This article explores the origin and evolutionary implications of anthropomorphism in the context of our relationships with animal companions. On the human side, anthropomorphic thinking enables animal companions' social behavior to be construed in human terms, thereby allowing these nonhuman animals to function for their human owners or guardians as providers of nonhuman social support. Absence of social support is known to be detrimental to human health and well being. Therefore, anthropomorphism and its corollary, pet keeping, have obvious biological fitness implications. On the animal side, anthropomorphism constitutes a unique evolutionary selection pressure, analogous to sexual selection, which has molded the appearance, anatomy, and behavior of companion animal species so as to adapt them to their unusual ecological niche as social support providers. Although such species undoubtedly have benefited numerically from the effects of this process, the consequences of anthropomorphism are less benign when viewed from the perspective of individual animals. Indeed, anthropomorphic selection probably is responsible for some of the more severe welfare problems currently found in companion animals.