Of course, as every reader of the headlines knows, addiction is an inherited disorder. We are not sure what the average writer or reader of headlines might understand by this, but what we understand is what we discussed in Chapter 7 as genetic quirks. Some people can have frequent evening cocktails, wine or beer with meals, and occasional weekend binges and never show a sign of alcohol addiction. A person with the relevant genetic quirk will, with the same alcohol intake, show a steady increase in drinking until he or she is spending prodigiously to support an ever-worsening addiction and is ever less able to work and maintain normal social relationships. The consequences of this genetic quirk would have been minimal until after such civilizing inventions as stills and six-packs. Alcoholism and much other substance abuse can justifiably be considered diseases of civilization.
DEVELOPMENTAL PROBLEMS FROM MODERN ENVIRONMENTS
Lack of adequate exercise may be expected to cause problems other than those associated with overweight and fatty foods. It makes no evolutionary sense, for example, for the human developmental process to cause a large proportion of the population to grow incisors in malfunctional positions and to suffer so many problems with wisdom teeth. If a large proportion of modern children need orthodontia and then later some require expensive and painful surgery on wisdom teeth, it implies that there is something wrong with their environment.
One possibility is a deficient demand for jaw exercise. No Stone Age ten-year-old would have been living on foods of anything like the tenderness and fragility of modern potato chips, hamburgers, and pasta. Their meals would have required far more prolonged and vigorous chewing than is ever demanded of a modern child. We wonder if deficient use of jaw muscles in the early years of life may result in their underdevelopment and indirectly in weaker and smaller associated bone structure. The growth of human teeth is more autonomous, but it assumes a jaw structure of a certain size and shape, one that might not be produced if usage during development is inadequate. Crowded and misplaced incisors and imperfectly erupted wisdom teeth may be diseases of civilization. Perhaps many dental problems would be prevented if prolonged vigorous biting were considered a prestigious athletic attainment for children. Perhaps chewing gum should be encouraged in schools!
Other abnormal behaviors during childhood might cause abnormal physical development. Sitting for hours at a time on chairs or benches in classrooms is unnatural, and nothing of the sort was ever demanded of Stone Age children. When they were sedentary, they would have been squatting, not sitting. Stone Agers must also have been able to shift from squatting to kneeling to walking or running or other sorts of activity. Might it not be that many of today’s sufferers from lower back pain owe their distress to the hours of abnormal posture imposed day after day during childhood? Maybe the later problems could be avoided by having children do more squatting and less sitting and giving them more exercise breaks or walks between classes.
University of Michigan physician Alan Weder and his colleague Nicholas Schork have tried to understand high blood pressure as a disease of civilization. Instead of emphasizing the high levels of salt in our diets, however, they note that blood pressure must be higher to supply the needs of larger bodies and that there is a mechanism that increases the pressure during adolescent growth spurts. In the ancestral environment, they argue, this mechanism would have made adjustments within a range of small body sizes. Today, our nutritionally rich environment yields fast growth and large body sizes that were rare in the past. The blood-pressure-regulating mechanism, pushed to adjust the system outside the range for which it was designed, often overshoots, causing high blood pressure.
Myopia is not the only ocular abnormality that may arise from novel environmental conditions early in life. Medical science has only recently become aware of ways in which eye usage in the first weeks and months after birth may be critical to the normal development of vision. Preferential use of one eye rather than the other, from whatever cause, may lead to changes in the allocation of brain regions to ocular functions so that a child may later prove incapable of using binocular cues for depth perception. Twenty-four-hour bright lights, sometimes used to treat neonatal jaundice, can cause color-vision defects not likely to be detected until much later. Would it be surprising to discover that constant exposure to loud noises, especially the unchanging sounds of modern machinery, can cause defective hearing development in some babies?
OTHER DISEASES RESULTING FROM MODERN ENVIRONMENTS
Cold weather can be considered a novel environmental factor. The spread of human populations to seasonally cold environments was facilitated by technological innovations, such as clothing and fire, which we achieved only a few tens of thousands of years ago. We still need these artificialities, or their modern equivalents, to survive the winter over much of the currently inhabited surface of the earth. Technology compensates for human biological inadequacies in dealing with such novel environmental threats as frostbite and hypothermia.
But low temperature is not the only stress imposed by high latitudes. Clothing and shelter that enable us to survive in places like Montreal and Moscow impose their own health problems. Our synthesis of vitamin D depends on our exposing our skins to sunlight. If we are indoors much of each day and largely covered with clothes when we are out, the amount of vitamin D we synthesize will be a tiny fraction of that made by a naked forager on the African savannah, and it could be grossly inadequate for our metabolic needs. Fortunately, our photosynthetic capability is not our only source of this material. We can also fulfill our vitamin needs by eating certain foods. Unfortunately, a seemingly adequate diet may in fact provide very little vitamin D, and a deficiency leads to health problems related mainly to abnormalities of calcium metabolism.
