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"OH WATERS, TEEM WITH MEDICINE TO KEEP MY BODY SAFE FROM HARM, SO THAT I MAY LONG SEE THE SUN." - Rig Veda
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.
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.
This poses an evolutionary conundrum for neo-Darwinist orthodoxy—if the sole target of natural selection is to maximize reproduction, then why has evolution allowed reproduction to fall to zero while so many remain alive? The rising fertility curves indicate increased reproduction with age, which is another kind of negative senescence. When you think about a tree that grows larger with each passing year, it’s not so surprising that it’s making more seeds the older it gets. The Spanish mountain plant in the third row is Borderea pyrenaica, a plant that grows out on the rocky cliffs of the Pyrenees mountains. If undisturbed, it can live to 300 years or more with no sign of aging; but notice that its fertility doesn’t really get going until it is more than 20 years old.
The message of this diagram is nature can do whatever she wishes with aging (or non-aging). Any time scale is possible, and any shape is possible, and each species is exquisitely adapted to its ecological circumstance. There are no constraints.