Perhaps it was metaphorical from them as well, going down into the caves to make their animal paintings.  They would have been leaving, even then, the world of words, the forms and logic of the ego behind, in the fire-lit darkness meditating in a way to reach the simple awareness of consciousness.  Dreams and reality they painted.  In the cave they were clear of the excess of thoughts and concerns.  They were close to nature, a part of it, again.