Item: A group of Tokyo researchers "...recently published a comprehensive study of over 40,000 years worth of cave paintings and found some pretty telling patterns (PDF). The spiral-like and labyrinthine designs that pop up in paintings from locations that are thousands of miles away from each other didn't just pop up by coincidence. Since these patterns are consistent with those that many humans see after taking hallucinogenic drugs, the scientists think that ancient cavemen had more in common than previously thought. They all loved to get high." Source: Gizmodo (2013). More here.
Item: "A cave mural in Spain suggests Europeans may have used hallucinogenic mushrooms in religious rituals 6,000 years ago, researchers say.
"The cave art may depict fungi with hallucinogenic properties, which would make them the oldest evidence of their use in Europe...
"The Selva Pascuala cave mural near the town of Villar del Humo has as its central feature a bull, but it is a row of 13 small mushroom-like objects that interests two researchers." Source: UPI (2011).
Comment: The purpose of prehistoric cave paintings is still a mystery, but some scholars think they had some kind of religious significance. For instance, the images of animals commonly found on cave walls could have been painted during shamanistic rituals designed to improve the tribe's chances during the hunt.
If the cave paintings did have a shamanistic purpose, then it wouldn't be surprising to learn that the painters used hallucinogenic plants to enter into visionary trance states while they created their rock art. However, I'm not sure if the common patterns found in these cave paintings really constitute evidence of the use of hallucinogens. After all, patterns like this could simply be a natural product of our neurobiology, the way the brain creates patterns in general.
The mushrooms in the Selva Pascuala cave mural are suggestive, but who knows why they're really there? Until some hard evidence emerges, all of this is just speculation, a matter of interpretation. If these prehistoric cave painters were taking hallucinogenic mushrooms, we won't know for sure until the remains of the actual mushrooms are discovered, preserved in fossils, perhaps. Organic matter can survive for long periods of time, but it's very rare.