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he North African jungles receded and gave way to savannas and grasslands near the end of the most recent ice age, 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 varieties 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 near the dung of ungulate herds that occupied the savannas and grasslands at that time. McKenna, referencing the research of Roland L. Fisher, 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, 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. Image: Food Of The Gods (Book Cover Detail)