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.
Today in Nature, evolutionary biologist Owen Jones and his colleagues have published a first-of-its-kind comparison of the aging patterns of humans and 45 other species. For folks (myself included) who tend to have a people-centric view of biology, the paper is a crazy, fun ride. Sure, some species are like us, with fertility waning and mortality skyrocketing over time. But 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.