Ancient cave painters may have been stoned, study says
According to a new paper in "Time and Mind: The Journal of Archaeology, Consciousness and Culture" by Tel Aviv University archaeologists, the humans who ventured into these subterranean enclosures during the Upper Paleolithic (50,000 to 12,000 years ago) would have needed to light torches in order to see what they were doing. In the process, they would have reduced the amount of oxygen in the caves, inducing hypoxia (oxygen-deprivation) in their brains. That, in turn, would have put them in a state of altered consciousness, experiencing euphoria, out-of-body experiences and perhaps even hallucinations.
The article referenced in the story from Haaretz in Israel is quite thorough but I realize that not everyone can access it. This Raw Story report gives the gist of the idea.