Then (1992), Food of the Gods
The Stoned Ape: Was McKenna Right?
Psilocybin mushrooms catalyzed human brain expansion, language, and consciousness. The doubling of the hominin brain over two million years was driven in part by psilocybin's effects on neuroplasticity and visual acuity.
It's Complicated
The neuroplasticity claim is partially supported: psilocybin does promote dendritic spine growth (Shao et al. 2021) and new synaptic connections. The specific evolutionary timeline lacks fossil or genetic confirmation. The mechanism McKenna proposed, improved hunting via visual acuity at low doses, is plausible but untested. The broad intuition (that the relationship between humans and psychoactive plants is ancient and significant) is well-supported by the archaeological record. The specific causal chain is not.