Perhaps the most intriguing of Terence McKenna's fascinating theories and observations is his explanation for the origin of the human mind and human culture.
To summarize: McKenna theorizes 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. The changes caused by the introduction of this drug to the primate diet were many -- 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, resulting in a new set of profound changes in our species as we reverted to pre-mushroomed and frankly brutal primate social structures that had been modified and/or repressed by frequent consumption of psilocybin.
McKenna's theory has great appeal and intuitive strength, but it is necessarily based on a great deal of supposition interpolating between the few fragmentary facts we know about hominid and early human history. In addition, because McKenna (who describes himself as "an explorer, not a scientist") is also a proponent of much wilder suppositions, such as his "Timewave Zero" theory, his more reasonable theories are usually disregarded by the very scientists whose informed criticism is crucial for their development.
This page links to resources that should help to fill in some of the gaps with data from the sciences and with other theories and myths about human origins.
The 20th century mind is nostalgic for the paradise that once existed on the mushroom-dotted plains of Africa, where the plant-human symbiosis occurred that pulled us out of the animal body and into the tool-using, culture-making, imagination-exploring creature that we are.
As you may or may not know, all primates -- and we certainly are primates -- have what are called 'male dominance hierarchies'. This means that the longest-fanged, meanest s.o.b. in a monkey tribe takes control of the group resources, the females, the weaker males, and this character runs the show, and this is pretty much how we do it today.
And there, in the manure of these ungulate animals that had evolved with the primates on the grasslands of Africa, was the mushroom, and the mushroom was acting as a tremendous force for directing the evolution of human beings away from that of the rest of the anthropoid apes and toward the unique adaptation that we see as special to human beings today.
People have attempted -- unsuccessfully -- to answer the question of how our minds and consciousness evolved from the ape. They've tried all kinds of things to account for this evolution, but to my mind, the key unlocking this great mystery is the presence of psychoactive plants in the diet of early man.
[T]he very forces which created the mushroom paradise in the grasslands of Africa, that same force which was the climatological drying out of the African continent and much of the rest of the planet, eventually turned those grasslands to desert
[I]ntelligence, not life, but intelligence may have come here in this spore-bearing life form... I think in a hundred years if people do biology they will think it quite silly that people once thought that spores could not be blown from one star system to another by cosmic radiation pressure.
[O]n one level, at the lowest dose, psilocybin increases visual acuity, which means better success at hunting. Then, at the middle dose level, it creates this hypersexual activity. Then, at still higher doses it creates the full-blown psychedelic experience, about which we are as uninformed and as easily amazed as our remote ancestors were. So, it was a 3 step process. It was basically a chemical that had been allowed into the diet that boosted us toward boundary dissolution, language acquisition, sexuality without boundaries, and so on.
There was a period when, because of the presence of psilocybin in the diet, the natural tendency to male dominance hierarchies was interrupted. It was in that moment that community values, altruism, language, long-term planning, awareness of cause and effect, all the things that distinguish us were established.
From 75,000 to about 15,000 years ago, there was a kind of human paradise on Earth. People danced, sang, had poetry, jokes, riddles, intrigue, and weapons, but they didn't possess the notion of ego as we've allowed it to crystallize in Western societies.
The way I analyze the modern predicament -- pollution, male dominance, there are a million ways to say it -- the overriding problems are brought on by the existence of the ego, a maladaptive behavioral complex in the psyche that gets going like a tumor. If it's not treated -- if there's not pharmacological intervention -- it becomes the dominant constellation of the personality.
[P]silocybin triggers this activity in the language-forming capacity of the brain that manifests as song and vision.