Author's Note: the purpose of this sequence is gears, i.e. physiology and molecular/cellular-level mechanisms, not evolution. This post contains only some bare-minimum background information as context for the rest; check out Will Bradshaw's series for a short intro which does better justice to evolutionary theories of aging in their own right.
Many species do not age: hydra, some turtles, rougheye rockfish, naked mole rats, and probably many others which we just haven’t sat around and watched long enough yet. This does not mean these organisms are immortal - in the wild, they get eaten or infected sooner or later. But their physiology does not change with age; post-development, older organisms look physiologically identical to younger organisms. They don’t get more wrinkles, or weaker muscles, or inflamed joints. In particular, non-aging species are not more likely to die soon as they get older. We call it “negligible senescence”.
Contrast to humans:
This is the Gompertz-Makeham Law of mortality: after development, humans’ annual death rate increases exponentially with age (doubling time ~8 years). For naked mole rats and other non-aging species, this curve would be completely flat after childhood. (Side note: that bump around age 20 is the car-accident bump).
This raises an evolutionary puzzle: aging obviously entails loss of functionality and increased death rate. Surely those things can’t be beneficial to organism fitness. But we know it’s physiologically possible for organisms to not age, so if aging isn’t evolutionarily beneficial, why do organisms ever age? Why haven’t all organisms evolved negligible senescence?
Well, once an organism has reproduced, evolution doesn’t really care what happens any more. Sure, slowly breaking down over time won’t be beneficial, but it’s not a significant disadvantage either, as long as the kids are grown. And in nature, the vast majority of organisms will die to predation or disease or starvation pretty quickly anyway. So, whenever there’s an opportunity to gain some early-life advantage in exchange for aging later in life, that’s going to be an evolutionarily advantageous trade-off. This is the “antagonistic pleiotropy” evolutionary theory of aging: most organisms age because there’s little selective pressure not to, and there are advantages to be had from trade-offs in favor of early life.
This ties in to the general theory of “life history strategies”: some creatures produce large numbers of offspring but don’t invest much in raising the children, while others produce just a few offspring and invest heavily in them. Creatures which invest heavily in their children will be more evolutionarily useful post-reproduction, so we should expect them to have longer lives; creatures which don’t invest much won’t gain as much fitness by living longer. At the extreme, we see organisms which spend their entire metabolic resources on reproduction, maximizing the number of young produced, and die shortly after.
There is some impressive evidence in favor of antagonistic pleiotropy in comparative (i.e. cross-species) aging - life history strategies do turn out to correlate quite heavily with lifespan, by multiple measures, and the measures which we’d expect to better reflect life history tend to screen off those which reflect it less well. I’m not going to go into the details here, but Robert Arking’s “The Biology of Aging” has a decent chapter on it (chapter 4).
For humans specifically, there’s some evidence that we have higher-than-usual evolutionary pressure against aging. In particular, humans are quite long-lived for animals our size. In general, larger animals have longer lifespans - mice live ~3 years, lions and gazelles live 10-15 years, elephants live 50-70… yet humans outlive elephants. Our long life span is typically attributed to very long gestational periods combined with a very high degree of parental investment in children. In the antagonistic pleiotropy <-> life history picture, high-parental-investment strategies are generally associated with long species life-spans, and vice-versa: when organisms don’t invest in their progeny, they are less evolutionarily useful in old age. Since we humans invest extremely heavily in our offspring, we are unusually useful in old age, and thus have unusually long lifespans.