WHAT IS HUMAN MORTALITY? Part 1

Identifying any fundamental pattern requires the appropriate metrics; understanding the evolutionary context for variation in human mortality patterns requires an evolutionarily relevant comparative baseline. Ideally, this baseline should approximate the average age-specific levels of mortality experienced through most of human existence and should not be limited to a specific calendar year or country (e.g., Sweden in 1751). Because of the hard work of field anthropologists, such a baseline exists, as the average mortality profile of ethnographically observed hunter-gatherers (3). Using this baseline as the standard for comparison provides the necessary means of gauging the rate and timing of mortality improvement in a broad evolutionary context. Many studies have demonstrated the remarkable ability of humans to prolong the length of life. Oeppen and Vaupel show that the best-case national life expectancy at birth has improved in a stunningly linear pattern between about 1840 and the present such that in the longest-lived national populations life expectancy has increased by about 3 mo per year. Similarly, Tuljapurkar et al. (5) show that the extensions in longevity are occurring because of rapid and steady progress in lowering mortality at all ages, and that the limits of these reductions are difficult to predict. The social, economic, and health implications of the reductions in mortality have been discussed extensively (6⇓–8). An element of the discussion that has been largely missing is the implication of mortality change for evolutionary theories of aging and, more generally, for biological understanding of plasticity of the mortality profile. This omission is odd given that age-specific mortality patterns are an essential driving force in the evolution of life histories (9). To better understand the evolutionary significance of the great changes in human mortality profiles and to more accurately describe the rate and magnitude of mortality reduction, we make a number of basic comparisons between the evolved human mortality profile and those of human populations from the present and recent historical contexts. We also make comparisons of the life expectancy extension achieved by humans with those achieved in the laboratory via experiments on model organisms. Such comparisons serve to frame human mortality improvement in a broad comparative light. We use this coarse-grained comparative approach to answer three basic questions about changes in human mortality profiles: (i) How can we place the observed human mortality improvement in a broader evolutionary framework? (ii) How much has mortality changed compared with the “typical” human mortality profile (where “typical” refers to the average mortality profile experienced during human evolution)? (iii) How does human mortality change compare with that observed in other species?