Why We Get Sick (22 page)

We have been discussing defects in the basic plan of the human body. These should not be confused with mere inadequacies of execution and random departures from optimal values. As a general rule for any readily measured physical feature, it pays to be in the middle of the pack, as we illustrated previously with the birds with longer- or shorter-than-average wings, which were especially likely to be killed in a storm. Unusually tall or short people tend not to live as long or as healthily as those of average height. Babies of average birth weight are usually better off than those who are much heavier or lighter. Everyone knows that high or low blood pressure is not as good as normal blood pressure. A high level of adaptive performance usually requires that many quantitative characteristics closely approach optimal values. While no individual is perfect, the various parameters sometimes combine to yield remarkable excellence. Yet even in near perfection there is substantial variation—as is well known to those basketball stars who played against Michael Jordan.

Many design features, while not maladaptive, are functionally arbitrary and explicable only as historical legacies. In mammals, the right side of the heart circulates the blood to the lungs, the left side to the rest of the body. In birds it is the other way around, for no better reason than that birds and mammals came from different reptilian ancestors that took arbitrarily different routes to cardiac specialization. Either way works equally well. Some arbitrary features can be advantageously exploited. Many people who are alive today would be dead except for the happenstance of everyone having two kidneys. When one fails or is donated, the other is able to do double service. By the same logic, many people die of having only one heart. The reason we have two kidneys and one heart is simply that, right from their origins, all vertebrates had two kidneys and one heart. This is pure historical legacy and has nothing to do with the advantage of having two of one organ or the disadvantage of having only one of another.

We have belabored what is wrong or arbitrary with the human body because the design flaws can cause many medical problems, but we hope that our readers will also realize that much about it is just right. Our oversize brains may be vulnerable to injury and may impede childbirth, but they make us the unchallenged leaders of the animal kingdom in cognitive capability and in all the social and technological advances that this makes possible. No other species in the history of our planet has ever controlled its environment to the extent that we have since the invention of agriculture. Similarly, our longevity is impressive in relation to that of any other mammal, except a few, such as elephants, that are far larger than we are. We can live about half again as long as any other primate.

Moreover, many of our other adaptations are equal or superior to those in other mammals. Our immune system is superb. Also, despite conspicuous design flaws and individual imperfections, our eyes and related brain structures incorporate layer upon layer of information-processing marvels that extract the maximum amount of usefulness from visual stimuli. If hawks, for example, have visual acuity that is in some ways superior to ours, this one kind of superiority must be purchased with some kind of trade-off. Animals that can see better than we can in the dark cannot see as well in the light. Normal human vision approaches a theoretical maximum of sensitivity and discrimination over a wide range of conditions. We are only
beginning to understand how it is that a face, seen from one angle at a certain distance, may later, from another angle and distance, be instantly recognized. No current computer can approach such feats. Our hearing is so sensitive to some frequencies that if it were more sensitive we would not hear as well. Informative sounds would be lost in the noise of random air molecules colliding with our eardrums.

W
e have been discussing mainly attributes that humans share with other vertebrates, other mammals, or other primates. Our discussions of our problems with upright stature also apply to extinct members of our genus,
Homo
. We now turn to more explicitly human legacies, with an emphasis on the evolutionary adjustments made in the period from about one hundred thousand to about ten thousand years ago. While natural selection has been changing us in many small ways in the last ten thousand years, this is but a moment on the scale of evolutionary time. Our ancestors of ten thousand or perhaps even fifty thousand years ago looked and acted fully human. If we could magically transport babies from that time and rear them in modern families, we could expect them to grow up into perfectly modern lawyers or farmers or athletes or cocaine addicts.

The point of the rest of this chapter, and the following one, is that we are specifically adapted to Stone Age conditions. These conditions ended a few thousand years ago, but evolution has not had time since then to adapt us to a world of dense populations, modern socioeconomic conditions, low levels of physical activity, and the many other novel aspects of modern environments. We are not referring merely to the world of offices, classrooms, and fast-food restaurants. Life on any primitive farm or in any third-world village may also be thoroughly abnormal for people whose bodies were designed for the world of the Stone Age hunter-gatherer.

Even more specifically, we seem to be adapted to the ecological and socioeconomic conditions experienced by tribal societies living in the semiarid habitat characteristic of sub-Saharan Africa. This is
most likely where our species originated and lived for tens of thousands of years and where we spent perhaps 90 percent of our history after becoming fully human and recognizable as the species we are today. Prior to that was a far longer period of evolution in Africa in which our ancestors’ skeletal features lead scientists to give them other names, such as
Homo erectus
and
Homo habilis
. Yet even these more remote ancestors walked erect and used their hands for making and using tools. We can only guess at many aspects of their biology. Speech capabilities and social organizations are not apparent in stone artifacts and fossil remains, but there is no reason to doubt that their ways of life were rather similar to those of more recent hunter-gatherers.

Technological advances later allowed our ancestors to invade other habitats and regions, such as deserts, jungles, and forests. Beginning about one hundred thousand years ago, our ancestors began to disperse from Africa to parts of Eurasia, including seasonally frigid regions made habitable by advances in clothing, habitation, and food acquisition and storage. Yet despite the geographic and climatic diversity, people still lived in small tribal groups with hunter-gatherer economies. Grainfield agriculture, with its revolutionary alteration of human diet and socioeconomic systems, was practiced first in southwest Asia about eight thousand years ago and shortly thereafter in Egypt, India, and China. It took another thousand years or more to spread to central and western Europe and tropical Africa and to begin independently in Latin America. Most of our ancestors of a few thousand years ago still lived in bands of hunter-gatherers. We are, in the words of some distinguished American anthropologists, “Stone Agers in the fast lane.”

