There are various hypothesis as to how humans became the dominant species. Perhaps the least-known and least-supported of these is the suspicion that our distant ancestors used natural psychedelic compounds that led to societal advances and bodily adaptations.

To the best of my knowledge, the idea has zero support among anthropologists and archeologists. It seems limited mostly to psychedelic proponents, the most prominent of whom is Terence McKenna, who outlined the idea in his book Food of the Gods.

Specifically, he wonders if as we became bipedal and made our way from the Horn of Africa to the savannah, we consumed psilocybin, which formed naturally on the ground. According to the tale, this fomented an ability to think abstractly, to develop toolmaking and fire-building skills, and fostered the first use of rudimentary language. As to why the likes of gazelles, primates, and other animals who had equal access to the compound had no corresponding advancements is left unexplained.

On another topic, McKenna attributes the tripling of brain size that took place in upright hominids over three million years to regular ingestion of psilocybin. He does this while claiming science offers no other explanation. First, even if that were true, it would merely be a secular variant of the god of the gaps fallacy. Secondly, anthropologists and archeologists have a good idea of why this trebling of the cerebrum and related parts occurred. 

They attribute it to the evolution of the opposable thumb and to cooking food, which made the victuals more nutritious. Skeptoid’s Brian Dunning noted, “Anthropologists have cemented these ideas with the expensive tissue hypothesis, which provides a metabolic and biochemical explanation for how protohumans were able to afford the greater energy requirements of a larger brain on the same basic energy budget, by reducing the relative size of the gut which became possible once food was being cooked to make its nutrients much more bioavailable.”

The brain and digestive systems require the most energy to function, and we see the results of this is humans and other animals. There exists a negative correlation between brain and gut sizes. As one shrinks, the other grows, and vice versa.

Dunning used the cow as an example, noting that our bovine buddies “consume only grass, a terrible diet virtually devoid of nutrition. So it needs four enormous stomachs and a great long digestive system, all energetically expensive tissue, leaving it with a tiny brain.”

We, by contrast, eat a far more energy dense diet – including a lot of cow meat and milk – and this enables the human digestive systems to be small and fuel-efficient. That leaves plenty of energy to fuel the brain, which is why it is of ample size to see the folly in a hypothesis predicated on stoned apes.