The most commonly recognized effect of vitamin D deficiency is rickets, a developmental disease of childhood. The symptoms are many, but the most important is defective growth of the bones. They become soft and weak from deficient calcium deposition and grow abnormally. The disease is essentially unknown in the tropics, where everyone gets abundant sunshine, and uncommon in Japan, Scandinavia, and other regions where traditional diets include good sources of vitamin D, such as fish. But at times it affected such large numbers of children in England that it was sometimes called the English disease.
Rickets was also a frequent malady in northern American cities prior to the 1930s, when vitamin D began to be routinely added to milk. Rickets struck black children at a higher rate than white. The adaptive significance of human racial differences is generally dubious, but the reduced vulnerability of pale-skinned people to rickets may be a valid example. Perhaps the first people who crossed the Mediterranean and later the Alps were quite dark. They found a land covered with trees under a sky often covered with clouds. During much of the year they spent long hours huddling in caves or drafty shelters. When they went outdoors they clothed themselves with animal skins or woven fabrics and exposed very little skin to the meager sunshine. The result, for many people, may have been depressed fitness because of vitamin D deficiency. Those who happened to have less heavily pigmented skins, which admitted more light for vitamin D synthesis, would have fared better than their darker neighbors.
In this way light skin may have evolved in perhaps a few hundred generations. The change may have been rapid because reductions of a trait are generally easier to evolve than increases or elaborations. Cave animals may lose almost all ability to make pigment in a few thousand generations, and this happens merely from relaxed selection for the maintenance of color. If there is an actual advantage to paler skin, the change should be much faster. The same evolutionary reduction of melanin synthesis may have happened, though to a lesser extent, in the colder parts of Asia, where forests give way to grasslands and deserts and winter days are more often sunny. The native peoples of Siberia and northern China are darker than those of central and northern Europe but paler than those of Africa or southern Asia. As a disease of civilization, rickets is more of a hazard for people wi
th highly pigmented skin, and pale skin may be recognized as especially adapted to a scarcity of sunshine. But then what happens when these pale people move back to sunny regions, such as Australia? Stay tuned for more on the sunshine problem (Chapter 12), and recall our discussion of sunburn in Chapter 5.
As noted above, the invention of agriculture led to population densities much greater than could be achieved by hunter-gatherer economies, and it permitted the support of great concentrations of people in cities. The spread of people into seasonally cold environments led to their prolonged concentration inside caves and buildings. Both these changes increased the number of people a given individual would contact in a short period of time and increased the closeness and duration of such contacts. New infectious diseases could then emerge that could be spread only by abundant personal contact.
Much of the natural selection taking place in these populations may have consisted of the weeding out of individuals whose genetic quirks made them vulnerable to smallpox, measles, or other contact-transmitted diseases. High-cost defenses against such tropical diseases as malaria, for example the sickle-cell trait, would have been lost rapidly. The effectiveness of the newly evolved defenses against such diseases as smallpox was tragically shown when settlers, carrying what for them were well-controlled pathogens, invaded parts of the world where native peoples had never been exposed to the diseases of civilization. Far more New World people were killed by European diseases such as smallpox and influenza than by European weapons.
In this chapter we have scarcely hinted at the many psychological problems that may arise from modern life. Despite the family-values rhetoric of politicians, children raised by nuclear families in single-unit suburban dwellings are experiencing a profoundly novel social environment, as are those being supervised by transient caretakers in day care centers. As adults and even as adolescents and children, we may have to deal more often with impersonal bureaucracies than with familiar individuals. Most of the people we encounter on what seems to be a normal day may be strangers. This was not the kind of world our ancestors evolved in. What about the prolonged winter darkness of high latitudes and, conversely, the hours of bright indoor lighting and resulting shortened periods of real darkness we actually experience? The cabin fever of snowbound Alaskan gold seekers is now a recognized malady that is getting attention from medical researchers. What about night-shift workers and the jet-lagged jet set? And then there are the psychological—as well as physiological—effects of offices without windows. We have just begun to explore the medical consequences of our novel modern environment.
CONCLUSIONS AND RECOMMENDATIONS
There is no Eden we can go back to even if such a move were desirable. What we can do is be alert to the modern dangers and take reasonable steps to forestall them. As with many other topics discussed in this book, our main recommendation for anyone faced with a problem of medical importance is to consider the question: What is its evolutionary significance? One possibility is that it is an adaptive mechanism, but this will normally mean adaptive in the Stone Age. Our cravings for sugar and fat, our tendencies to be lazy, and our eye-growth adjustments that result in myopia are evolved adaptations, but in modern environments they cause difficulties for many people. Other evolved attributes, such as senescence and susceptibility to sunburn, are adaptive in no environment but may represent costs of other adaptations. Again and again we harp on the themes that all benefits have costs and that many benefits are worth their associated costs.
