The Stoned Ape at the Edge of History

“The mushroom spoke, and our symbiotic relationship with it is what has given us souls.”

Terence McKenna's most famous hypothesis, that psilocybin mushrooms catalyzed the evolution of human consciousness, that they sit at the origin of language and culture and the doubled brain, is almost certainly wrong in its mechanism and possibly right in its direction.

The specific claim is this: as hominids migrated out of the forest into the grasslands of Africa roughly two million years ago, they began following herds of cattle. The cattle grazed. The cattle produced dung. In the dung, particularly in the tropical lowland conditions that pertained along migration routes, grew Psilocybe cubensis mushrooms. Hominids who ate the dung, as many primates will opportunistically eat the dung of other animals for nutritional and microbiological reasons, encountered the mushrooms. At low doses, the psilocybin improved visual acuity, potentially providing an advantage in hunting. At higher doses, it produced the states of visual, auditory, and conceptual richness that McKenna associates with the origins of religious experience, language, and art.

McKenna supported this argument with data on human brain expansion: the hominin brain doubled in size over approximately two million years, from around 530 cubic centimeters in Homo habilis to around 1450 in anatomically modern humans. This expansion is real. It is one of the most dramatic brain size increases in evolutionary history. McKenna argued that psilocybin, by stimulating synaptic density and the growth of new neural connections, could plausibly have contributed to this expansion.

The problem is that this argument has two levels, and only one of them can be evaluated with current science.

The evolutionary claim, that mushroom consumption drove the doubling of the hominin brain, lacks fossil evidence and genetic confirmation. We have no mushroom residue in early hominin coprolites. We have no genetic signature of psilocybin exposure in ancient DNA. We do not have the kind of evidence that would allow us to distinguish McKenna's hypothesis from dozens of competing explanations for human brain expansion, including increased caloric density from cooking, social complexity and its cognitive demands, the selective advantages of symbolic thinking, or simple sexual selection for intelligence.

The neuroscientific claim, that psilocybin increases synaptic density and promotes the growth of new neural connections, is supported. David Nichols's 2016 Pharmacological Reviews comprehensive survey of psychedelic science documents the mechanism: psilocybin acts as an agonist at 5-HT2A receptors, with secondary effects on glutamate signaling, and produces measurable changes in cortical connectivity. More recently, research from Washington University (Shao et al., 2021) and other groups has shown that psilocybin promotes dendritic spine growth in mouse frontal cortex, the structural substrate of new synaptic connections, and that this effect persists for weeks after a single exposure. The neuroplasticity claim, in other words, is real. The evolutionary timeline it implies is speculative.

What McKenna was most right about was something more diffuse than the specific mechanism. He was right that the relationship between human beings and psychoactive plants is ancient, ubiquitous, and poorly understood. He was right that the standard scientific framework for thinking about drug use, as pathology, as addiction, as a behavior to be explained in terms of reward circuits hijacked by foreign molecules, does not begin to account for the role these substances have played in human cultural history. The ethnobotanist Richard Evans Schultes, whose 1967 NIMH symposium paper documented the extraordinary depth and sophistication of indigenous psychopharmacological knowledge, estimated that the relationship between humans and psychoactive plants predates recorded history by orders of magnitude. Seventy plant species used for intoxication globally, each with its own cultural context, ceremonial tradition, and accumulated empirical knowledge, this is not the behavior of a species stumbling accidentally into drugs. It is the behavior of a species that has organized major portions of its cultural life around these substances.

McKenna was right about that pattern. His error was in the specificity of the causal claim. He wanted to identify the exact molecule that made us human, as if there were such a thing, rather than describing the broader truth that our relationship with plant medicines is woven into what we are. The stoned ape hypothesis fails as a specific scientific claim and succeeds as a rhetorical provocation: it forces you to take seriously the possibility that something important happened at the intersection of human evolution and psychoactive plants, even if what exactly happened cannot yet be specified.

The mushroom spoke. McKenna was confident about that. What it said, and when, and what exactly it did to the brains of the creatures who ate it, those questions remain open, waiting for the kind of evidence that the fossil record may never provide.

Sources

• McKenna, T. Food of the Gods (1992)
• Nichols, D.E. Psychedelics. Pharmacological Reviews, 68(2) (2016)
• Medical History of Psychedelic Drugs. Cambridge HPS dissertation (2007) [Source not found; under review]