In humans, increases in life expectancy in the past century have been ascribed overwhelmingly to reductions in environmental causes of mortality that are extrinsic to the aging process. Longevity in evolutionary terms is difficult to reconcile with the findings, in recent decades, that, in humans, both mortality and disability rates have dropped for all older men and women, across virtually all social classes and ethnic groups, and in most developed and developing countries. It is possible that other factors, in addition to "physical fitness and reproductive capability," may be subject to positive selection such as the "grandparents hypothesis," that is, the important contribution of the grandparents in raising the young (Chapters 2 and 5). Comparison of life spans of several species (selective longevity) may show some differences in longevity and allow us to draw some relationships between the life span and the selected physiologic characteristics (physiological correlates of longevity).
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