You Are Not a Car: Your Body Does Not "Wear Out"

Just as the constant increase of entropy is the basic law of the universe, so it is the basic law of life to be ever more highly structured and to struggle against entropy.

— Václav Havel

I'm an extraordinary machine.

— Fiona Apple

Doesn't it seem strange that our bodies build themselves miraculously from single egg cells to fully formed, perfectly functioning adults — but then they can't seem to maintain themselves in good repair, as they gradually deteriorate and ultimately fail? It's as if the teen next door, who pieced together a complete 2002 Toyota Camry out of scrap from a junkyard, ran the car into a pothole and couldn't figure out how to change a tire.

Surely it is not a question of can't but won't. The body knows perfectly well how to repair and maintain itself, but that is not part of its genetic program, not part of its evolutionary mission. Modes of repair are shut down progressively as we age.

How Do You Think About Aging As We Begin?

Before I speak to people about aging, I always ask what ideas they have come to on their own. Everyone has thought about aging, at least enough to come to terms with it in their loved ones and their own lives. What is aging, and where does it come from?

When I am speaking to an audience of evolutionary biologists, the majority answer that "aging is a pleiotropic side effect of genes for fertility." This is what we referred to as "answer number two" to the mystery of aging in the preface. But outside university evolution departments, I have never found people to respond in this way. Instead, there are two popular notions about aging. About half the educated public already has the right idea (as I shall argue) about the significance of aging; this chapter is addressed to the other half.

The right idea is that aging has been programmed into our genes by evolution, an adaptation to make room in the niche for the next generation to grow up. The effect is to democratize, to keep the community diverse and resilient, and above all to stabilize the ecosystem against lopsided growth of any one species. The wrong idea is that bodies wear out for the same reason that machines do — gradually rusting, accumulating little nicks and dents. If that is the way you think about aging, my goal in this chapter is to convince you otherwise.

Things wear out. Nothing lasts forever. This is the oldest and still the most pervasive idea about what aging is. It is seductive because some aspects of aging fit with this picture; but the idea is also deeply flawed. It is a misapplication of basic physical law, and it also fails to account for some familiar facts about aging.

Bodies Versus Machines

The joints and bearings in your car become pitted and rusted over time, and they continue to work, but with less freedom and more squeaking. Isn't that just what happens to our arthritic knees and shoulders as cartilage wears away? Knives get dull, and the blades develop nicks and chips — just like our teeth. The plumbing pipes in your house become corroded over many decades, and deposits build up on the interior walls, impeding the flow. This sounds a lot like atherosclerosis — coronary heart disease. Often the performance of an older automobile engine suffers because the piston rings wear away, leaking exhaust within the cylinders. Our athletic performance declines with age, and it is natural to imagine it is for similar reasons. Biochemists might speak of "leaks in the electron transport chain" of our mitochondria, which are mini power plants inside each cell. In snowy, salty winters of the American Northeast, car chassis rust out over the years; as New Englanders might say, "The car has cancer." Even computers that have no moving parts are subject to performance degradation with age, because more and more apps are running in the background, each grabbing a chunk of the processor's "attention." Our immune systems fail in a similar mode, as our bloodstream accumulates memory T cells, white blood corpuscles that are expert at responding to diseases from our past, but there is a shortage meanwhile of naïve T cells that can respond to the next challenge.

Most compelling of all are the big problems that appear suddenly and send your car into the shop. The transmission fails, or the brakes wear out, or a rusting exhaust line finally leaks through into the passenger compartment. These problems are much more frequent in an old car than in a new car, and they are the reason that we retire the old car and buy a new one. Our bodies, similarly, become more vulnerable with age. Your chances of suffering a heart attack are fifteen times higher at age eighty than at age forty. You are twenty times more likely to be diagnosed with cancer at age eighty, compared to forty (and ten times more likely to die of cancer). Incidence of ordinary infectious diseases also rises with age, and though the increase is not so dramatic, the same diseases can be far more serious in an elderly person. Taken together, pneumonia and influenza are the eighth leading cause of death in the United States, and almost all these deaths are in people over seventy, most over eighty.

These surface similarities mask some major differences. If you leave your car in the garage most of the time and drive only two thousand miles a year, it will last a lot longer. But if you stay at home and don't use your muscles, you are risking rapid aging and early death. Exercise is the best thing you can do to extend your life. Why doesn't exercise wear your body out the way fast driving wears out a car?

This is our first clue that there is something very different about aging in a living body compared to wear in a machine. The body can fix itself in a way that the machine cannot. So for the body, its state of repair depends on the difference between the damage that is done in living and the repair that is accomplished internally, automatically, by grace of complex evolved physiology.

Better Than New

You might imagine that the body is always doing its best to repair itself, subject to some limits imposed by an energy budget. If the injuries and the damage are at a manageable level, then the body's automatic repair service will...