As we find ourselves in the beginning of a new millennium, we are faced with challenges to our survival as a human population. Some of the greatest threats to our survival are sweeping epidemics that affect millions of individuals worldwide. Drug addiction, although often regarded as a personality disorder, may also be seen as a worldwide epidemic with evolutionary genetic, physiological, and environmental influences controlling this behavior. Globally, the use of drugs has reached all-time highs. On average, drug popularity differs from nation to nation. The United Nations Office on Drugs and Crime identified major problem drugs on each continent by analyzing treatment demand [1]. From 1998 to 2002, Asia, Europe, and Australia showed major problems with opiate addiction, South America predominantly was affected by cocaine addiction, and Africans were treated most often for the addiction to cannabis. Only in North America was drug addiction distributed relatively evenly between the use of opiates, cannabis, cocaine, amphetamines, and other narcotics. However, all types of drugs are consumed throughout each continent. Interpol reported over 4000 tons of cannabis were seized in 1999, up 20% from 1998, with the largest seizures made in Southern Africa, the US, Mexico, and Western Europe [2]. Almost 150 tons of cocaine is purchased each year throughout Europe and in 1999 opium production reached an estimated 6600 tons, the dramatic increase most likely due to a burst of poppy crops throughout Southwest Asia. This rapid increase in drug use has had tremendous global effects, and the World Health Organization cited almost 200,000 drug-induced deaths alone in the year 2000 [3]. The Lewin group for the National Institute on Drug Abuse and the National Institute on Alcohol Abuse and Alcoholism estimated the total economic cost of problematic use of alcohol and drugs in the United States to be $245.7 billion for the year 1992, of which $97.7 billion was due to drug abuse [4]. The White House Office of National Drug Control Policy (ONDCP) found that between 1988 and 1995, Americans spent $57.3 billion on drugs, of which $38 billion was on cocaine, $9.6 billion was on heroin and $7 billion was on marijuana.
Among the different approaches for diagnosis, prevention, and treatment of drug addiction, exploring the evolutionary basis of addiction would provide us with better understanding since evolution, personality, behavior and drug abuse are tightly interlinked. It is our duty as scientists to explore the evolutionary basis and origins of drug addiction so as to uncover the underlying causes rather than continuing to solely focus on the physiological signs and global activity of this epidemic. Too often the treatment of addiction simply works to alleviate the symptoms of addiction, dealing with overcoming the physiological dependence and working through withdrawal symptoms as the body readjusts to a non-dependent state of homeostasis. However, we must not only concentrate on this aspect of addiction when considering global treatments and preventative programs. We must take into consideration that it is not purely the physiology of addiction we are battling.
Drug addiction is thought of as an adjunctive behavior, or a subordinate behavior catalyzed by deeper, more significant psychological and biological stimuli. It is not just a pharmacological reaction to a chemical but a mode of compensation for a decrease in Darwinian fitness [5]. There are three main components involved in substance addiction: developmental attachment, pharmacological mechanism, and social phylogeny including social inequality, dominance, and social dependence [6]. Developmental attachment created by environmental influences, such as parental care or lack thereof, may influence children's vulnerability to drug addiction. Evolutionarily speaking, children that receive care that is more erratic may focus more so on short-term risks that may have proved to be an adaptive quality for survival in ancient environments. Compounding that attachment, the pharmacological mechanism describes the concept of biological adaptation of the mesolimbic dopamine system to endogenous substance intake. These factors combined with the influence of social phylogeny create a position for predisposition to drug addiction. They attribute to the common belief that many substances of abuse have great powers to heal, and that is often the driving motivation for overuse and addiction. Evolutionary perspective shows an intermediate and fleeting expected gain associated with drug addiction correlated with the conservation in most mammals of archaic neural circuitry [7], most often being a falsified sense of increased fitness and viability related to the three components of drug abuse [5, 8]. The chemical changes associated with fitness and viability are perceived by mammals as emotions, driving human behavior.
Human behavior is mediated primarily by dopaminergic and serotonergic systems, both of ancient origins probably evolving before the phylogenetic splits of vertebrates and invertebrates [9]. 5-HT (serotonin), stimulated by a small range of drugs, mediates arousal. It is believed to be inhibited by hallucinogens and also helps control wanting for ethanol and cocaine consumption. The cortico-mesolimbic dopaminergic system, on the other hand, is believed to be the target of a wide range of drugs, including marijuana and cocaine, increasing the transmission of dopamine to the nucleus accumbens [10]. This system mediates emotion and controls reinforcement, and is the primary pathway acted on by antipsychotic drugs such as chlorprothixene and thioridazine. Problematic use of drugs develops into addiction as the brain becomes dependent on the chemical neural homeostatic circuitry altered by the drug [7]. No matter the theory of drug addiction, there remains one constant: withdrawal is inevitable. As a drug is administered continuously and an individual becomes addicted, the brain becomes dependent on the presence of the drug. With an absence of the drug, withdrawal symptoms are experienced as the brain attempts to deal with the chemical changes. There are believed to be evolutionary origins of drug addiction, which will be discussed further, as well as a link between physiological addiction and the evolution of emotion.