The fitness costs of senescence: The evolutionary importance of events in early adult life

Summary The increased mortality caused by ageing represents a fitness cost to organisms. This paper develops techniques for determining the proportions of that cost that accrue at each age. A variety of analyses using several different sources of data on human ageing—palaeodemographic life tables and life tables from more recent societies with high mortality rates—all suggest that the fitness cost of ageing was high during most of our evolutionary history, and was largely due to physiological changes occurring early in adult life. These results imply that predictions about the nature of senescence based on evolutionary theory should be tested using data from middle-aged individuals. They also have implications about the relative importances for human evolution of the pleiotropy and mutation-accumulation theories of the evolution of senescence, and for the validity of Gompertz Law' for the shape of the relationship between mortality and age. An analysis of a life table of the African buffalo suggests that the costs of ageing early in adult life are relatively high in at least one non-human species in its natural environment.