Why Are Humans Long-Lived?
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We humans are already unusually long-lived when compared to many other mammals of our size, though nowhere near as long-lived as some of us would like to be. It is something of an open question as to what has influenced evolution to produce a number of social species that live far beyond their reproductive years - see, for example, the grandmother hypothesis:

How is it, then, that a woman's lifespan can greatly exceed her childbearing and childrearing years? Is this phenomenon simply a byproduct of improved standards of living, or do older women - grandmothers in particular - play a measurable role in increasing their family members' biological success?

As it turns out the same questions can be asked of killer whales. They also live long in comparison to many other mammals, far past the age at which they can no longer reproduce. I noticed a very readable paper recently that looks at the question of this sort of evolved longevity:

Menopause is a seemingly maladaptive life-history trait that is found in many long-lived mammals. There are two competing evolutionary hypotheses for this phenomenon; in the adaptive view of menopause, the cessation of reproduction may increase the fitness of older females; in the non-adaptive view, menopause may be explained by physiological deterioration with age. The decline and eventual cessation of reproduction has been documented in a number of mammalian species, however the evolutionary cause of this trait is unknown.

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By extending their lifespans after reproduction ceases, post-reproductive females who help daughters or other kin raise offspring increase their own [chances of evolutionary success]

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Although existing data do not allow us to examine evolutionary tradeoffs between survival and reproduction for this species, we were able to examine the effect of maternal age on offspring survival. Our results are consistent with similar studies of other mammals - oldest mothers appear to be better mothers, producing calves with higher survival rates. Studies of juvenile survival in humans have reported positive benefits of grandmothers on newly weaned infants; our results indicate that 3-year old killer whales may experience a positive benefit from helpful grandmothers.

I would expect there to be some evolutionary balance struck between older parents being better parents and older parents being more damaged by aging, though it isn't clear just how much of a selection effect is applied when you're looking at older parents. Perhaps only the best survive to older ages to be those better parents.

This paper doesn't firmly answer any of the questions posed, but is is a good overview of current thinking on how we humans ended up aging the way we do. As the broad diversity of the animal kingdom shows, there are plenty of other options on the table for natural life span, all too few of them better in terms of years of health attained. While it is interesting to look at our origins, it is also important to remember that we already have the knowledge and capabilities to step beyond them. Aging to death will one day be a barbaric thing of the past, eradicated along with smallpox. Just how soon that comes about is entirely up to our collective efforts.

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