Human Coevolution with Psychoactive Drugs

So why do these drugs have the effect they do on humans?  Many scientists attribute the effects to a mismatch, a chance occurrence.  However, because animals and human use of these substances has occurred for such a long time, some experts, like Sullivan and Hagen, believe that humans and plants have coevolved and that plants are actually designed to mimic human neurotransmitters and that humans evolved to use plant chemicals.

Generally, lifeforms such as plants and animals develop chemicals to assure that they do not return for more.  It can be easily believed that plants that interfere with the central nervous system could have that effect.   What is curious is that even though these plants have this potential warding off effect, human and other animals continue to return.   It is believed by some that animals, including humans, have evolved to exploit these defensive mechanisms of the plants.

Evidence of a long time evolution between plants and animals extends beyond the fact that humans have been using drugs for millennia and that certain plant chemicals closely resemble the neurotransmitters in nerve cell synapses. It is also seen in the adaptations of humans to use the chemicals.  These adaptations include enzymes designed to metabolize the chemicals, the ability to smell certain chemicals, and vomiting to expel an overabundance of the substances.  Humans are also programmed with detoxification behaviors such as geophagy, the seeking and consuming of dirt and similar materials in order to absorb some of the chemicals (Sullivan and Hagen 2002). 

So we can see now that humans seem to have evolved to use these chemicals, but what would push humans to pursue psychoactive substances to the extent that they evolved these mechanisms?  In other words, why do humans pursue drugs?  Ronald K. Siegel held that drugs were and are a biological need.  He says the pursuit of drugs is a basic human drive he calls the “fourth drive,” a drive so powerful it falls in line with the need for food, water, and sex (Seigel). 

Sullivan and Hagen give two possible reasons for this evolution.   One is that by ingesting these neurotransmitter simulating chemicals, the body could save energy producing them on its own.  The body is already known to intake many chemicals, like proteins, rather than make them itself in order to save energy.  In addition, much of the modern day use of substances by societies comes from environments where there are few nutrients, like the deserts.  The other explanation is that when humans are stressed, neurotransmitters are depleted and allowed for prolonged survival during stressful periods of early human existence.  This is similar to diseases like schizophrenia and depression today which are thought to be caused by abnormalities in neurotransmitter levels.  People with schizophrenia and depressing are known to self medicate with psychoactive substances (Sullivan and Hagen 2002).

Another explanation is offer by Nesse and Berridge who claim that psychoactive substances cause changes in emotion which influence motivation, learning and decisions.  These changes have the potential to temporarily increase fitness, or at least give the illusion of increased fitness, by stimulating positive emotions and negating negative emotions (Nesse and Berridge 1997).

Whatever the reason, it certainly seems likely that humans have evolved alongside plants and the substances they create.  It also seems likely that we crave the substances that they, sometimes to an unnecessary extent.


Nesse Randolph M., Berridge Kent C.  “Psychoactive Drug Use in Evolutionary Perspective.”  Science  278  (1997):  63-66. Print.

Siegel, Ronald K.  Intoxication: life in pursuit of artificial paradise.  New York: Penguin, 1989.  Print.

Sullivan R. J., Hagen E.H.  “Psychotropic substance-seeking: evolutionary pathology or adaptation?”  Addiction  97  (2002): 389-400.  Print.