Humans are the only primate species that has a very long post-reproductive lifespan. In most primates, as in most animals and plants, an organism that has finished reproducing has nothing more to contribute to its offspring. But the extended human lifespan seems to be an evolutionary anomaly, and seems to require an evolutionary explanation.
Enter the Grandmother Hypothesis. Grandmothers, though long past their reproductive age, could help to take care of grandchildren while the mothers and fathers searched and hunted for food. Furthermore, grandmothers were what science writer Natalie Angier called “Alexandrian libraries of preliterate cultures.” The brains of old people were filled with cultural lore that could be passed on to the next generation. Humans are weak but smart, and our adaptation to hostile environments (which is, in some way, nearly all of them) is principally cultural. According to the hypothesis, natural selection has favored old age in humans as a way of passing on a lifetime of memorized culture.
But there is a problem with this, according to evolutionary gerontologist (yes! there is such a thing!) Bruce Carnes. In primitive cultures, grandparents were only about 35 years old, and most people did not live past 40. The Grandmother Hypothesis is correct, but it does not explain the characteristics of humans past the age of 40.
The unfortunate conclusion, then, is that most of the characteristics of middle- and old-aged humans (which includes me) come from the breakdown of human physiology. Our bodies could live longer, if we lived more mildly, but natural selection has favored humans that lived hard and died young, though not as young as squirrels and birds. The mild mannered Paleolithic man might live a little longer but would probably leave fewer offspring. Natural selection has seldom had a chance to act, positively or negatively, upon 53-year-old men like me. We are just lucky that our physiological processes do not simply collapse at age 40. I, for one, am thankful for each day of life in which my body does not simply stop working.