By Gary L. Wenk, Professor of  Psychology, Neuroscience and Molecular Virology, Ohio State University

A long time ago, our ancestors discovered that some plants produced effects that ranged from unpleasant, or even lethal, to incredibly pleasurable. Many of our most popular psychoactive drugs were discovered in plants. Plants contain chemicals that are nutritious, psychoactive, or both. Why do plants have such profound effects upon us? Why are plants trying to control humans? Tobacco, caffeine, and opiates have almost succeeded in taking over the world.

In truth, plants have no interest in humans at all.  For the past one hundred and twenty thousand years, since the origin of our species, we have been, and will likely remain in spite of our role in global warming, almost entirely irrelevant to them. Why?  The Earth is home to over one trillion different species; invertebrates such as insects, spiders, and mollusks make up eighty percent of all of those species while plants make up about seventeen percent of all species.  In terms of the number of species and total biomass, plants and insects are the dominant two species on the surface of the planet.  For the past 400 million years, plants and insects have established a complicated symbiotic relationship: plants both need the insects for their own survival and procreation and also must avoid being eaten by the insects. The problem for plants is that they are not mobile; they cannot simply run away from the bugs or swat them with a limb.  Their solution was to synthesize a large variety of chemicals to influence the insects’ behavior to serve the needs of the plant.  These chemicals are produced only for the plant’s interactions with insects.  Plants do not produce chemicals for our benefit, nutrition or entertainment.  Humans are simply bystanders to the tug of war between plants and insects; we can either benefit from their battle or become casualties.

Why do our brains respond so profoundly to the chemicals in plants?  To discover the answer, we need to go back in time to about 1.3 billion years ago when the last common ancestor of both plants and animals lived on the planet.  Today, humans and plants share over three thousand genes that are critical to our survival that were bequeathed to us by this simple creature.  This shared genetic message due to a shared evolutionary history explains why our human brains respond to the contents of plants.  Plants, insects and human brains all produce and utilize chemicals that are the basis for our normal brain function, such as the familiar neurotransmitters acetylcholine, dopamine, serotonin and GABA that profoundly affect our thinking and mood.  Interestingly, our human brains also synthesize many of the same psychoactive chemicals that exist in plants, including chemicals similar to morphine and the hallucinogens DMT and bufotenin.  All of these neurotransmitters and psychoactive chemicals were already present over a billion years ago in the last common ancestor of plants, insects and humans.  The chemicals in plants influence our brain function because of our shared genetic history.

Therefore, whether you choose to eat a bunch of broccoli or a baked potato, the chemicals they contain may alter how your neurons function and, therefore, how you feel and think.  We have all experienced the consequences of our shared evolutionary history with the plants we eat. For example, unripe bananas contain high levels of the neurotransmitter serotonin. When you eat an unripe banana, its serotonin is free to act upon the serotonin neurons within your intestines. The consequence is likely to be increased activation of the muscles in the wall of your intestines, usually experienced as diarrhea.

Plants are not the only source of chemicals that can act upon your brain. The fact that you share an evolutionary history with insects and reptiles also underlies the ability of venoms, which often also contain serotonin, to produce their unpleasant effects. Our shared history with plants and animals on earth leads to some interesting predictions. For example, consider the following science fiction scenario: A spaceman is walking on an earth-like planet and is suddenly bitten by an unfriendly and grizzly looking creature. The spaceman can see that he is injured and that a liquid substance was injected under his skin by the beast. Does he die? No, he does not die, because his species and that of the creature on this foreign planet do not share an evolutionary past or a common ancestor. Their independent evolutionary paths have made it highly improbable that they use similar neurotransmitter molecules within their respective bodies. Thus, every spaceman, from Flash Gordon to Captain Kirk to Luke Skywalker, should feel safe walking around any planet (except their own) with impunity from animal and plant toxins. For this same reason, the intoxicating drinks and powerful medicines that always seem to be popular in these foreign worlds in science fiction movies would also be completely without effect on the brains of our plucky spaceman.

Gary L. Wenk is Professor of Psychology & Neuroscience & Molecular Virology, Immunology and Medical Genetics at the Ohio State University and Medical Center. He is the author of “Your Brain on Food” (3rd Edition, 2019, Oxford University Press)