The causes of disease – Mystery and Myth

Why, in a body of such exquisite design, are there a thousand flaws and frailties that make us vulnerable to disease? If evolution by natural selection can shape sophisticated mechanisms such as the eye, heart, and brain, why hasn’t it shaped ways to prevent nearsightedness, heart attacks, and Alzheimer’s disease? If our immune system can recognize and attack a million foreign proteins, why do we still get pneumonia? If a coil of DNA can reliably encode plans for an adult organism with ten trillion specialized cells, each in its proper place, why can’t we grow a replacement for a damaged finger? If we can live a hundred years, why not two hundred?

We know more and more about why individuals get specific diseases but still understand little about why diseases exist at all. We know that a high-fat diet causes heart disease and sun exposure causes skin cancer, but why do we crave fat and sunshine despite their dangers? Why can’t our bodies repair clogged arteries and sun-damaged skin? Why does sunburn hurt? Why does anything hurt? And why are we, after millions of years, still prone to streptococcal infection?

The great mystery of medicine is the presence, in a machine of exquisite design, of what seem to be flaws, frailties, and makeshift mechanisms that give rise to most disease. An evolutionary approach transforms this mystery into a series of answerable questions: Why hasn’t the Darwinian process of natural selection steadily eliminated the genes that make us susceptible to disease? Why hasn’t it selected for genes that would perfect our ability to resist damage and enhance repairs so as to eliminate aging? The common answer—that natural selection just isn’t powerful enough—is usually wrong. Instead, as we will see, the body is a bundle of careful compromises.

The body’s simplest structures reveal exquisite designs unmatched by any human creations. Take bones. Their tubular form maximizes strength and flexibility while minimizing weight. Pound for pound, they are stronger than solid steel bars. Specific bones are masterfully shaped to serve their functions—thick at the vulnerable ends, studded with surface protrusions where they increase muscle leverage, and grooved to provide safe pathways for delicate nerves and arteries. The thickness of individual bones increases wherever strength is needed. Wherever they bend, more bone is deposited. Even the hollow space inside the bones is useful: it provides a safe nursery for new blood cells.

Physiology is still more impressive. Consider the artificial kidney machine, bulky as a refrigerator yet still a poor substitute that performs only a few of the functions of its natural counterpart. Or take the best man-made heart valves. They last only a few years and crush some red blood cells with each closure, while natural valves gently open and close two and a half billion times over a lifetime. Or consider our brains, with their capacity to encode the smallest details of life that, decades later, can be recalled in a fraction of a second. No computer can come close.

The body’s regulatory systems are equally admirable. Take, for instance, the scores of hormones that coordinate every aspect of life, from appetite to childbirth. Controlled by level upon level of feedback loops, they are far more complex than any man-made chemical factory. Or consider the intricate wiring of the sensorimotor system. An image falls onto the retina; each cell transmits its signal via the optic nerve to a brain center that decodes shape, color, and movement, then to other brain centers that link with memory banks to determine that the image is that of a snake, then to fear centers and decision centers that motivate and initiate action, then to motor nerves that contract exactly the right muscles to jerk the hand away—all this in a fraction of a second.

 

Bones, physiology, the nervous system—the body has thousands of consummate designs that elicit our wonder and admiration. By contrast, however, many aspects of the body seem amazingly crude. For instance, the tube that carries food to the stomach crosses the tube that carries air to the lungs, so that every time we swallow, the airway must be closed off lest we choke. Or consider nearsightedness. If you are one of the unlucky 25 percent who have the genes for it, you are almost certain to become nearsighted and thus unlikely to recognize a tiger until you are nearly its dinner. Why haven’t these genes been eliminated? Or take atherosclerosis. An intricate network of arteries carries just the right amount of blood to every part of the body. Yet many of us develop cholesterol deposits on the walls of our arteries, and the resulting blockage in blood flow causes heart attacks and strokes. It is as if a Mercedes-Benz designer specified a plastic soda straw for the fuel line!

Dozens of other bodily designs seem equally inept. Each may be considered a medical mystery. Why do so many of us have allergies? The immune system is useful, of course, but why can’t it leave pollen alone? For that matter, why does the immune system sometimes attack our own tissues to cause multiple sclerosis, rheumatic fever, arthritis, diabetes, and lupus erythematosus? And then there is nausea in pregnancy. How incomprehensible that nausea and vomiting should so often plague future mothers at the very time when they are assuming the burden of nourishing their developing babies! And how are we to understand aging, the ultimate example of a universal occurrence that seems functionally incomprehensible?

Even our behavior and emotions seem to have been shaped by a prankster. Why do we crave the very foods that are bad for us but have less desire for pure grains and vegetables? Why do we keep eating when we know we are too fat? And why is our willpower so weak in its attempts to restrain our desires? Why are male and female sexual responses so uncoordinated, instead of being shaped for maximum mutual satisfaction? Why are so many of us constantly anxious, spending our lives, as Mark Twain said, “suffering from tragedies that never occur”? Finally, why do we find happiness so elusive, with the achievement of each long-pursued goal yielding not contentment, but only a new desire for something still less attainable? The design of our bodies is simultaneously extraordinarily precise and unbelievably slipshod. It is as if the best engineers in the universe took every seventh day off and turned the work over to bumbling amateurs.