Has the Brain Adapted to Drugs?

Has the Brain Adapted to Drugs?

by Fenna Andriessen, Sophie Bacon, Dionne Liberia, Stine Løven, and Alyssa Mattson

Drugs have become so ingrained in our culture we have almost become numb to their existence. Celebrities like Miley Cyrus and Snoop Dogg sell hit songs about dancing on Molly and it’s no secret that college campuses have become a space where individuals have the opportunity to experiment with recreational drugs and alcohol.

Our ancestors used to seek drugs for survival purposes. However, now it seems we use drugs primarily for recreational use. Unfortunately, some individuals eventually become dependant on drugs, as exemplified by events such as the 1980s crack cocaine epidemic and the current U.S. opioid crisis.

With that said, there are a few questions that we would like to address. How does context play a role in drug usage? What evolutionary mechanisms have humans developed that lead us to seek out drugs? How does substance use progress to addiction?

Let’s talk about drugs!

Why do we use drugs?

Throughout our lives, we engage in behaviors to seek pleasure, such as food or sex. Because behaviors like these were beneficial to our ancestors’ survival and reproduction, natural selection made them rewarding, or pleasurable, so that we continue to perform them.  Our natural rewards system (mesolimbic dopamine system; MDS) is responsible for these pleasurable feelings. For example, if you are eating a cookie, you are ingesting a tasty, calorie-dense food — something that our ancestors, in their food-scarce habitat, would have considered highly pleasurable and valuable to survival. In our modern-day environment, the feelings of pleasure associated with food and other stimuli remain regardless of their original evolutionary intent or the disadvantages they may now present. As it turns out, many drugs activate this same brain system, stimulating dopamine release and thus, causing a pleasant feeling that we want to experience repeatedly–despite the fact that the drug itself may not be increasing our ability to survive and reproduce. When we use a drug, even with the understanding that it may harm us, there is a risk that this behavior can become an addiction due to the false sense of benefit.

There are two theories that attempt to explain drug use in humans – the hijacking hypothesis and the neurotoxin regulation hypothesis. First, the hijacking hypothesis claims that our drug use is a by-product of the MDS. Drugs would “hijack”, or stimulate the MDS artificially, triggering similar pleasant feelings in users. Despite the widespread acceptance of this theory among researchers, there are several issues concerning its accuracy. Evidence supporting the hijacking hypothesis doesn’t show the whole truth. As the famous rat park experiment findings undermine the claim that drugs hijack decision-making machinery because rats chose the alternative option (sweetened water) instead of the drug. So, it appears as we would choose the cookie over cocaine. Additionally, supporters of the hypothesis claim that our ancestors could not have been exposed to drugs as we currently know them because our drug use is only a byproduct and not an adaptation. They claim that drugs are novel and nowadays we are exposed to them a lot more.

A competing explanation, the neurotoxin regulation hypothesis paints a somewhat different picture. This theory suggests that our ancestors may have used certain drugs to treat illness. Many plants–including those from which many common drugs are derived–have mechanisms designed to discourage consumption (e.g., by insects). For instance, when thinking about tobacco, certain symptoms such as the bitter taste, nausea, vomiting, and diarrhea can occur resulting in someone not wanting to use it again. According to the neurotoxin regulation hypothesis, humans may have evolved to use these drugs to their advantage. Tobacco, for example, is effective against certain parasites that were vastly present in our ancestor’s environment. Thus, in contrast to the hijacking hypothesis, which suggests that drugs are rewarding more or less by accident, the neurotoxin regulation hypothesis claims that drug use is rewarding because it helped our ancestors to reduce the threat of disease. 

One study conducted at Claremont Graduate University in 2003 compared the acute lethal toxicity of many commonly abused drugs. In their findings, it was concluded that an average dose of a drug, while very small, is only slightly below what would be a lethal dose. For the hijack hypothesis to be true, we would expect the MDS system to reinforce large intakes of these drugs – however, humans appear to be capable of monitoring their intake of drugs, thus their consumption of neurotoxins. The neurotoxin regulation hypothesis explains human drug use despite cues to toxicity and how most humans successfully measure intake and avoid an overdose. An additional study suggests that the historical consumption of certain plant neurotoxins provided ancestral humans with benefits including improved cognitive ability, neurological functions and other fitness benefits.

In addition, H. Ahmed and G. F. Koob conducted a similar study in October of 1998 investigating whether short-term and  long-term access had an influence on rats’ ability to self-administer the drug. The study showed that long-term access rats self-administered more cocaine compared to the short-term access rats. It suggests that escalated cocaine use (long-term access) resulted from a change in the hedonic set-point–a higher baseline for when they experience an effect from the drugs. Results found that rats that were given shorter access to cocaine displayed an ability to limit use, suggesting that drugs do not “hijack” the brain, but rather that animals have the capability to regulate use to prevent undesirable fitness outcomes. This study supports the neurotoxin regulation hypothesis in which humans have evolved to regulate and optimize benefits from drugs.


Explaining drug addiction

By examining drug addiction from an evolutionary perspective, we can gain some insight into why these psychological mechanisms occur and how to approach designing treatment programs. Given this perspective, drug addiction is best understood by examining internal influences, including our biology and psychological mechanisms, as well as external influences, such as social and familial factors in our modern environment.

In a historical context, primitive humans sought drugs for cultural ceremonies, religious purposes, medicinal use, and during times of food and resource scarcity given their perceived fitness benefits. However, given the toxicity of these substances, there were serious side effects associated with their use. As they are designed to deter consumption, plant neurotoxins (drugs), may induce headache, nausea and vomiting, diarrhea, and, in extreme cases, erratic or violent behavior, severe physical and mental health problems, and even death. However, in the modern-day, human manufacturing and refinement of these drugs have reduced the immediate severity of side effects while increasing their strength.

For example, cocaine is typically ingested today in a very pure and refined form. Traditionally, however, it was ingested by chewing on a coca leaf. Not only would the concentration of cocaine have been much lower in this form (making overdose less likely), but people would have been ingesting it along with a number of other toxins produced by the plant which would have produced aversive physical symptoms (making overconsumption and abuse less likely). Thus, the neurotoxin regulation hypothesis could explain how our ancestors would have regulated intake and avoided an overdose, and help us understand why addiction, abuse, and overdose are more common today.

In addition to content, research proposes a link between the intense growth of human brains and the utilization of psychotropic plants. Evidence for this notion stems from the belief that in the past, ancient civilizations viewed psychotropic plants as food sources when resources were low. The use of these drugs began to signal false signs of increased fitness and happiness, resulting in the adaptation that perpetuates human drug use.

Looking at drug addiction from an evolutionary perspective allows us to understand the mechanisms that we have adapted for drug use and develop more effective rehabilitative strategies.


We wish we could explain drug use and addiction to you but we don’t have a clear answer–however, the neurotoxin regulation theory has stronger evidence compared to the hijacking hypothesis. In terms of future research, it would be interesting to look at the self-administration of drugs as well as the histories of addicted individuals in order to account for different contexts of drug use.

With that said, it’s important to take into consideration the learning process associated with drug-use and evolution in general. This includes the fact that contrary to popular beliefs regarding evolution, adaptations aren’t set in stone. We aren’t helpless and we can still make a conscious decision to say no to drugs.

We can all agree that our environment has changed drastically, as drugs are all around us–at the pharmacy, on the streets, at parties, you name it, it’s hard to ignore. It’s safe to say that using drugs now isn’t about “survival” it’s more about being cool and fitting in. With all of this said, now you know doing a line in the bathroom is very different from chewing on a coca leaf in the jungle.

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