The "Stoned Ape" theory of human evolution
Perhaps the most famous of Terence McKenna's theories and observations is his explanation for the origin of the human mind and culture. McKenna theorized that as the North African jungles receded toward the end of the most recent ice age, giving way to grasslands, a branch of our tree-dwelling primate ancestors left the branches and took up a life out in the open — following around herds of ungulates, nibbling what they could along the way.
Among the new items in their diet were psilocybin-containing mushrooms growing in the dung of these ungulate herds. 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)[14] [15] [16] [17], claimed enhancement of visual acuity as 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 a physical sexual arousal (again, not reported as a typical effect in scientific studies) — and in still larger doses, ecstatic hallucinations and glossolalia — gave evolutionary advantages to those tribes who partook of it. There were many changes caused by the introduction of this drug to the primate diet. McKenna theorizes, 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 the mushroom from the human diet, which McKenna argued to result in a new set of profound changes in our species as we reverted to pre-mushroomed and brutal primate social structures that had been modified and/or repressed by frequent consumption of psilocybin.
McKenna did not attempt to defend his hypotheses through rigorous scientific evidence; he consciously self-identified as a type of shaman, or ethnobotanist. McKenna and his followers view his theories as speculation that is at a minimum scientifically feasible and arguably gifted by special knowledge due to psychedelic plants. His hypothesis that psilocybin induced a phase change in human evolution is necessarily based on a great deal of supposition interpolating between the few fragmentary facts we know about hominid and early human history. But perhaps its most significant problem is its inconsistency with natural selection (the central concept of evolutionary theory) which cannot favor any variations, no matter how adaptive, unless they result from an allele or genetic factor. A live recording of his "Stoned Ape" theory can be found on the CD Conversations on the Edge of Magic (recorded live at the Starwood Festival).