Stolen from Wikipedia:
http://en.wikipedia.org/wiki/Terrence_mckenna Perhaps the most famous of Terence McKenna's theories and observations is his explanation for the origin of modern human consciousness and culture. McKenna theorized that as the North African jungles receded, near the end of the most recent ice age, giving way to savannas and grasslands, a branch of our tree-dwelling primate ancestors left the forest canopy and began to live in the open areas outside of the forest. There they experimented with new varities of foods as they adapted, physically and mentally, to their new environment.
Among the new food items found in this new environment were psilocybin-containing mushrooms growing in the dung of ungulate herds that occupied the savannas and grasslands at that time. McKenna, referencing the research of Roland L. Fisher, Ph.D. (College of Optometry and Departments of Psychiatry and Pharmacology, College of Medicine, The Ohio State University), claimed that enhancement of visual acuity was an effect of psilocybin at low doses, and supposed that this would have conferred an adaptive advantage. He also argued that the effects of slightly larger doses, including sexual arousal (not reported as a typical effect in scientific studies) - and in still larger doses, ecstatic hallucinations and glossolalia - gave selective evolutionary advantages to members of those tribes who partook of it. There were many changes caused by the introduction of this psychoactive mushroom to the primate diet. McKenna hypothesizes, for instance, that synesthesia (the blurring of boundaries between the senses) caused by psilocybin led to the development of spoken language: the ability to form pictures in another person's mind through the use of vocal sounds.
About 12,000 years ago, further climate changes removed psilocybin-containing mushrooms from the human diet. McKenna argued that this event resulted in a new set of profound changes in our species as we reverted to the previous brutal primate social structures that had been modified and/or repressed by frequent consumption of psilocybin.