I
magine what it must have been like in that idyllic era. You were born into a nomadic band of forty to a hundred people. Whatever its size, it was a stable social group. You grew up in the care of various close relatives. Even if your local band consisted of a hundred or more people, many of them were distant cousins. You knew them all and knew their genetic and marital connections to
yourself. Some you loved deeply and they loved you in return. If there were those you did not love, at least you knew what to expect from them, and you knew what everyone expected of you. If you occasionally saw strangers, it was probably at a trading site, and you knew what to expect of them too. In a sparsely peopled world the necessities of life—plant and animal foods uncontaminated by pesticides—were there for the taking. You breathed the pure air and drank the pure water of a preindustrial Eden.

Having asked you to imagine an idyllic past, we now urge that you be more realistic. Like other Golden Age legends, such as the age of chivalry or that delightful antebellum world into which Scarlett O’Hara was born, it is a fabricated myth. Enjoy it in fantasy or fiction, but do not let it mislead serious thought on medicine or human evolution. The unpleasant fact is that our hunter-gatherer ancestors lived with enormous difficulty and hardship. Simple arithmetic on the rates of death and reproduction makes this conclusion inescapable. Death always balanced reproduction, even though people reproduced at something approaching the maximum feasible rate.

In most primitive social systems, women start bearing children as soon as they are able to do so, which, because of nutritional limitations, is often delayed until about age nineteen. Pregnancy and childbirth are followed by two or three years of lactation, which inhibits ovulation. Then the mother is soon pregnant again, whether this is medically advisable or not. In the unlikely event that she remains fully fertile and survives to menopause, she will probably produce about five babies. Having more children would require shortened lactation periods, and this is unlikely given the limited foods available for babies in preagricultural societies.

But even if hunter-gatherer women averaged only four children before succumbing to sterility or death, only half their babies could have survived to maturity. Otherwise the human population would have steadily increased, and this obviously did not happen. Even an increase of 1 percent per century would cause a population to become a thousand times as numerous in less than seventy thousand years, but populations remained extremely sparse until the invention of agriculture. The conclusion is thus quantitatively inescapable that deaths almost precisely kept up with births for nearly all of human history. The extraordinarily low death rates of the last few centuries, and especially in the last few decades in Western
societies, show that we live in times of unprecedented safety and prosperity. It is no doubt difficult for most readers of this book to appreciate the harshness and insecurity of human life under natural conditions.

Mortality rates in the Stone Age, like those of today, were highest in infancy and declined throughout childhood. Many early deaths in some groups were from infanticide, motivated by parents’ economic hardship or imposed by patriarchs. While fictional accounts of Stone Age conditions probably exaggerate the ravages of predation and other wild-animal attack, lions, hyenas, and venomous snakes were ever-present hazards and took a steady toll, with children especially vulnerable. Death rates from poisoning and accidents were far higher than they are now.

The infectious diseases, which were probably the most important source of mortality for all age groups, were not the same bacterial and viral diseases that afflict us today. Most of today’s infections depend on rates of personal contact only possible in abnormally dense populations. Back then, vector-borne protozoa and worms were common causes of prolonged sickness and ultimate death. Many of these diseases are not merely lethal but most unpleasantly so. Some readers will know how unpleasant malaria can be, from personal experience or from knowing someone who has had the disease. It is a lark compared to other protozoan diseases such as kalaazar, which slowly destroys the liver and other viscera; parasites such as lungworms, which cause death by suffocation; hookworms, which are seldom fatal but can make children grow into physically and mentally defective adults; and filaria, which among other things cause elephantiasis. The name comes from the swelling of the limbs and scrotum to elephantine proportions because the parasites block the lymphatic vessels.

Food was often abundant for hunter-gatherers, but memories of bounteous fruit harvests or an occasional big kill must have been a poor solace during the regularly recurring famines. Climatic variations induce fluctuation in resources. Even in the most stable climates, food abundance varies because of plant and animal diseases. Prior to the invention of reliable preservation techniques, temporarily abundant food could not be saved for leaner times. Even foods preserved by drying or smoking could be attacked by pests that could frustrate the most careful planning for future emergencies.

Shortages of vital necessities were not only directly stressful, they also encouraged strife. Imagine that people from a hill tribe were suffering from a protein shortage, while people in the valley were feasting on the abundant fish from their lake. The people from the hills would no doubt insist that their leaders take them to that lake, no matter how loudly the valley people asserted their exclusive fishing rights. If catching the fish means killing the fishermen and appropriating their fishing gear, that is what the hill people might decide to do. Even in the absence of economic necessity, human nature often finds excuses for armed robbery and attendant taking of life. Fortunately for early tribal societies, they lacked the technology of transport and communication that permitted banditry on the scale practiced by Genghis Khan or Alexander of Macedon.

Human nature has, of course, its nobler aspects. There are such things as love and charity and honesty. Unfortunately, the evolutionary origins of such qualities are rooted in their utility in parochial tribal settings. Natural selection clearly favors being kind to close relatives because of their shared genes. It also favors being known to keep one’s promises and not cheating members of one’s local group or habitual trading partners in other groups. There was, however, never any individual advantage from altruism beyond these local associations. Global human rights is a new idea never favored by evolution during the Stone Age. When Plato urged that one ought to be considerate of all Greeks, not merely all Athenians, it was a controversial idea. Today, humanistic sentiments still face formidable opposition from parochialism and bigotry. In fact, these destructive tendencies are aggravated by what we just now called the “nobler” aspects of human nature. As Michigan biologist Richard Alexander so neatly put it, today’s central ethical problem is “within-group amity serving between-group enmity.”