Consumption of Underground Plant Foods

Plants that sequester nutrients in underground storage organs (USOs) are staples in many hunter-gatherer diets. Baboons also exploit near surface corms, but do not take deeply buried tubers. With the exception of digging residues that may indicate USO feeding by chimpanzees, no other living hominids are known to use these resources. Australopithecines, however, are characterized by molar enlargement that may reflect heavy USO consumption. The tubers routinely taken by ethnographically known hunter-gatherers include species that young children cannot handle efficiently, requiring them to depend on shares of these resources acquired by others. This observation is the basis for a grandmother hypothesis that links exploitation of tubers with novel fitness opportunities for older females. As their own fertility was ending, food sharing with grandchildren would improve the youngsters’ survival and allow their mothers to shorten birth intervals. The more robust elders who supplied more help had more descendants, resulting in the evolution of longer human life spans and our distinctive form of cooperative breeding. Ecological changes in the Plio-Pleistocene to drier, more seasonal environments would have favored plants with USOs, and the changes outlined above have been hypothesized to mark the appearance and spread of genus Homo. Nutrient availability from some USOs can be improved by cooking. This step in handling increases the return rate advantage for adults compared to children. It is also central to another hypothesis about the evolution of genus Homo, one that emphasizes the wider array of resources that can be consumed with cooking and proposes that the smaller teeth and larger body size of genus Homo are consequences of the advent of this practice roughly two million years ago. Resources collected in quantity for cooking become vulnerable to expropriation. The cooking hypothesis proposes that ancestral females enlisted protectors by forming pair-bonds, which in turn is said to account for reduced sexual dimorphism in Homo compared to the australopithecines.