"Stoned ape" theory of human evolution

In his book Food of the Gods, McKenna proposed that the transformation from humans' early ancestors Homo erectus to the species Homo sapiens mainly had to do with the addition of the mushroom Psilocybe cubensis in its diet.

According to his theory, this took place around 100,000 BCE, which is when he believed that the species diverged from the Homo genus).  McKenna based his theory on the main effects, or alleged effects, produced by the mushroom, while citing studies by Roland Fischer et al. 

McKenna stated that due to the desertification of the African continent at that time, human forerunners were forced from the increasingly shrinking tropical canopy in search of new food sources.  He believed they would have been following large herds of wild cattle whose dung harbored the insects that, he proposed, were undoubtedly part of their new diet, and would have spotted and started eating magic mushrooms.

McKenna's hypothesis was that low doses of psilocybin improve visual acuity, meaning that the presence of psilocybin in the diet of early pack hunting primates caused the individuals who were consuming psilocybin mushrooms to be better hunters than those who were not, resulting in an increased food supply and in turn a higher rate of reproductive success.

Then at slightly higher doses, he contended, the mushroom acts to sexually arouse, leading to a higher level of attention, more energy in the organism, and potential erection in the males, rendering it even more evolutionarily beneficial, as it would result in more offspring.

At even higher doses, McKenna proposed that the mushroom would have acted to "dissolve boundaries," promoting community bonding and group sexual activities.  Consequently, there would be a mixing of genes, greater genetic diversity, and a communal sense of responsibility for the group offspring.  At these higher doses, McKenna also argued that psilocybin would be triggering activity in the "language-forming region of the brain", manifesting as music and visions, thus catalyzing the emergence of language in early hominids by expanding "their arboreally evolved repertoire of troop signals."  He also pointed out that psilocybin would dissolve the ego and "religious concerns would be at the forefront of the tribe's consciousness, simply because of the power and strangeness of the experience itself."

Therefore, according to McKenna, access to and ingestion of mushrooms was an evolutionary advantage to humans' omnivorous hunter-gatherer ancestors, also providing humanities first religious impulse.  He believed that psilocybin mushrooms were the "evolutionary catalyst" rom which language, projective imagination, the arts, religion, philosophy, science, and all of human culture sprang.