The Story Told One Last Time, from Beginning to End

As soon as your reproductive role has been accomplished, you’re disposable.

After sexual maturation, deterioration in peak efficiency occurs because, as Harold Morowitz, a professor of biology at George Mason University, says, “perfect order requires infinite work.” Also, deterioration builds on itself.

In the late nineteenth century, August Weismann, a German biologist, made a distinction between “the immortality of reproductive cells, the cells in the body that carry genes forward to the next generation, and the mortality of the rest, which will age and die.” Death takes place, he said, “because a worn-out tissue cannot forever renew itself, and because a capacity for increase by means of cell-division is not everlasting but finite.”

Once a body’s mission is accomplished, nature has little interest in what happens next. Reproductive life spans of members of a species work as perfectly as possible to match the time an individual of that species might expect to survive before dying. In other words, physiological resources go into reproduction, not into prolonging life thereafter.

The force of natural selection declines with age. Natural selection has shaped human biology in such a way that aging and death become increasingly likely by the time you reach your 40s. If a disaster strikes a person who has passed the age of reproductive fitness, the consequences are by and large unimportant to the survival of the rest of the species.

Nature favors the accumulation of genes that do beneficial things early in life, even though they might do harmful things late in life, since—under normal conditions—most animals do not live long enough for the harmful effects to cause a problem. The same general mechanism that protects against cancer protects against aging. Long-lived species, with their better cellular protection, get cancer later than short-lived species.

The pineal gland is your internal clock. It knows how old you are, and it knows when you’re past your reproductive prime. As soon as it senses that you’re too old to reproduce effectively—around age 45—it begins to produce far lower levels of melatonin, which signals all of your other systems to break down and the aging process to begin. (Women’s larger pineal gland is another reason why women age more slowly than men, and it may be why they live longer.)

These low levels of melatonin cause, for instance, your immune system to shut down and your endocrine system to produce fewer sex hormones. Lower levels of sex hormones in turn lead to the atrophy of sexual organs in both men and women, to a decrease as well in sexual interest and the ability to perform. At 90, my father’s equipment finally quit.

In the late stages of adulthood, moths mimic the movements of juvenile moths, leading predators away from young moths and sacrificing their own lives, in order to benefit the species. What I’ve been trying to get to all along, in a way, is this: The individual doesn’t matter. You, Dad, in the large scheme of things, don’t matter. I, Dad, don’t matter. We’re vectors on the grids of cellular life. We carry 10 to 12 genes with mutations that are potentially lethal. These mutations are passed on to our children—you to me, me to Natalie. Aging followed by death is the price we pay for the immortality of our genes. You find this information soul-killing; I find it thrilling, liberating. Life, in my view, is simple, tragic, and eerily beautiful.