We all know that particular plants and mushrooms can affect the mind – how we perceive the world, how we feel and how we think. Some of these plants can induce stimulating effects, including: tobacco, the seeds of Coffea arabica (used to make coffee), the coca leaf (used to make cocaine), betel, khat, the seeds of cocoa (used to make chocolate), the leaves of Camellia sinensis (used to make tea), plants of the Ephdra species (which contain ephedrine, which has a similar effect to adrenaline) and kola seeds (used to make Coca Cola, a drink which also contained cocaine when it was invented in 1886). Other psychoactive plants can offer more sedative and euphoric effects, including the opium poppy (Papaver somniferum). Opium contains 26 different alkaloids, of which two are recognisable: codeine (which is prescribed for pain relief) and morphine (which is used medicinally for pain relief, but which is also converted into heroin).

The more interesting plants with psychoactive properties are those which can offer drastic alterations in perception, mood and thought. These are the psychedelic plants and mushrooms. They include cannabis (which can be viewed as a very mild psychedelic), psilocybin-containing mushrooms, the Amanita muscaria mushroom (which contains muscimol), Hawaiian baby woodrose seeds and morning glory seeds (both of which contain LSA – closely related to LSD), various plant species containing bufotenin, various plant species containing DMT or 5-MEO-DMT, the Peruvian Torch, San Pedro and Peyote cacti (which all contain mescaline), Salvia divinorum (which contains Savlinorin A), Tabernanthe iboga (which contains ibogaine) and the blue Egyptian water lily (Nymphaea caerulea), which may have been used sacramentally in ancient Egypt.

So it seems nature is abundant with plants and mushrooms which can cause profound changes in consciousness. But then we have to ask ourselves: why do some plants and mushrooms have these psychoactive properties? Why would some plants contain compounds which, when consumed, can transform reality as we know it, induce feelings of ecstasy, enhance creativity and offer us personal, philosophical, existential and ecological insights? These plants also can allow us to have mystical experiences, involving strange phenomena such as a sense of ineffability, contact with a ‘divine intelligence’, ego death and that elusive feeling of ‘oneness’, unity or interconnectedness with humanity, nature or the universe.

These dramatic effects have led some to assume that these plants are gateways into a kind of ‘spiritual reality’. Of course, this explanation won’t sit well with people who demand reliable evidence, regardless of someone’s strong conviction about an experience they’ve had. Others assume that these compounds exist to teach humans about their true place in nature, in order to live harmoniously with nature and with each other. However, this assumes that plants are purposeful and have a ‘plan’ for us. But why else then would these plants have psychoactive properties? Could it really just be accidental?

It will be helpful to answer this question from an evolutionary perspective. One of the ways that plants ward off predators is through chemical defences, the other two forms of defence being nutritional (i.e. making metabolism of the plant difficult for insects) and physical (having characteristics such as thorns). Many plants have developed chemical defences in order to avoid being consumed by insects. These chemicals achieve this by creating adverse physiological effects in the insect, such as a bad taste or even poisoning. Nicotine, contained in the tobacco plant, for example, kills insects by paralysing their muscles. It would do the same for humans if the concentration was high enough. And cocaine, contained in the coca plant, kills insects by inhibiting their feeding. Again, a similar hunger-inhibiting effect is well established in humans, but the effect is not fatal, unless of course the dose is frequently administered.

While these compounds can induce elation, euphoria and altered states of consciousness in humans, they serve completely different functions for the plant. Thus, psychoactive drugs are accidentally psychoactive for humans – the compounds are essentially part of the plant’s mechanism for survival. In some cases, these compounds act as insecticides (as the examples above show), but in many cases, there is no known function of the compounds. Many of these compounds have similar structures to human neurotransmitters – indeed this is precisely why they can affect our minds in the way they do. Tryptamines, for example, are a group of alkaloids which include neurotransmitters (i.e. serotonin), hormones (i.e. melatonin) and psychedelic compounds (i.e. psilocybin, psilocin, DMT, 5-MeO-DMT and bufotenin).

These psychedelic tryptamines are serotonergic substances because they produce effects via the serotonin system in the brain. Because tryptamines have a strong structural similarity to serotonin (DMT especially), they bind to 5-HT2A receptors in the brain, which serotonin also binds to. We have to ask again: Is this just a coincidence? Or is psilocybin, psilocin (which is what psilocybin converts to in the body) and DMT similar to brain chemistry because mushrooms and plants containing these compounds evolved for human consumption?

Again, it may just be the case that these alkaloids in mushrooms and plants are toxic to insects and other small animals when metabolised, but for humans are not harmful. This may be because psychoactive doses for humans – at least the range of psychoactive doses we find desirable – are not high enough to produce toxic effects. Another possibility is simply that these particular plants did not evolve chemical defences to ward off humans.

In any case, compounds such as psilocybin and DMT are the least toxic of all psychedelic drugs, which Terence McKenna argued was due to their structural similarity to ordinary brain chemistry. The fact that these compounds can bind to 5-HT2A receptors in the human brain could be just a complete coincidence. Conversely, some plants have chemical defence mechanisms which ward of some animals (including humans) but not others. Poison ivy, for example, contains urushiol, the oily resin in the plant which cause that infuriating rash. But for some animals urushiol does not cause these undesirable effects – deer, goats, horses, cows and beetles can happily eat poison ivy with no ill-effects.

Another possibility is that these alkaloids are just a useless waste product of some other essential chemical process going on in the plant. A further possibility, still, is that these alkaloids are psychoactive for other animals, but that these altered states are undesirable for the animal. For example, since water is such a scarce source in the desert, the Peyote cactus may have developed mescaline as a defence mechanism to protect its precious water supply. When an animal consumes mescaline, the disorientation and altered sensory perception may not only be unpleasant (leading the animal to avoid the cactus in the future), but it may, of course, make the animal more vulnerable to attack by predators. Psilocybin and other psychoactive compounds may have similar undesirable effects for animals when consumed. The fact that some plants have desirable properties for us does not mean that they evolved these properties for us.

It’s the same with medicinal plants. For instance, the bark of willow trees contains salicyclic acid (the active metabolite of aspirin) and salicyclic acid is known for its ability to ease aches and pains and reduce fevers – hence why it was used to synthesise aspirin. This chemical also has anti-inflammatory properties (which is why it is used in acne creams), and it is also known to have anti-cancer properties. But the willow tree did not evolve to produce salicyclic acid so that humans could use it medicinally. The willow tree does not benefit by having its bark stripped away. Salicylic acid is a plant hormone with multiple functions, including the activation of plant pathogen defence systems.

In conclusion, some plants have medicinal and psychoactive properties, not because we evolved symbiotically with them, but because we stumbled upon active ingredients in the plants which were desirable, so we continued to use them, either for medicinal or ceremonial purposes.