Mind-Altering Drug Use

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Human Uniqueness Compared to "Great Apes": 
Likely Difference
Human Universality: 
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Given the nebulous definition of the term "mind," mind-altering drug use can be defined either narrowly, as the intentional consumption of substances for their ability to produce profound perceptual and cognitive alterations, or more generally, as the intentional consumption of any psychoactive substance, regardless of marked perceptual changes. Since consumption of these substances, largely phytochemicals, is difficult to trace archeologically due to degradation, it is likely that consumption of psychoactive substances, both psychedelic and otherwise, dramatically predate archeological indications, potentially existing alongside the behavioral modernization of humans. The ingestion of psychoactive substances is universal among human populations, and evidence of early psychedelic use has been discovered on nearly every continent (excluding Antarctica). The latter practice is also universal across human populations, though it is often confined to smaller subsets of these diverse populations.

Mind-altering drug use in non-human animals is difficult to quantify, as the intention of achieving psychoactive effects via drug consumption cannot be easily assessed empirically, particularly in the wild, and it cannot be ascertained whether the animal recognizes or experiences the substance as psychoactive. As defined, this behavior does require intentionality, and its presence in animals may thus be inextricably linked to that characteristic. Still, well-documented anecdotes of regular self-exposure to psychoactive substances do exist in non-humans, and often result in observed intoxication. Despite these isolated instances of non-human drug use, however, human consumption of psychoactive substances is unique in the diversity of substances consumed, the regularity of consumption, and the universality of the practice across populations.

Though uncharacteristic of natural settings, rodents and primates will self-administer most psychoactive substances under certain circumstances, indicating similar reward from the majority of highly-rewarding drugs used by humans. Interestingly, however, almost no published studies have been able to sustain self-administration of psychedelics in rodents or primates. Though one study was able to produce a transient bout of psychedelic self-administration in rhesus monkeys, this required an extensive history of MDMA self-administration in the subjects preceding psychedelic exposure, and was reasonably inconsistent. This experiment was the only documented case of sustained self-administration of any classical psychedelic in a non-human model organism. It is unclear whether the inability to produce sustained self-administration of classical psychedelics in non-human animals is consequent to the laboratory setting, reflective of common human patterns of use, which are sparse and intermittent (i.e. infrequently compulsive), or due to a unique rewarding property of these substances that differentiates humans from other species.


Timing of appearance of the difference in the Hominin Lineage as a defined date or a lineage separation event. The point in time associated with lineage separation events may change in the future as the scientific community agrees upon better time estimates. Lineage separation events are defined in 2017 as:

  • the Last Common Ancestor (LCA) of humans and old world monkeys was 25,000 - 30,000 thousand (25 - 30 million) years ago
  • the Last Common Ancestor (LCA) of humans and chimpanzees was 6,000 - 8,000 thousand (6 - 8 million) years ago
  • the emergence of the genus Homo was 2,000 thousand (2 million) years ago
  • the Last Common Ancestor (LCA) of humans and neanderthals was 500 thousand years ago
  • the common ancestor of modern humans was 100 - 300 thousand years ago

Possible Appearance: 
150,000 thousand years ago
Probable Appearance: 
2,000 thousand years ago
Definite Appearance: 
200 thousand years ago
Background Information: 

Human consumption of psychoactive substances can be traced back nearly 45,000 years via commonplace aboriginal consumption of duboisia (pituri), a nicotine-containing plant abundant in central Australia, and likely preceded these estimates. Archeological evidence for the consumption of psychedelics, a class of drugs more amenable to the narrow definition of mind-altering drug use, dates back over 5,000 years (again, a highly conservative estimate). For instance, mescaline-containing buttons of the peyote cactus collected by Native Americans have been carbon dated to between 3780-3660 BCE, and indigenous cultures in Mexico and Guatemala produced rock paintings and sculptures of mushroom deities that resembled hallucinogenic species. Written record of ceremonial practice with psychedelics in India and Greece also date back nearly 4,000 years, and the practices themselves likely preceded written record.

The Human Difference: 

Although various non-human species consume substances known to be psychoactive in humans, humans differ from other animals in the regularity with which we consume these substances, the diversity of substances consumed, and the universality of their consumption across populations. Furthermore, with limited exception, humans are the only species known to willfully consume classical (i.e. serotonergic) psychedelics with regularity. Our ability to predict resulting subjective effects following consumption and, consequently, to intentionally consume psychoactive substances specifically for their psychoactive effects, may also be unique.

