Why Do the Contents of Simple Plants Affect My Big Brain?

We all learned in high school biology class that plants and animals belong to completely different kingdoms of life. Given this, how is it possible that the chemicals produced by plants can influence the normal function of a species from the animal kingdom?

It is common knowledge that the contents of many different plants, from belladonna to foxglove to willow bark, can affect health. Many of the nutrients we require and many of our psychoactive drugs were discovered inside plants. Nutritional neuroscientists and psychopharmacologists have been investigating the mechanisms that underlie how the contents of plants alter brain chemistry and brain function.

Basically, plants contain chemicals that are nutritious, psychoactive or both. Everything that humans consume can, and often does, affect brain function in subtle and profound ways and influences how we think and feel. I recently wrote about a study that discovered that consuming daily water extracts from the forsythia plant might one day alleviate memory deficits in patients with Alzheimer’s disease.

Why do plants have such profound effects on us? Are plants trying to control humans? In truth, plants have no interest in humans at all. For 200,000 years since the origin of our species, we have been, and will likely remain despite our role in global warming, almost entirely irrelevant to them. Why?

Earth is home to more than 1 trillion different species; invertebrates such as insects, spiders, and mollusks make up 80 percent of all those species, and plants make up approximately 17 percent. In terms of the number of species and total biomass, plants and insects are the dominant two species on the surface of the planet (single-celled organisms are the dominant species in Earth’s crust).

For the past 400 million years, plants and insects have had a complicated symbiotic relationship: Plants need insects for their own survival and procreation and must avoid being eaten by them. 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 has been to produce a large variety of chemicals to influence the insects’ behavior to serve the needs of the plants.

These chemicals are called secondary metabolites because they do not play a primary role in a plant’s biological processes related to its daily existence–they are produced simply for the plant’s interactions with insects. Plants do not produce these secondary metabolites for our benefit 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 approximately 1.3 billion years ago when the last common ancestor of both plants and animals lived on the planet. Humans and plants still share more than 3,000 genes that are critical to the survival that was bequeathed to us by this 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 the chapters that follow, including acetylcholine, dopamine, serotonin, γ-aminobutyric acid, glutamate, opiates, and prostaglandins. Human brains synthesize many of the same psychoactive chemicals that exist in plants, including morphine and the hallucinogens dimethyltryptamine and bufotenin. All these chemicals already existed more than one billion years ago in the last common ancestor of plants, insects, and humans. Consuming these ancient molecules can influence our brain function because of our shared genetic history.

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 receptors within your intestines. The consequence is likely to be increased activation of the muscles in the wall of your intestines, usually experienced as diarrhea.

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Our shared history with plants on Earth leads to some interesting predictions for the future. For example, consider the following science fiction scenario: A spaceman visiting an Earth-like planet walks into a bar to enjoy a beverage of a local fermented plant. Does he get drunk or die? No, he does not die, and the chemicals in the beverage are unlikely to affect his brain. The reason is that the spaceman and the plant on this foreign planet do not share an evolutionary past.

Although their amino acids might have first evolved in space, as is now believed, since that distant time, their independent evolutionary paths have made it highly improbable that they use similar molecules within their respective cells. Thus, every spaceman, from Flash Gordon to Captain Kirk and Luke Skywalker, should not bother visiting the local bars.