If you don’t know where you are going, any road will get you there.
Do all animal species have built-in expiration timers? Some fish and reptiles may not, but most creatures — and especially mammals — do seem to have an inner clock that triggers every individual’s decline to frailty after the middle years of fight-flight-and-reproduction run their course. (…) The same holds true across nearly all mammalian species. Few live to celebrate their billionth pulse. (…)
Most mammals our size and weight are already fading away by age twenty or so, when humans are just hitting their stride. By eighty, we’ve had about three billion heartbeats! That’s quite a bonus.
How did we get so lucky?
Biologists figure that our evolving ancestors needed drastically extended lifespans, because humans came to rely on learning rather than instinct to create sophisticated, tool-using societies. That meant children needed a long time to develop. A mere two decades weren’t long enough for a man or woman to amass the knowledge needed for complex culture, let alone pass that wisdom on to new generations. (In fact, chimps and other apes share some of this lifespan bonus, getting about half as many extra heartbeats.)