Universality in Human Populations: 

Evidence of the consumption of psychoactive substances has been found on every inhabited continent, and all populations consume psychoactive substance in some form. There are, however, a large number of individuals who abstain from drug use, and the frequency, ubiquity, cultural significance, and variety of drug use vary between populations.

Mechanisms Responsible for the Difference: 

The unique human capacity for intentionality and our capacity to learn from others and/or effectively transmit information between individuals/generations may allow us to better predict resulting subjective/toxic effects following consumption and intentionally consume these substances for their psychoactive effects. The human-unique constructs of spirituality and existential contemplation, which may derive in part from our capacity for metacognition, could contribute to the human-specific consumption of classical psychedelics, as evocation of spiritual experience and introspection are commonly cited reasons for recreational use, and psychedelic consumption plays a role in religious ceremonies in numerous populations.

Humans may also possess specific metabolic machinery that facilitates the detoxification and metabolic processing of a variety of psychoactive substances. For example, in Asian and north African populations where opium and khat (which contains various amphetamine-like compounds) are indigenous, there exists a high frequency of ultrametabolizing polymorphisms to CYP2D6, the cytochrome P450 (CYP) primarily responsible for opiate- and amphetamine-like metabolism, and reduced-function mutations in CYP2A6, the CYP primarily involved in nicotine metabolism. Conversely, this trend is reversed in Caucasian populations, which originate from tobacco-rich and opium/khat poor regions. In a similar manner, humans may possess various unidentified modifications to their metabolic systems that facilitate diverse drug consumption.

Possible Selection Processes Responsible for the Difference: 

The use of psychoactive substances can often result in profound detriment to personal health (e.g. cancer, neurodegeneration, liver/kidney/heart failure, overdose, addiction/compulsive drug seeking at expense of self-preservation, etc.) and numerous social ills, which, prima facie, would reduce evolutionary fitness. To address this paradox of drug reward, a number of speculative evolutionary explanations exist for the prevalence of human psychoactive drug use. The "evolutionary mismatch" hypothesis, posited by Nesse and Berridge, suggests that human drug use is more likely an exaptation alongside highly sensitive neural reward circuitry that creates a strong drive for evolutionarily beneficial behaviors such as feeding and sex. They contend that drugs simply highjack circuitry designed to reward adaptive behavior, and that our widespread use of these substances reflects our unique capacity to intentionally exploit this reward circuitry (i.e. video games exploit rewards associated with hunting and fighting, junk foods exploit reward associated with evolutionarily constructed nutritional drives for high salt, fat, and sugar content, etc.). Their hypothesis is predicated on the pretense that our evolutionary landscape did not provide access to pure/potent drug, and we thus did not develop adequate defenses against their allure or adapt to avoid them due to deleterious effects.

While this hypothesis may well address prevalence of compulsive pure drug use in modern humanity, it is of dubious value for explaining the evolutionary persistence of the behavior. Not only is the hypothesis antithetical to the evolution of psychoactive phytotoxins in plants as defense mechanisms (i.e. exploitation of reward circuitry by these chemicals would encourage, rather than deter consumption), but it also neglects evidence of specific human adaptations indicative of evolution alongside psychoactive substances (e.g. population-specific cytochrome P450 polymorphisms that facilitate detoxification and degradation of regionally prevalent psychoactive compounds), and recent studies demonstrating averse responses upon first-time use of many of the most widely consumed psychoactive substances (e.g. nicotine).

