When viewed in evolutionary perspective, biogerontology becomes primarily the biology of longevity rather than the biology of aging and death. It is shown here that the life-span of mammals is fundamentally related to two quantitative characteristics: a specific measure of relative brain size, the cephalization coefficient; and a measure of lifetime metabolic activity that has the physical dimensions of action. Recent wqrk by Hart and Setlow (1974) is reviewed which shows that species life-span has a high correlation with the capacity of cultured cells to repair damage to their DN A. As an alternative to the widely accepted “senescence gene hypothesis,” the hypothesis is here proposed that natural selection acts to maintain and extend mammalian longevity by modifying a set of longevity assurance mechanisms that all mammals have in common.