“Nothing in biology makes sense except in the light of evolution”
“Revolutionary new drugs have rarely been developed by the pure insights of molecular and cellular biology . . . Rather, the pathway of discovery has usually been the reverse: the presence of the drug is first detected in whole organisms, and the nature of its activity is subsequently tracked down to the molecular and cellular levels. Then the basic research begins”
-Edward O. Wilson
Psychoactive substances are chemicals capable of inducing changes in mood, perception and related subjective states in humans. Many species of animals consume certain types of plants that induce psychotropic effects. If we think about it, one of the important questions about this is: Why would a plant synthesize psychoactive substances? After all, plants do not have a nervous system, and as far as we know, a nervous system is necessary to produce the phenomenon of sensation. Therefore, how does the production of such substances benefit plants?
In one word: Evolution!
Nature is the best chemist. During the course of evolution, through literally millions of years, a wide variety of organisms have developed substances used for defense against predators or to become predators themselves (this sentence is the very first sentence of my PhD dissertation… (:-)…). As part of this process, chemical structures which prove beneficial for the survival of the organism are conserved. These molecules may act as toxins against proteins and other macromolecules that control important physiological processes in other organisms.
There is abundant evidence suggesting that plants use chemicals as defense from insects. A very well-known example is nicotine, produced primarily by tobacco plants; this compound is probably one of the most addictive drugs used by humans. Nicotine has been described as a natural insecticide, which interferes with the normal physiology of an insect’s nervous system. Insects are much smaller than humans; therefore, an insect that tries to feed on a tobacco plant is exposed to nicotine; this exposure will repel the insect and can even kill it (think overdose). On the other hand, a human (a much bigger organism) exposed to the same amount of nicotine can experience psychoactive effects, why?
The reason, again, has to do with an evolutionary concept, namely common descent. In the same way that many defensive molecules are conserved for their positive effects on survival, their targets are conserved too. In other words, vertebrates, including humans, will express targets very similar to the ones present in insects. In this example, a human that smokes or chews tobacco, will be exposed to nicotine, which will interact with these targets and induce pleasurable sensations. The similarity between many insect and vertebrate molecular targets is why invertebrate animal models have provided important insights about human physiology.
In this context, it has been suggested that many types of plants are engaged in a “chemopsychological war” against animals. These plants can have an evolutionary edge at several levels. The first one, as I mentioned above, is the immediate protective effect againts insects. A second example is more subtle. Humans, upon experiencing pleasurable sensations from the use of a given plant species, will start a selection process. In this process, plants with desirable properties are protected from insects or grazing animals and eventually actively cultivated. Eventually, a population of plants with the desired traits is obtained. This is a practice still widely performed in agriculture and animal husbandry today.
It has been suggested that these human-initiated processes are major components of evolutionary change in nature. Some schools of thought propose that psychotropic substances coevolved with humans; for example, the ingestion of neurotransmitter-like molecules derived from plants could act as substitutes for bioenergetically expensive neurotransmitters under nutritional deprivation states.
How where psychoactive plants discovered? We will probably never know for sure; this happened thousands of years ago, before history began to be recorded. Nonetheless, we can imagine a few scenarios. The first one would be when a person ate a plant. If the plant produced a substance capable of inducing a psychoactive effect, it would be apparent right away. A second possibility is when someone took a stroll through a field and nibbled on a blade of grass; the person may have noticed a peculiar flavor, a funny sensation or even a numbing sensation in the mouth. This was the most likely way by which coca leaves were discovered to have anesthetic properties. It has been documented that South American natives were aware of this, as they used to chew on coca leaves to alleviate toothaches. The coca plant, of course, is the natural source of cocaine, a potent psychoactive and addictive compound, as well as the first example of a local anesthetic.
Finally, let’s suppose that after the discovery of fire, a group of humans needed some kindling (like dry leaves) to start a campfire. They found some dry marijuana leaves and flowers, started the fire, gathered around it, inhaled the smoke and well…
The rest, as they say, is history.
If you want to know more
Eisner T and Meinwald J (eds.) Chemical Ecology: The Chemistry of Biotic Interaction. National Academy Press, Washington D.C. 1995.
Elrich PR and Raven PH (1967) Butterflies and plants. Scientific American (June).
Firn R (2010) Nature’s Chemicals: The Natural Products that Shaped Our World. Oxford University Press.
Pagán OR (2005) PhD Dissertation. Cornell University.
Sullivan R. J., Hagen E.H (2002) Psychotropic substance-seeking: evolutionary pathology or adaptation? Addiction 97:389-400.