There are some who believe that fungi and plants like marijuana and psilocybin mushrooms actually co-evolved with mammalian nervous systems, as opposed to the m

There are some who believe that fungi and plants like marijuana and psilocybin mushrooms actually co-evolved with mammalian nervous systems, as opposed to the more common theory that their psychoactive chemicals evolved as pesticides (like caffeine, which is deadly to insects). For example, one theory is that psilocybin developed to inspire herbivorous animals to break out of their routines and seek new ranges, defecating over a wider area and creating a larger habitat for the mushrooms. Terrence McKenna, though it should be said that he was far from a skeptical researcher, believed that psilocybin actually drove human evolution; for example, drug-induced synesthesia, mapping concepts to symbols, is his explanation for the emergence of language. (However, language has been shown to emerge naturally in the absence of psychedelic chemicals, so I find this to be particularly far-fetched.)