this way intentionally; these individuals wore bandages wrapped tightly around their heads up until about age five, while their skulls were still soft.

Artificial cranial deformation—or the practice of intentionally changing the shape of a person’s skull—has been practiced by Neanderthals of 40,000 years ago until very recently, maybe even still today. People on every continent except Antarctica have done it, making heads more cylindrical, cone-shaped, ridgier, bumpier or flatter depending on the region. The reason, most archaeologists believe, was pretty much the same reason we modify our bodies today: to show an association with a particular social group.

“Cranial deformation had to do with ideas of beauty, what would be socially acceptable and desirable to look like, and that differed between groups around the world,”  Mercedes Okumura, an anthropologist at the Federal University of Rio de Janeiro in Brazil, said in an interview.