There is a recent article in Phys Rev Letters (“Programed Death is Favored by Natural Selection in Spatial Systems”) arguing that aging is an adaptation – natural selection has favored mechanisms that get rid of useless old farts. I can think of other people that have argued for this – some pretty smart cookies (August Weismann, for example, although he later abandoned the idea) and at the other end of the spectrum utter loons like Martin Blaser.
If this were the case, it would be a good thing, because we could then hope to significantly extend the human lifespan just by interfering with key steps in the programmed aging mechanism.
We would be able to easily identify those key steps because of natural experiments – people born with mutations that screwed up their kill switch and therefore lived lots longer than normal – you know, like those inventions that get released by mistake (the gasoline pill, the 100 mpg carburetor, etc.) . The problem is, I have never heard of any such slow-aging genetic syndrome. I don’t think any exist in humans. Too bad. And humans are the species to look at – not just because we hope to apply this to humans, but also because we know an enormous amount about human genetics. Milo of Crotona couldn’t lift a current paper edition of OMIM .
On the other hand, there may well be cases in which something like this happens in other species, particularly in semelparous organisms – those that reproduce only once, like Pacific salmon. Normally those salmon go back to their original spawning grounds, breed and die. In dying, they create a nutrient-rich environment for their off spring, and for other offspring that are on average closely related. You can see how programmed death might pay off in this case. A simple change does slow down senescence in those salmon: castration before they flame out triples their life span.
There might could be mutations that significantly extended lifespan but had consequences that were bad for fitness, at least in past environments – but that isn’t too likely if mutational accumulation and antagonistic pleiotropy are the key drivers of senescence in humans. As I said, we’ve never seen any.
It is possible that old people were once useful (materially contributed to the fitness of near relatives), especially in preliterate days. Back in the ice age climate could change rapidly (Dansgaard-Oeschger cycles) – old farts may have remembered previous climates, and that might have been useful. When Peter Panum studied the 1846 measles epidemic in the Faeroes, he found that quarantine could prevent contagion – but the old farts that had lived through the 1781 epidemic already knew that. People who listened to them, about a quarter of the population, were shielded from the epidemic.