High infant and child mortality rates are suggested to be one of the most enduring and important features of ancestral human environments, referred to as the Environment of Evolutionary Adaptedness (EEA). These rates contrast with the very low rates of infant and child mortality among many industrialized nations since the 19th and 20th centuries. The authors compare data from recent hunter-gatherer and agricultural societies, historical records, and non-human primates in attempt to quantitatively describe infant and child mortality rates during the EEA.