Ancient cave art shows us many things, from the artistic capabilities of Neanderthals to how ancient humans used the stars. But a new study finds a much more gruesome story in cave paintings, namely those that depict mutilated hands.

Publishing their findings in the Journal of Paleolithic Archaeology, researchers looked into why many hand paintings from the Upper Palaeolithic era appear to lack digits. And their conclusion is pretty grim. The researchers think that people may have cut off their own fingers.

Various rock paintings depict hands missing a finger or two. The Grotte de Gargas in France contains 231 hand images, 114 of which don’t have five digits. Meanwhile, Cosquer Cave, also in France, depicts 49 hands, 28 lacking fingers. A smaller number of similar images have been discovered at three other sites in France, while more have been found in Spain.

Why the fingers are missing has always been a mystery. Were they simply folded away? Did the artists suffer frostbite? Do they depict sign language?

The paintings at the Grotte de Gargas appear to have been created by hands being pushed flat against the wall, suggesting the fingers weren’t simply folded away. Meanwhile, the researchers argue that if frostbite was to blame, why are fingerless paintings not more widespread?

The team looked at various ethnographic documents, travel journals, and expedition archives and to their surprise found that 121 groups across the continents have carried out finger removal in the past. A few still do it, but the practice is dying out.

“I was pretty shocked,” study co-author Mark Collard told New Scientist. “It seems like such a debilitating practice that I couldn’t imagine signing up to do it myself. I still can’t. Yet, we kept finding group after group that did it.”

The team found various reasons why people might cut off their fingers voluntarily. They could be appealing to a deity for help, expressing extreme grief, attempting to heal an ailment, or communicating group identity or marital status.

Some women used to swallow parts of their own fingers in an attempt to get pregnant and would even swallow the tips of their babies’ fingers to give the baby good luck. 

Women belonging to the Khoikhoi of southern Africa would remove a finger before marriage. If they became widowed and wanted to marry again, they would cut off another finger to release their first husband’s spirit. Women of the Dani tribe in Indonesia remove parts of their fingers to express grief as they mourn, although this has now been banned by the government. 

The researchers note that the cave paintings were created by men, women, and children, so marriage isn’t the best explanation. Mourning and sacrifice were the most common examples, and therefore the most likely.

Many believe the cave paintings to be the “remnants of religious rituals”, so the team thinks sacrifice is the best bet. They argue that these “dysphoric rituals” may even have caused groups to become very cooperative, while being hostile to outsiders, as people tend to bond strongly through shared trauma. The darkness of the caves along with the sinister paintings would have added to this dysphoria.

The researchers note that their explanation is merely a hypothesis and could be proved wrong, but it is nonetheless intriguing to think that for many cultures around the world, hacking off your fingers was once a part of life.