Sullivan and Hagen posit an alternative hypothesis, the "deep time relationship" hypothesis, that asserts evolution of modern humans alongside psychoactive substances, and puts forth some possible evolutionary roles for their use. One such explanation is that many of these psychoactive phytotoxins occur in plants that are fairly nutrient rich but are toxic to most other species, presenting a low-demand food source to humans capable of withstanding their toxicity. Over time, this benefit may have developed into a reward response to the psychoactive substances, as they would be indicative of a valuable nutrient source that is readily available for consumption due to decreased interspecies competition. Another conjecture by Sullivan and Hagen is that the use of psychoactive chemicals, which often resemble endogenous neurotransmitters, could substitute for metabolically costly neurotransmitters and preserve some neural function when synthesis is impeded or existing supplies are depleted due to absence of dietary precursors in marginal or variable environments (akin to psychiatric self-medication). They also hypothesize that these drugs could have been used to curb maladaptive stress responses during long bouts of stress (e.g. during long hunts), or as defenses against human pathogens, such as gastrointestinal parasites. The latter function appears most analogous to drug consumption by other species. Though the evolutionary benefits of psychedelics are rather obscure due to their potentially debilitating perceptual and behavioral effects, low doses of these drugs can heighten arousal, enhance novelty detection, and increase energy, possibly enhancing capability for food-seeking or hunting. Additionally, these substances tend to structurally resemble more metabolically costly neurotransmitters, fitting well with Sullivan and Hagen's neurotransmitter-substitution hypothesis.

Implications for Understanding Modern Humans: 

Mind-altering drugs are, by definition, substances that alter the conscious state of living organisms. Given the presumed uniqueness of human consciousness (i.e. enhanced cognitive capacity, metacognition, theory of mind/intersubjectivity, intentionality, etc., etc.), our particular proclivity for consciousness-altering drugs with stereotyped physiological/pharmacological and subjective effects may provide insight into the biological mechanisms underlying our unique conscious experience.

Occurrence in Other Animals: 

Various anecdotes of regular self-exposure to psychoactive substances exist in non-human animals of diverse lineages. Wallabies in Tasmania, for example, have been reported to invade commercial poppy fields and consume large quantities of the opium-containing plants, often subsequently running in circles, exhibiting gate indicative of intoxication, and falling asleep. Reindeer in northern Europe have been documented repeatedly consuming the hallucinogenic fly agaric mushroom (Amanita muscaria) and appearing intoxicated. There are also reports of reindeer consuming the urine of humans who have consumed fly agaric (active compounds of fly agaric are passed relatively unchanged in urine), indicating a particular affinity for the psychoactive compound itself, rather than the nutritional properties of the mushroom. Additionally, many species have been documented to consume fermented fruits. While popular reports of elephants consuming fermented fruit from the marula tree for intoxication have been thoroughly debunked, evidence of pen-tailed treeshrews, bats, and Vervet monkeys consuming fermented fruit in quantities that could produce intoxication in humans does exist. In the former two cases, no overt signs of intoxication were detected, likely due to increased metabolic efficiency for the breakdown of alcohol. Vervet monkeys, however, do show intoxicated behavior similar to humans, including loss of coordination and increased social behavior.

Some species, such as lemurs, capuchins, and dolphins, will experience visible signs of intoxication following intentional interactions with toxin-producing animals. Lemurs and capuchins often willingly pick up and agitate poisonous millipedes before rubbing them on their bodies, coating themselves in their secretions, which contain a host of toxic compounds including cyanide and many benzoquinones. It is widely believed that this practice serves to deter insect approach (e.g. mosquitoes), but these primates have been said to enter into what appears to be a "blissful intoxication" following the practice. Though the alternative function may disqualify this as an exemplar of intentional drug use for psychoactive effects, it is possible that these primates have evolved a reward response to these chemicals due to the conferred advantage of reduced pest contact. Dolphins have been encountered toying with inflated pufferfish, which produce the highly toxic compound tetrodotoxin, before entering into a "trance-like" state of lethargy, often floating belly up in the water, and losing motor coordination. Though dolphins have been seen mutually interacting with and "passing" these inflated pufferfish in communal settings, it is possible that their interactions are not an instance of active drug use, but rather result from exploration of a novel object. The apparent intoxication in this case may simply be a motor deficit induced by the toxin.

The only reports of repeated consumption of classical psychedelics in non-humans pertain to the ingestion by jaguars in the Amazonian jungle of Banisteriopsis caapi, a plant containing, among other active alkaloids, the highly potent psychedelic DMT. Banisteriopsis caapi is one of two plants commonly used by indigenous Amazonian cultures to produce the hallucinogenic brew ayahuasca, which is known to produce emetic and diarrheal effects. It is thus speculated that jaguars use this plant to cleanse their digestive track, reducing their risk of possible parasites. Nonetheless, profound intoxicating effects have been noticed in many jaguars immediately following consumption, and the possibility of intentional consumption for psychoactive effects cannot be ruled out.

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