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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For centuries, ever since the legendary Ponce de Leon went searching for the elusive Fountain of Youth, people have been looking for ways to slow down the aging process. Medical science has made great strides in keeping people alive longer by preventing and curing disease, and helping people to live healthier lives. Average life expectancy keeps increasing, and most of us can look forward to the chance to live much longer lives than our ancestors.