The Stoned Ape Theory which was formulated by Terrence Mckenna is one of the most creative and intriguing. It investigates and gives an explanation as to how humans gained consciousness. Basically the idea is that at one point all of North Africa jungles were turned to grasslands after the end of the last Ice Age. This caused humans to modify the way they interacted with their environments. Namely they would follow large herds of game and took on a more nomadic lifestyle, eating what they found along the way. One of the new components of their diet were the mushrooms that would be growing underneath the animal poop. These mushrooms, as many know, contained the psychoactive compound psilocybin. The result of taking the psilocybin (He theorizes) causes the senses to collapse. Perhaps some readers of this blog are familiar with mushrooms and their effects, such being able to see sounds, or hear smells. These effects are called synesthesia. These effects McKenna posits could have been a contributing factor to the development of language. Which is largely dependent upon the idea that when we are speaking we are essentially creating pictures in someone else’s mind through the use of vocal sounds.
It should be noted that McKenna didn’t call himself a scientist, and while many scientists did respect him for his creativity, he didn’t have much proof as there is very little regarding how early hominids developed language.
And here’s a video of Terrence McKenna talking about the stoned ape theory.