Clays employed historically in the consumption of astringent acorns plus seven edible clays from Africa were examined in relation to the functional significance of human geophagy. On the basis of sorptive maxima for tannic acid ranging from 5.6 to 23.7 mg/g, we conclude that adsorption of tannic acid in traditional acorn preparation methods in California and Sardinia helped make these nuts palatable. Calcium available in solution at pH 2.0 and 0.1 mol NaCl/L was 2.10 and 0.71 mg/g for the Sardinian and Californian clays, respectively. The African clays released calcium, copper, iron, magnesium, manganese, or zinc in amounts of nutritional significance from some clays but not from others. A clay recovered from an archaeological site occupied by Homo erectus and early H. sapiens was indistinguishable mineralogically, in detoxification capacity and in available minerals, from clays used in Africa today. We suggest that the physiological significance of geophagy made it important in the evolution of human dietary behavior.