Death may be difficult to accept, but it is a clear state of affairs: when an organism no longer lives, it has died. Aging, however—wrinkling, weakening, deteriorating and the rest of the assault—seems less self-explanatory. Why does it take place? Do all living things go through it?
Not all species do. The paths that organisms follow after maturity vary enormously. Some plants live for one year only, others come back every season. Bacteria clone themselves and don’t die from age at all but from hostile organisms or conditions in their environment. Seabirds age very slowly; as long as they can fly, they can stay ahead of most predators. Lobsters don’t age; they can continue to grow and remain fertile for 45 years or more in the wild, dying only when they can no longer molt and grow a larger shell.
How and why the declines of aging are included in the final phases of some species’ lives is complex. Wikipedia’s “Senescence” introduces the range of theories and uncertainties. Here are three insights from the evolutionary perspective that make sense to me.
One is that certain harmful genetic mutations switch on later in life after the end of an organism’s reproductive period. Many cancers in humans do, for example. Because they don’t impact the number or health of the offspring, such genes do no harm to the persistence of the species and so they are unlikely to be lost over the generations. The diseases of the elderly get passed along by the young.
Even more unfortunately, some mechanisms in our bodies boost our health when we’re young and then come back to bite us when we get older. Digesting calcium, for instance, builds strong bones early on but helps clog and stiffen arteries decades later. As long as such a function improves our fitness to make and raise babies, whatever damage it does later on doesn’t matter much in the very long run.
A third way in which selection seems indifferent to the pains of aging is statistical: even if natural selection did reduce the ravages of aging and prolong the fertile period, the population of such organisms would still decline with age as accidents and predators took their inevitable toll. The body invests its resources where they are the most effective for the future, in youth and early reproduction, not in a comfortable old age.
In these ways and others, aging is linked to the importance of reproduction and the dangers of predators and other external forces. For primates, including me, we reproduce early because the big cats—leopards, jaguars, cougars, tigers—stalked us for millions of years in the forests and grass lands. And for most other species as well, the safest bet for continuity is simple: reproduce early. Still, the exceptions are fascinating. Lobsters in their suit of armor run little risk from ancient predators, so they can reproduce throughout their lives without ever aging into genetic irrelevance.
So, armed with such insights, do I experience my weakening muscles, declining sexuality, distracted thinking, and dulled senses with any less resentment? Yes, a little. Knowing that the decline has its place, even though it’s a melancholy one, in the evolution that brought me to being in the first place is some consolation.