friction, like that at the top edge of a shoe or the tip of a cellist’s finger, grows the thicker skin we call a callus. This adaptive growth, an induced defense, not only minimizes mechanical injury, it also prevents breaks in the skin that could provide entrances for pathogens.
Some of our most useful hygienic behaviors help maintain the skin’s barrier. The most obvious are behaviors that keep nasty things off the skin. Scratching and other grooming maneuvers remove external parasites, important sources of discomfort and disease transmission for most people during most of human history and still problems in less fortunate societies. Benjamin Hart, a veterinarian from the University of California at Davis, has shown just how crucial grooming is to preventing illness in animals. An animal that cannot groom is quickly infested with fleas, ticks, lice and mites, and willlose weight and fall ill. The mutual grooming of monkeys is not just a ritual, it is preventive health care.
P AIN AND M ALAISE
J ust as an itch can motivate defensive scratching, pain is an adaptation that can lead to escape and avoidance. The skin, sensibly enough, is highly sensitive to pain. If it is being damaged, something is clearly wrong, and all other activities should be dropped until the damage is stopped and repair can begin. Other kinds of pain can also be helpful. While an abstract realization that chewing is impaired because of an abscessed tooth might possibly lead to more chewing with other, unimpaired teeth, the tormenting pain of a toothache far more effectively prevents the pressure on the tooth that would delay healing and spread bacteria. Continued pain from infection or injury is adaptive because continued use of damaged tissue may compromise the effectiveness of other adaptations, such as tissue reconstruction and antibody attacks on bacteria. Pain motivates us to escape quickly when our bodies are being damaged, and the memory of the pain teaches us to avoid the same situation in the future.
The simplest way to determine the function of an organ like the thyroid gland is to take it out and then see how the organism malfunctions. The capacity for pain cannot be removed, but very occasionally someone is born without it. Such a pain-free life might seem fortunate, but it is not. People who cannot feel pain don’t experience discomfort from staying in the same position for long periods, and the resulting lack of fidgeting impairs the blood supply to the joints, which then deteriorate by adolescence. People who cannot feel pain are nearly all dead by age thirty.
Generalized aches and pains, or merely feeling out of sorts (malaise, in medical terminology), are also adaptive. They encourage a general inactivity, not just disuse of damaged parts. That this is adaptive is widely recognized in the belief that it is wise to stay in bed when you are sick. Inactivity also likely favors the effectiveness of immunological defenses, repair of damaged tissues, and other host adaptations. Medication that merely makes a sick person feel less sick will interfere with these benefits. This is fine when patients arewell informed about the risks and realize that they are sicker than they feel and should make a special effort to take it easy. Otherwise, a drug-induced feeling of well-being may lead to activity levels that interfere with defensive adaptations or repairs.
D EFENSES B ASED ON E XPULSION
T he body must have openings for breathing, for the intake of nutrients and expulsion of wastes, and for reproduction. Each of these openings offers pathogens an invasion route, and each is endowed with special defense mechanisms. The constant washing of the mouth with saliva kills some pathogens and dislodges others so they can be destroyed by the acid and enzymes in the stomach. The eyes are washed by tears laden with defensive chemicals and the respiratory system by antibody and enzyme-rich secretions that are steadily propelled up to the throat, where
Some of our most useful hygienic behaviors help maintain the skin’s barrier. The most obvious are behaviors that keep nasty things off the skin. Scratching and other grooming maneuvers remove external parasites, important sources of discomfort and disease transmission for most people during most of human history and still problems in less fortunate societies. Benjamin Hart, a veterinarian from the University of California at Davis, has shown just how crucial grooming is to preventing illness in animals. An animal that cannot groom is quickly infested with fleas, ticks, lice and mites, and willlose weight and fall ill. The mutual grooming of monkeys is not just a ritual, it is preventive health care.
P AIN AND M ALAISE
J ust as an itch can motivate defensive scratching, pain is an adaptation that can lead to escape and avoidance. The skin, sensibly enough, is highly sensitive to pain. If it is being damaged, something is clearly wrong, and all other activities should be dropped until the damage is stopped and repair can begin. Other kinds of pain can also be helpful. While an abstract realization that chewing is impaired because of an abscessed tooth might possibly lead to more chewing with other, unimpaired teeth, the tormenting pain of a toothache far more effectively prevents the pressure on the tooth that would delay healing and spread bacteria. Continued pain from infection or injury is adaptive because continued use of damaged tissue may compromise the effectiveness of other adaptations, such as tissue reconstruction and antibody attacks on bacteria. Pain motivates us to escape quickly when our bodies are being damaged, and the memory of the pain teaches us to avoid the same situation in the future.
The simplest way to determine the function of an organ like the thyroid gland is to take it out and then see how the organism malfunctions. The capacity for pain cannot be removed, but very occasionally someone is born without it. Such a pain-free life might seem fortunate, but it is not. People who cannot feel pain don’t experience discomfort from staying in the same position for long periods, and the resulting lack of fidgeting impairs the blood supply to the joints, which then deteriorate by adolescence. People who cannot feel pain are nearly all dead by age thirty.
Generalized aches and pains, or merely feeling out of sorts (malaise, in medical terminology), are also adaptive. They encourage a general inactivity, not just disuse of damaged parts. That this is adaptive is widely recognized in the belief that it is wise to stay in bed when you are sick. Inactivity also likely favors the effectiveness of immunological defenses, repair of damaged tissues, and other host adaptations. Medication that merely makes a sick person feel less sick will interfere with these benefits. This is fine when patients arewell informed about the risks and realize that they are sicker than they feel and should make a special effort to take it easy. Otherwise, a drug-induced feeling of well-being may lead to activity levels that interfere with defensive adaptations or repairs.
D EFENSES B ASED ON E XPULSION
T he body must have openings for breathing, for the intake of nutrients and expulsion of wastes, and for reproduction. Each of these openings offers pathogens an invasion route, and each is endowed with special defense mechanisms. The constant washing of the mouth with saliva kills some pathogens and dislodges others so they can be destroyed by the acid and enzymes in the stomach. The eyes are washed by tears laden with defensive chemicals and the respiratory system by antibody and enzyme-rich secretions that are steadily propelled up to the throat, where
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