People have wondered why humans made those cave paintings tens of thousands of years ago. One thing I would think is for sure; that they did so because they discovered they could do it. But, I feel I've found another reason; the pictures are monuments to their humanity like later architecture and artwork were made for. At that time, they hadn't thought to work with stone, clays, or even dirt. The later use of stone and clays to make temples and statues was a 'generalization' of the discovery of drawing artwork in caves. I'd site the drawing of their hands as proof of what these cave drawings are about.