Archaeologists have come up with a gruesome theory to explain why so many ancient cave paintings of hands have missing fingers.
Handprints are an extremely common form of cave art which are found in several different continents. But two caves in France, known as Gargas and Cosquer, have unusual-looking handprints on their walls.
Of the 231 handprints in Gargas, 114 are missing at least one finger. Cosquer has 49 handprints, and 28 of those are missing fingers. Caves elsewhere in Europe are also known to have handprints lacking one or more digits.
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Experts are divided on why such a high percentage of the French cave handprints are missing fingers, but a group of Canadian archaeologists have recently posed a startling new idea.
They say Stone Age Europeans may have deliberately cut off their fingers during religious ceremonies.
"Finger amputation was a reasonably common behaviour in many regions in the recent past," Mark Collard from Simon Fraser University told NewScientist.
He and his colleagues examined studies of early modern human societies, and made the "shocking" discovery that 121 regularly practised finger amputation.
"It seems like such a debilitating practice that I couldn't imagine signing up to do it myself," he said. "Yet we kept finding group after group that did it."
The group isolated ten reasons that humans might have amputated fingers, including as a form of punishment, a mourning ritual and a marker of group identity.
Dr Collard believes fingers were mostly removed as a sacrifice during religious rituals which may have involved taking mind-altering drugs.
"Cave art is often in dark, hard-to-access parts of caves, which is consistent with them being part of some sort of dysphoric ritual."
If the theory is true, it likely means that Stone Age Europeans lived in tight-knit communities that regularly competed with each other, as studies show going through unpleasant experiences with others can enhance cooperation and bonding.
The University of Sydney's Ian Gilligan isn't convinced by the amputation theory, and has suggested that early modern Europeans lost fingers to frostbite instead.
However Paul Pettitt from Durham University, England, disputes the idea that many people from that time and region had missing fingers at all.
He said Stone Age communities may have represented numbers on their cave walls by folding down their fingers and pressing their hands against the surface.
"The argument that it's not bent fingers has usually been made on the basis that you can't bend one or more fingers back and then create a sharp-outlined hand stencil," he told NewScientist.
"If you spend half an hour learning to create these things, you can do very sharp outlines with bent fingers."