11
ALLERGY
Many people in temperate parts of North America dread the day in August when ragweed first releases its pollen, causing sneezing and wheezing and reaching for handkerchiefs and antihistamines. The poor ragweed plant is just trying to reproduce, but we are the ones who suffer. A single plant may release a million grains of pollen a day, mostly between 6 and 8 A.M., perfect timing to maximize the likelihood that those sex cells will find their way to receptive ragweed flowers on the morning breeze. A square mile of ragweed plants can produce sixteen tons of pollen in a year, but an allergic response can be provoked by one millionth of a gram. The notorious pollen grain is tiny, a sphere twenty microns in diameter that contains two living ragweed sex cells, accompanied by proteins and other nutrients. One of the proteins, Amb a I, makes up only 6 percent of the protein but causes 90 percent of the allergic activity. And what a lot of unfortunate activity it is. From the middle of August, those who suffer from ragweed allergy look forward to the day a few weeks before the first hard freeze when the ragweed plants will die and stop broadcasting their pollen.
Ragweed is, of course, not the only culprit. Allergies are also provoked by inhaling other pollens, fungal spores, animal danders, and mite feces, by skin contact with many different substances, by eating certain foods or drugs, and by injections of drugs or toxins like bee venom. A quarter of the modern American population suffers from some allergy or another. You or a relative or friend may well have sought help from an allergist. If so, you probably had skin tests to try to identify the substance (allergen) that caused the allergy. Two kinds of advice were then forthcoming: avoid the allergen and relieve the symptoms with this or that anthistaminic drug.
Avoiding the allergen makes sense, but what about relieving the symptoms? We dealt with that kind of advice in discussing the treatment of infectious disease. Could taking antihistamine for your allergy be analogous to taking acetaminophen for fever or giving mice a pill to keep them from smelling cats? At the moment we know that the system that gives rise to allergy is a defense, but we do not know for sure what it is supposed to defend us against. We can be sure that the capacity for an allergic reaction is a defense against some kind of danger, or else the underlying mechanism, the immunoglobulin-E (IgE) part of the immune system, would not exist. It is perhaps conceivable that our IgE system is a remnant of a system that was useful for other species, but this is unlikely because systems of this complexity degenerate quickly if they are not maintained by natural selection and even more quickly if they cause any harm. It is much more likely that the IgE system is somehow useful.
This need not mean that every allergy attack is useful. In fact, an evolutionary view of inexpensive defensive reactions suggests that most individual instances will be harmful even though the system as a whole is adaptive. This is a manifestation of the smoke-detector principle. Smoke detectors are designed to warn people when a dangerous fire is in progress, but few of them ever perform this service. They hang there year after year doing nothing or only sounding an occasional false alarm from a cigar or smoky toaster. Yet the annoying false alarms, and the costs of the smoke detector and its occasional battery change, are well justified by the protection they provide against a major fire. More on this principle when we discuss anxiety in Chapter 14.
Your allergist probably did not give you a discussion about the utility of the IgE system and the evolution of its regulation. If you asked why you have to be allergic to cats or oysters or whatever, your allergist probably said something like “Well, as in everything else, people vary tremendously in their sensitivities to different allergens, and you happen to be excessively sensitive to something in cat dander. This excess in your sensitivity must be treated by avoiding cat dander and suppressing the defensive reaction it triggers.”
There are two serious difficulties with the excess-sensitivity theory. First, an allergy is not just a matter of degree. Allergic people react to minute traces of their allergens, while nonallergic people have no apparent reaction to enormously greater quantities. In this respect allergy is quite different from an excess sensitivity to sunshine or motion sickness. The second difficulty is more serious. Allergy is not an extreme action of some normally well behaved system with an obvious function. IgE antibody seems to do almost nothing, at least in modern industrial countries, except cause allergy. It would appear that we evolved this special IgE machinery for no better reason than to punish random individuals for eating cranberries or wearing wool or inhal
ing during August.
Despite these problems, this explanation of allergies as a result of excess sensitivity is widely employed. For instance, a 1993 New York Times article on asthma describes it as an excessive immunological reaction, one to be solved by finding a drug that can “interfere with the asthmatic process” by “keeping the lungs from responding to allergens in the first place.” Nowhere is the possibility considered that the lungs (or their IgE-carrying cells) may know something that we don’t. A widely used textbook of immunology describes allergy in a chapter entitled “Hypersensitivity” and also makes no effort to explain why the IgE cells exist at all.
THE MYSTERY OF THE IGE SYSTEM
On finding a complicated feature characteristic of a species or larger group, one of the first things biologists want to know is what it does. They assume that if it did not do something important it would not have been produced and maintained in evolution. A short digression offers a vivid illustration. The snouts of sharks contain a cluster of flask-shaped organs (the ampullae of Lorenzini, named for the Renaissance anatomist who first described them). These complicated structures have a rich nerve supply. For three centuries people guessed that the ampullae of Lorenzini regulated buoyancy or amplified sounds, but no serious biologist suggested that they were “just there.” The question stayed on the table until some adequate experiments finally showed that the ampullae of Lorenzini detect minute electrical stimuli, thus allowing sharks to detect muscle activity in potential prey hidden in total darkness or buried in the sand. This discovery was made only because some biologists, habitués of the adaptationist program, assumed that the ampullae of Lorenzini must be an adaptation.
Why We Get Sick Page 19
