By Josh Mitteldorf & Dorion Sagan
Humans age gradually, but some animals do all their aging in a rush at the end of life, while others don’t age at all, and a few can even age backward. The variety of aging patterns in nature should be a caution sign to anyone inclined to generalize—particularly the generalization that aging is inevitable.
Bacteria reproduce symmetrically, just dividing in two. What could “aging” mean for bacteria since, after reproduction, there is no distinction between parent and child? Single-cell protists like the amoeba also reproduce symmetrically, but curiously, they invented a way to age nevertheless. And even among macroscopic life forms, life spans of organisms are immensely variable in a way that is finely tuned to local ecologies and reproduction rates. This can hardly be the result of a universal, inexorable process; in fact, such fine-tuning to circumstance is the signature of an adaptation.
Life spans range from Methuselans great and small to genetic kamikazes that die of a spring afternoon. Submerged dragonflies live four months, adult mayflies half an hour. We live some 70-odd years; but the meristem of the ginkgo may be millions of years old. This range becomes all the more impressive when we realize that the genetic basis for aging is widely shared across different species, from yeast cells on up to whales. Somehow, the same genetic machinery, inherited from our common ancestors at the dawn of life on Earth, has been molded to generate life spans ranging from hours (yeast cells) to thousands of years (sequoia trees and quaking aspen).
And it is not only the length of life but the pattern of deterioration within that time that varies widely. Aging can occur at a steady pace through the course of an entire lifetime (most lizards and birds), or there can be no aging at all for decades at a time, followed by sudden death (cicadas and century plants).
Our own “inner assassin” works with stealth, like an evil empress gradually poisoning her husband; but other species have inner killers that do their deed far more quickly, and still others appear to have no genetic death programs at all. Such variety is a sure signal for a feature molded by active natural selection, not an immutable law of entropy.
Picture: Catherine Scott (Matti) [CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0)%5D, via Wikimedia Commons