Evolutionary models of drug use seek to explain human drug usage from the perspective of evolutionary fitness. Plants for instance, may provide fitness benefits by relieving pain. Proponents of this model of drug use suggest that the consumption of pharmacological substances for medicinal purposes evolved in the backdrop of human-plant coevolution as a means of self-medication. Humans thus learned to ignore the cues of plant toxicity (e.g. bitter taste) because ingesting the bioactive compounds of plants in small amounts was therapeutic.
The hijack model of substance addiction suggests that Psychoactive drugs act on ancient and evolutionarily conserved neural mechanisms associated with positive emotions that evolved to mediate incentive behavior. They induce emotions that in human evolutionary history signaled a benefit for the group. Modern drugs tap into these emotions without passing on any evolutionary advantage. This may explain the modern problems of overuse and addiction.
Hijack hypothesis
The dominant paradigm of drug abuse focuses on human neurobiology and suggests that drug use is the result of reward-related behavior and that drug addiction is a consequence of drug interference with natural reward systems. Specifically, this tradition postulates that the chemical compounds humans seek out increase brain dopamine levels and thereby effectively usurp the mesolimbic pathway, a system originally intended to motivate/reward fitness enhancing behaviors such as those that increase access to food and sex.
History
Ideas concerning the neural bases of motivation and reinforcement in behavior can be traced back to the 1950s. In 1953, Olds and Milner published findings implicating a brain region, specifically a cluster of dopamine neurons, with reward-based learning. Addictive substances were later discovered to increase dopamine in the region of the brain associated with reward-based-learning (see: brain stimulation reward).
Proximate mechanisms
Molecular pathways
Research on the molecular pathways of addiction suggests that addictive substances, despite their diverse chemical substrates, converge on a common circuitry in the brain's limbic system. Specifically, drugs are thought to activate the mesolimbic dopamine pathway, facilitating dopamine transmission in the nucleus accumbens, via disinhibition, excitation, uptake blockade, etc. to produce a dopamine-like, yet dopamine independent effect.
Emotional pathways
The hijack model of substance addiction explains that drugs that elicit positive emotion mediate incentive motivation in the nucleus accumbens of the brain. Put another way, addictive substances act on ancient and evolutionarily conserved neural mechanisms associated with positive emotions that evolved to mediate incentive behavior. Psychoactive drugs induce emotions that in human evolutionary history signaled increases in fitness. Positive emotions such as euphoria and excitation are tools chosen by natural selection to help direct the behavior and physiology of an individual towards an increase in Darwinian fitness. For example, in the environment of evolutionary adaptation, humans would feel positive euphoric emotions in response to a successful foraging session or in the event of a successful breeding. Many psychoactive substances provide this same feeling and yet do not produce fitness benefits.
Example: Alcohol
Researchers have shown how emotional disposition is correlated with problematic use of alcohol, wherein if the reason for alcohol consumption is positive, the user is thought to drink to enhance positive feelings with greater control of the substance than if the user's emotional disposition prior to alcoholic consumption was negative. In these cases, the individual is drinking to cope and is shown to have less control over his/her own use. Alcohol mediates negative feelings by their suppression but also encourages the habituated continuance of positive emotion. Recovering alcoholics often report that the reason for relapse is often related to the impulse to compensate for negative feelings, resulting in a motivation to cope and therefore drink.