Detoxification as the adaptive function of geophagy is demonstrated from field and historical data associating clay consumption with the domestication of potentially toxic potatoes. In vitro analyses showed that the glycoalkaloid, tomatine, was effectively adsorbed by four classes of edible clays over a range of simulated gastrointestinal conditions. These results, in conjunction with reports of geophagy by nonhuman primates, suggest geophagy as a solution to the impasse chemical deterrents pose to the process of domestication and to chemical constraints on plant exploitation by non-fireusing hominids. The inorganic component of the chemical environment deserves increased attention from chemical ecologists.