Geophagy has been hypothesized to be an adaptive behavior, either as a means to allay nutrient deficiency or to protect against ingested pathogens and toxins. Others have proposed that geophagy is non-adaptive, occurring either to allay hunger or as an epiphenomenon of nutrient deficiencies. This paper evaluates these hypotheses using 482 published cultural-level accounts of human geophagy and 330 accounts of geophagy among 297 species of mammals, birds, and reptiles. Information was extracted from reports of human geophagy to permit statistical analysis; reports of non-human geophagy were tabulated. Human geophagy did not parallel changes in nutrient requirements, occurred most frequently among children and pregnant women and in tropical areas (where pathogen densities are highest), and was associated with ingestion of toxic substances and gastrointestinal distress. Earth ingested by humans was craved and carefully selected and prepared; it had high clay content, but few bioavailable mineral nutrients. In primates, geophagy was associated with both protection from toxins and obtaining nutrients, whereas in other vertebrates it was associated mainly with obtaining nutrients. Our results indicate that human geophagy is best explained as providing protection from dietary chemicals, parasites, and pathogens, whereas animal geophagy may involve both micronutrient acquisition and protection.