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The evolution of drug abuse

New research chal­lenges tradi­tional accounts of why we wallow in chemical gratification

March 21, 2008
World Science staff

Why do peo­ple abuse drugs? It’s not only a ques­tion wor­ried par­ents ask their way­ward, sub­stance-dab­bling teenagers. It’s al­so a deeper ques­tion asked by bi­ol­o­gists.

In gen­er­al, na­ture has de­signed all crea­tures as ex­quis­ite machines for their own pro­tec­tion and propaga­t­ion. Yet we’re easily and of­ten drawn in­to self-destruction by noth­ing more than life­less chem­i­cal lures. This weak­ness seems such a jar­ring ex­cep­tion, such a dis­mal Achilles’ heel, that it calls out for ex­plana­t­ion.

A new study pro­poses that hu­mans and other animals have a long ev­o­lu­tion­ary re­la­tion­ship with brain-in­flu­enc­ing drugs. Shown above is the plant Can­na­bis sativa, which pro­duces the psy­cho­ac­tive in­gre­di­ent in ma­ri­jua­na. (Im­age cour­te­sy Mis­souri Dept. of Trans­por­ta­tion) 
Sci­en­tists typ­ic­ally of­fer the fol­low­ing one. Drugs are chem­i­cals that in­ap­pro­pri­ate­ly trig­ger ac­ti­vity in brain cir­cuits de­signed for very dif­fer­ent pur­poses: to pro­vide a sense of re­ward for hav­ing sat­is­fied or­di­nary needs, health­fully. 

The brain has few de­fenses against this chem­i­cal de­cep­tion, the stand­ard ac­count goes, be­cause drugs were un­known in the nat­u­ral en­vi­ron­ment that shaped hu­man ev­o­lu­tion.

This tra­di­tion­al view, though, is com­ing un­der at­tack. A new study pro­poses the brain evolved to ac­count for and even ex­ploit drugs. Al­though their abuse is still un­healthy, the au­thors sug­gest it’s wrong to think they cheat the brain in the sense tra­di­tion­ally theo­r­ized.

“Ev­i­dence strongly in­di­cates that hu­mans and oth­er an­i­mals have been ex­posed to drugs through­out their ev­o­lu­tion,” wrote the sci­en­tists in the stu­dy. The re­search, by an­thro­po­lo­g­ist Rog­er Sul­li­van of Cal­i­for­nia State Uni­ver­s­ity and two col­leagues, ap­peared March 19 on­line in the jour­nal Pro­ceed­ings of the Roy­al So­ci­e­ty B: Bi­o­log­i­cal Sci­ences.

The most pop­u­lar drugs of abuse are plant tox­ins that evolved to pro­tect plants from preda­tors, as ev­o­lu­tion­ary bi­ol­o­gists have “con­vinc­ingly ar­gued,” Sul­li­van and col­leagues wrote. For ex­am­ple, nic­o­tine, the key ad­dic­tive in­gre­di­ent of cigarettes, helps ward off an ar­ray of in­sects, mam­mals and other creat­ures from munch­ing on to­bacco plants. Fur­ther ev­i­dence of the fun­da­men­tally poi­son­ous na­ture of drugs of abuse, the three sci­en­tists ar­gued, is that first-time users of­ten re­port un­pleas­ant re­ac­tions.

Since plants long pre­date hu­mans, the pres­ence of these sub­stances in plants would seem to in­di­cate we and our an­ces­tors have long dealt with them, the re­search­ers con­tin­ued. But fur­ther ev­i­dence of this, they added, is in our own make­up. All an­i­mals pro­duce mo­le­cules known as cy­to­chromes, whose func­tions in­clude de­tox­i­fy­ing in­gested plant poi­sons. Cy­to­chromes that spe­cif­ic­ally neu­tral­ize brain-affecting plant tox­ins have re­mained a con­sist­ent fea­ture of hu­man ev­o­lu­tion, Sul­li­van and col­leagues wrote.

All this shows “our an­ces­tors were reg­u­larly ex­posed to plant neurotox­ins,” they added, so the view of our brains as un­sus­pect­ing vic­tims of the new chem­i­cal threat is un­ten­able. 

It remains unclear what might be the true ev­o­lu­tion­ary ex­plana­t­ion of drug abuse, they wrote: the “para­dox” stays of why sub­stances de­signed as poi­sons, are pleas­ur­a­ble to so many.

One pos­si­bil­ity, the sci­en­tists sug­gested, is that an­i­mals co-opted some plant tox­ins and used them for their own de­fenses against para­sites. If this is true, then ev­o­lu­tion, the pro­cess by which spe­cies adapt and change to meet en­vi­ron­men­tal de­mands, might have de­signed our brains to en­cour­age some drug use. This could in­volve shap­ing our brains to as­so­ci­ate drug in­take with feel­ings of re­ward. “But there are, of course, oth­er pos­si­bil­i­ties,” the re­searchers wrote.

* * *

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There are substance abuse treatment programs designed for people from all walks of life, because addiction can befall just about everyone.

Why do people abuse drugs? It’s not only a question worried parents ask their wayward teenagers who dabble in dangerous substances. It’s also a deeper question asked by biologists. In general, nature has designed all creatures as exquisite machines for their own protection and propagation. Yet we’re easily and often drawn into self-destruction by nothing more than lifeless chemical lures. That seems such a jarring exception, such a dismal Achilles’ heel, that it demands explanation. Scientists typically offer the following one. Drugs are chemicals that inappropriately trigger activity in brain circuits designed for very different purposes: to provide a sense of reward for having healthfully satisfied ordinary needs. The brain has few defenses because drugs were unknown in the natural environment that shaped human evolution. This traditional account, though, is coming under attack. A new study proposes the brain actually evolved to account for, and even exploit drugs. Although many drugs are still unhealthy, the authors suggest, it’s wrong to think they cheat the brain in the sense biologists traditionally claim they do. “Evidence strongly indicates that humans and other animals have been exposed to drugs throughout their evolution,” wrote the scientists in the study. The research, by anthropologist Roger Sullivan of California State University and two colleagues, appeared March 19 online in the journal Proceedings of the Royal Society B: Biological Sciences. The most popular drugs of abuse are plant toxins that evolved to protect plants from predators, as evolutionary biologists have “convincingly argued,” Sullivan and colleagues wrote. For example, nicotine, the key addictive ingredient of cigarettes, protects helps the tobacco plant from an array of plant-eaters ranging from insects to mammals. Further evidence of the fundamentally poisonous nature of drugs of abuse, the three scientists argued, is that first-time users often report unpleasant reactions. Since plants long predate humans, the presence of these chemicals in plants would seem to indicate we and our ancestors have been exposed to them, the researchers continued. But further evidence of this, they added, is in our own makeup. All animals produce molecules known as cytochromes, whose functions include detoxifying ingested plant poisons. Cytochromes that specifically neutralize brain-affecting plant toxins have remained a consistent feature of human evolution, Sullivan and colleagues wrote. In short, “our ancestors were regularly exposed to plant neurotoxins,” they added, so the view of our brains as unsuspecting victims of the new chemical threat is intenable. What the true evolutionary explanation of drug abuse is remains unclear, they wrote: the “paradox” remains of why substances designed as poisons, are pleasurable to so many. One possibility, the scientists suggested, is that animals co-opted some plant toxins and used them for their own defense against pathogens. Thus evolution, the process by which species adapt and change to meet environmental demands, might have designed our brains to encourage some drug use. This could involve shaping our brains to associate drug intake with feelings of reward. “But there are, of course, other possibilities,” they wrote.