An evolutionary perspective; did humans evolve to take drugs?
About 4000 plants and fungi are known to yield psychoactive substances. It is though that these plants evolved to produce toxins that would repel predators, deterring threats from herbivores and pathogenic invasions. They evolved to discourage those who chanced to eat them by producing such a disturbing or unpleasant effect that they wouldn’t return. Yet strangely, humans (and other animals) have kept coming back for more.
Evidence points to a long history of co-evolution between humans and plants, and it is thought that just as plants developed the ability to synthesise chemicals so closely aligned with brain neurotransmitters, over time, humans developed certain adaptations which allowed them to make use of the effects of such substances. In other words, some co-evolutionary activity between mammalian brains and psychotropic plants occurred, meaning they interacted ecologically and therefore responded to one another evolutionarily.
To understand how this might have occurred, we need to look beyond the concept of psychoactive substances as just drugs of pleasure, medicine or abuse.
In the early evolutionary development of our ancestral hominids, there may have been great advantage in the ingestion of such plants, as psychoactive substances may have been an additional source of neurotransmitters for their large brains. Those individuals whose brains had a heightened response to emotion-linked neurotransmitters (such as dopamine and serotonin) may have been better suited to survival. (g)
There are many perceived evolutionary advantages offered by psychoactive substances:
- The plants were used as nutritional sources providing vitamins, minerals, and proteins rather than recreational psychotropic substances inducing inebriation. Due to limited resources within ancient environments drugs became food sources to prevent decreased fitness from starvation and death, therefore increasing both their fitness and viability. (f)
- 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. (d)
- when humans are stressed, neurotransmitters are depleted and possibly ingestion of some psychoactive plant substances enhanced the chances of 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 (d))
- they 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).
- Psychoactive drugs induce emotions that at one point in mammalian evolutionary history signalled increased fitness, not happiness. In ancient environments positive emotion correlated with a sign of increased fitness, such as successful foraging sessions or successful breeding and an induced euphoria may have had some increasing effects on fitness levels in ancient mammalian species. (Modern drug addiction fundamentally indicates a false increase of fitness, leading to increasing drug abuse to continue gain, even if the gain is realized as being false. This is the quintessential paradox among drug addicts.)(f)
- tolerance to thermal fluctuations, increased energy, and decreased fatigue, all advantageous to fitness by allowing longer foraging session as well as greater ability to sustain in times of limited resources. (f)
- Ethanol is found widely in ripe wild fruit. When wild yeast lands on the fruit and feeds on the sugars, fermentation occurs. The riper the fruit, the more alcohol it produces. Our primate ancestors, depended heavily being able to seek out fruit prior to domestication of plants, and they may have learned to find this food source quickly by following the scent of ethanol. There is often a big nutritional reward associated with being attracted to ethanol. (h) In tropical forests, wild fruit trees are not always abundent. The appearance of ripe fruit is highly seasonal. To disperse their seeds at the right time, it’s important for fruit bearing plants to attract consumers who will eat seed bearing fruit and ensure its distribution over a wider area, improving survival chances for the plants.(o)
- Ingestion of alcohol laced fruits and berries may have favored adaptations in the way humans metabolize small amounts of alcohol to make the best use of its physiological benefits, a possible explanation why the consumption of moderate amounts of alcohol lowers cardiovascular disease and appears to be associated with a longer life span. (o)
There is compelling evidence that coevolution has occurred.
For example, the mammalian brain has evolved receptor systems for plant substances, such as the opioid receptor system, not available by the mammalian body itself. The mammalian body has also evolved to develop defenses against overtoxicity, such as exogenous substance metabolism and vomiting reflexes.(f)
Although many modern animal species can tolerate plant toxins, different species possess different detox function levels. Among humans (and indeed other species, inebriation in birds and mammals has often been observed) from different geographic locations, these detox functions differ. Often, human populations with greater numbers of toxin-metabolizing genes originate from parts of the world that contain an abundance of those plants. For example, human populations in and near Turkey have a very high frequency of enzymes that can metabolize opiates, and the opiate poppy is native to the Turkish region.(n)
About half of all people of Asian descent share a genetic trait that causes a prompt reddening of the face in response to drinking alcohol — the result of an enzyme deficiency that interferes with alcohol metabolism and causes the temporary build-up in the body of a toxic chemical product, alcohol dehydrogenase. According to a new study in BMC Evolutionary Biology, the genetic mutation that causes the reaction first appeared about 10,000 years ago in Southern China, at about the same time residents began farming rice along the Yangtze River. The study’s authors hypothesize that the alcohol intolerance associated with facial flushing may have evolved as a survival strategy enabling ancient populations to enjoy the positive effects of alcohol derived from fermented rice — it can be used as a disinfectant and preservative — while imbibing in moderation. (p)
So whilst there is much evidence for the survival advantages gained from the use of psychoactive substances throughout human evolution, and evidence for the ways in which our bodies have evolved to cope with the toxicity of such substances, what of the intoxicating effects of alcohol and other mind altering psychoactives?
Seeking intoxication seems to defy the logic of natural selection, as the altered state invariably diminishes the capacity to utilise the normal repertoire of survival skills. (q)
Several theories have been suggested:
The consumption of intoxicants satisfies a basic need to alter the normal ego-centred consciousness, and it seems whether by the use of brain altering substances, meditation, or even the playground roundabout, humans will find a way.
To fend off boredom and misery. After the Neolithic revolution, most people had exchanged the hunter gatherer life in small communal groups for lives of grinding poverty in crowded, oppressive and disease ridden societies, and much as animals in captivity are more likely to use intoxicants than animals in the wild, so too are humans more likely to seek the experience of the temporary relief of euphoria regardless of the cost.(q)
Other theories of the evolution of drug use and intoxication are discussed in “social”.