Why would plants make drugs1 that are used by humans? More specifically, why would plants make drugs at all? Perhaps this may be explained by natural selection2. The concept of natural selection was proposed by Charles Darwin in the late 1800’s as a cornerstone of his theory of evolution. The theory of natural selection stated that organisms survive by passing on traits that are desirable and promote survival. In the case of plants, they need to ward off predators such as insects to avoid being eaten. Thus, plants developed three types of defenses against predators; 1) nutritional, 2) physical, and 3) chemical. A nutritional defense produced by plants is to contain low nitrogen levels or an unfavorable balance of amino acids, making the metabolism difficult if the insect eats the plant. Second, plants can have physical characteristics (e.g. thorns) that make them difficult to hold, manipulate, and consume by insects. Third, and most relevant to our discussion, a plant can synthesize chemicals to ward off insects. For example, a plant can produce substances that result in adverse physiological effects in the insect, such as a bitter taste or even poisoning. Nicotine, contained in the tobacco plant, is an excellent insecticide, producing death in insects by paralyzing their muscles (it would do this in humans, too, if they are exposed to high enough concentrations). Cocaine, contained in the coca plant, kills insects by inhibiting their feeding (a similar anorexic effect of cocaine in humans is well-established).
Why would plants make compounds that are psychoactive3? While these compounds can make humans “feel good”, they may serve completely different functions for the plant. In some cases, these compounds act as insecticides, but in many cases, no known function of these compounds exists for the plant. Many of the compounds in plants have similar structures to those chemicals (especially neurotransmitters) found in humans. Humans who seek psychoactive properties from these plants have contributed to Darwinian “survival of the fittest”. Over decades and centuries, humans have selectively cultivated the plants with the most desirable properties. A good example is breeding marijuana plants (i.e., they are genetically-modified) with high levels of THC today compared to the marijuana in the 1960’s.
1 a substance that affects the structure or function of a cell or organism.
2 theory of evolution proposed by Charles Darwin stating that organisms survive by passing on traits that are desirable and promote survival.
3 pertains to drugs that act in the brain to produce changes in mood, perceptions and behavior.