Right, let's settle this once and for all. What are those cave paintings about? The ancient ones, I mean. What do they say? Why were they done? What the deuce? William McNeill recounts one explanation in the New York Review of Books—that of zoologist R. Dale Guthrie, who says that most of the paintings were probably scrawled on the walls by young adolescents, and are mostly pretty puerile:

The other evidence Guthrie offers is the subject matter of the graffiti that survive abundantly but have attracted scant attention since they lack significant artistic value. Crude, sometimes unfinished or corrected outlines of animal forms are numerous; so are images of male and especially of female sexual organs—exactly what adolescent boys would be most acutely interested in.

Guthrie then devotes an entire chapter to explaining the effects of testosterone on human consciousness and behavior and imagining how small groups of boys, emancipated from their mothers' supervision, spent surplus energy and spare time in risky, scary caves, leaving behind innumerable scratch marks and painted images that expressed their adolescent hopes and fears.

Hmmm. Or maybe cave paintings were the equivalent of modern-day bathroom graffiti: you're taking a leak, you're bored, you've got a bit of chalk, might as well scribble something with your free hand. Most of the paintings were done in relatively remote caves, small and dark, after all. Paleolithic latrines? But no, okay. Here's what seems to be McNeill's preferred explanation, a bit more high-minded:

Everyone agrees that many of the masterworks of cave art were constructed around preexisting marks and curves on natural surfaces. In that sense, we can say that human intervention did not create the animals. Instead, it assisted them to emerge from the stone, just as newborns emerged from their mothers' wombs, ready to provide a disembodied spirit with a new home.

This is not art as we know it. Nor was the most impressive cave painting the work of adolescent boys. Rather, it seems to me likely that adult hunting parties sought to make peace with dispossessed animal spirits by bringing all those surprisingly accurate images of horses, bison, and mammoths to birth in the warm darkness of the caves, so the spirits of animals they had slain could find safe and lasting homes.

Here's an earlier news account of Guthrie's theory, which sounds plausible enough, although it's not like I would really know one way or the other.