"Why Do We Age? A 46-Species Comparison"

"Since the early 1950s, evolutionary biologists have come up with a few explanations, all of which boil down to this: As we get older, our fertility declines and our probability of dying — by bus collision, sword fight, disease, whatever — increases. That combination means that the genetic underpinnings of aging, whatever they are, don’t reveal themselves until after we reproduce. To use the lingo of evolutionary biology, they’re not subject to selective pressure. And that means that senescence, as W.D. Hamilton wrote in 1966, “is an inevitable outcome of evolution.”

Except when it’s not...

lots of species show different patterns —bizarrely different. Some organisms are the opposite of humans, becoming more likely to reproduce and less likely to die with each passing year. Others show a spike in both fertility and mortality in old age. Still others show no change in fertility or mortality over their entire lifespan...

What the new study didn’t find, notably, is an association between lifespan and aging. It turns out that some species with pronounced aging (meaning those with mortality rates that increase sharply over time) live a long time, whereas others don’t. Same goes for the species that don’t age at all. Oarweed, for example, has a near-constant level of mortality over its life and lives about eight years. In contrast, Hydra, a microscopic freshwater animal, has constant mortality and lives a whopping 1,400 years."

http://phenomena.nationalgeographic.com/2013/12/08/why-do-we-age-a-46-species-comparison/