During the altered state of consciousness, perhaps these duuudes were communing with ancestors, spirits, animals, it’s hard to know. What we know is that they left paintings that touch and speak directly to us across a temporal chasm of tens of thousands of years.
So that was the first "high", wasn't it? :-)
Joking aside, I've always seen cave paintings as human beings' natural, innate, in-built creative side. Those paintings were not mere representations, but the first displays of aesthetic thinking. :-)