scientists led by archaeologist Prof Mark Collard of Simon Fraser University in Vancouver say the truth may be far more gruesome. “There is compelling evidence that these people may have had their fingers amputated deliberately in rituals intended to elicit help from supernatural entities,” said Collard.
Up until 12kyr ago was the last glacial period, and stone age painting by definiton were mostly around that time. Maybe it was just cold and people out all day in the cold hunting deers lose their finger?
But if cave paintings from tropical regions like Africa and south asia also have missing digits then I'll believe it might a ceremonial thing.