Selective pressures in our ancestral environments were likely not on addiction, but rather on behaviors that were biologically rewarding (i.e. mate or food finding, avoidance of harmful stimuli). Given the role of nicotine in neurological function, it is possible that, in the case of nicotine addiction, the phenotype on which natural selection was working was related to enhancements in memory or cognition. The addiction phenotype would have hitchhiked along because it acts through the same or related mechanisms. The addiction phenotype was likely not selected against in ancestral environments because the availability and opportunity for prolonged use of purified drugs was negligible.
