
In a press release for the University of Utah study, Lisa Potter writes:
The authors hoped that psilocybin’s evolutionary history would clarify the most basic question—what does psilocybin do for mushrooms? The psilocybin-producing gene clusters likely have some benefit, but no one knows what it is.
Nevertheless, the authors suggested some hypotheses related to defence against predation, and they aren’t the first to suggest them. Potters state:
The molecular structure of psilocybin mimics serotonin and binds tightly to serotonin receptors, especially at 5-HT2A, a famous receptor onto which many psychedelic drugs bind. When a chemical binds to these receptors in mammals and similar ones in insects and arachnids, they produce unnatural and altered behaviors. Some have proposed that this altered mental state might be a direct deterrent to predation. It’s also possible that psilocybin functions as a laxative or induces vomiting to spread spores before they are fully digested. However, psilocybin mushrooms often occur infrequently in the wild, making it unlikely that animals could learn to recognize them. An alternative theory is that psilocybin is a chemical defense against insects. However, empirical studies are lacking, and the authors’ observations confirm that psilocybin-containing mushrooms regularly host healthy, thriving insect larvae.
She adds:
The authors are preparing experiments to test an alternative theory that they call the Gastropod Hypothesis. The timing and divergence dates of Psilocybe coincide with the KPg boundary, the geological marker of the asteroid that threw Earth into a brutal, prolonged winter and killed 80% of all life. Two lifeforms that thrived during the darkness and decay were fungi and terrestrial gastropods. Evidence, including the fossil record, shows that gastropods had a massive diversification and proliferation just after the asteroid hit, and it’s known that terrestrial slugs are heavy predators of mushrooms. With the study’s molecular dating of Psilocybe to around 65 million years ago, it’s possible that psilocybin evolved as a slug deterrent. They hope that their feeding experiments will shed some light on their hypothesis.
In my book, I note that these potential functions of psilocybin were suggested in Kerry Ogame and L.G. Nicholas’ book Psilocybin Mushroom Handbook, published back in 2006. I also covered the topic in a blog post back in 2014. Whiteman observes:
It seems like these chemicals were tailor-made for us. Yet, no humans were around when the organisms producing the chemicals first evolved the ability to make them. A closer look reveals that in many cases we seem to have simply intercepted chemical weapons deployed in what naturalist Charles Darwin called “the war of nature.” Quite simply, many baroque chemicals we use and abuse appeared on the planet because they enhance the survival odds of the organisms that make them or absorb them through their diet or microbiomes.
He adds:
Why do some organisms make psychedelic chemicals? Perhaps because what is good for the goose is good for the gander: like botulinum toxin, penicillin, caffeine, Taxol, and ziconotide, natural psychedelics may serve as chemical weapons that plants, fungi, and animals use in defense against enemies.
In my book, I point out (as Whiteman does) that psychoactive drugs like caffeine and cocaine evolved as a form of defence against insects. They act as toxins; they inhibit the hunger of insects, and they cause paralysis and eventually death. Psychedelic exceptionalism may motivate some psychedelic users to concede that many psychoactive compounds exist purely as insecticides, and we just so happen to enjoy their effects, but this cannot possibly apply to psychedelics. The latter are seen as producing effects simply too profound and transformative to be accidental. I understand this line of thought, but it nonetheless requires evidence to support it.
A 2018 study, published in Evolution Letters, also arrives at the conclusion that psilocybin may have been created to ward off insects. While others had suggested this possible function previously, this study – authored by a team of researchers at Ohio State University – helps to support the hypothesis. These researchers compared several species of mushroom containing psilocybin with those that did not. What they discovered was that the psilocybin mushrooms shared a cluster of five genes absent in the non-psychedelic ones.
The many different species of psilocybin mushrooms may not be closely related, but they nonetheless all like to grow in animal faeces and rotting wood.

These are all habitats where you can find many species – of insects and other creatures – that like to feed on mushrooms. While we experience psychedelic effects when we consume these mushrooms, this doesn’t mean that insects and other small animals trip as well. The authors suggest that psilocybin may cause the insects to lose their appetite (the same effect that caffeine and cocaine has on insects).
Despite being distantly related, various species of mushrooms have been able to share this pest control technique through a process known as ‘horizontal gene transfer’. This involves the movement of genetic material between organisms that are not in a parent-offspring relationship. Bacteria are able to share antibiotic-resistant genes in this way, for instance. This is a tactic commonly employed by single-celled organisms, but it is much rarer to observe it in complex organisms like mushrooms.
The process is typically set in motion by a stressor, such as an attack by a predator. In the case of psilocybin mushrooms, attempts by insects to eat them may have been the catalyst that led the mushrooms to start producing psilocybin. In addition, the insects are not only predators of mushrooms; they also compete with them for food. Termites would be the major fungal competitors inside decaying wood. This would rule out the notion that psilocybin is just a useless waste product of some essential process going on in the mushroom.
Writing on the research for Scientific American, Jennifer Frazer writes (in agreement with what has been said so far about natural psychoactive compounds):
[T]he majority of naturally-produced recreational drugs – caffeine, nicotine, cocaine, morphine, and psilocybin evolved to be, if not quite insecticides, then scramblers of insect brains. The fact that our brains are enjoyably scrambled by them too is sheer coincidence, but also speaks to the uncomfortable truth that your brain is not so different from a cockroach’s as you might like to think.
Psilocybin produces psychedelic effects in us because its metabolite, psilocin, activates the same receptors (5-HT2A) receptors as serotonin. However, we are, of course, not the only creature that produces this neurotransmitter. Insects produce serotonin as well. So psilocybin may also affect the brains of insects, just not in the way we’re familiar with (and which we like) when we consume these mushrooms. As Frazer puts it:
Psilocybin may help tilt the playing field in the fungus’s favor by causing insects to, I don’t know, maybe blank on what they went in that log for again? Another serotonin receptor antagonist called 5HT-2A causes Drosophila fruit flies to somehow neglect to eat that fruit they’re sitting on.
This, then, would amount to an eating-inhibiting effect, even if not a hunger-inhibiting one (although perhaps psilocybin serves that function as well). It is also possible that in larger (but non-human) animals, psychedelics have different psychoactive effects, but these are still felt to be unpleasant. It is true that there are many species of animal that enjoy getting high, but this does not mean every animal would experience the consumption of psychedelic plants and mushrooms as pleasant. They may feel disoriented and in a fear-ridden state, which we can see in a 1960 film showing a cat on LSD. Moreover, animals in this altered state of mind would be more vulnerable to attack, which adds a further reason for them to avoid consuming psychedelic plants and mushrooms.
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