Why We Get Sick (39 page)

W
hy has it taken more than a hundred years to apply Darwin’s theory systematically to disease? Historicans of science will eventually address this question, but from this close perspective several explanations seem likely: the supposed difficulty in formulating and testing evolutionary hypotheses about disease, the recency of some advances in evolutionary biology, and some peculiarities of the field of medicine. Biologists have long tried to figure out the evolutionary origins and functions for organismic characteristics, but it has taken a surprisingly long time to realize that this enterprise is fundamentally different from trying to figure out the structure of organisms and how they work. Harvard biologist Ernst Mayr, in
The Growth of Biological Thought
, traces the parallel development of the two biologies. Medicine, while at the forefront of proximate biology, has been curiously late in addressing evolutionary questions. This is, no doubt, in part simply because the questions and goals are so different. It takes a wrenching shift to stop asking why an individual has a particular disease and to ask instead what characteristics of a species make all of its members susceptible to that disease. It has seemed a bit odd until now even to ask how something maladaptive like disease might have been shaped by natural selection. Furthermore, medicine is a practical enterprise, and it hasn’t been immediately obvious how evolutionary explanations might help us prevent or treat disease. We hope this book convinces many people that seeking evolutionary explanations for disease is both possible and of substantial practical value.

If we are to assign blame for the tardiness of medicine in making use of relevant ideas in evolutionary biology, it rests as much with evolutionary biologists as with the medical profession. It took evolutionists an inexcusably long time to formulate the relevant ideas.
Given the powerful insights of Darwin, Wallace, and a few others in the middle of the nineteenth century, and the Mendelian revolution in genetics in the early years of the twentieth, why was it not until Fisher’s book of 1930 that we had the first fruitful idea about why the number of boys and girls born is nearly equal? Why was it not until Medawar’s midcentury work that we had any idea why there is such a thing as senescence? Why was it not until Hamilton’s publications in 1964 that there was any realization that kinship would have some relevance to evolution? Why was it not until the 1970s and 1980s that we had useful ideas on how parasites and hosts, or plants and herbivores, influence each other’s evolution? We believe that the answers to these and related questions will be found in a persistent antipathy to evolutionary ideas in general and to adaptation and natural selection in particular (even among some biologists). Meanwhile, we will simply note that medical researchers can hardly be blamed for failing to use the ideas of other sorts of scientists before those scientists developed them.

Medical scientists may also hesitate to consider functional hypotheses because of their indoctrination in the experimental method. Most of them were taught early, firmly—and wrongly—that science progresses only by means of experiment. But many scientific advances begin with a theory, and much testing of hypotheses does not rely on the experimental method. Geology, for instance, cannot replay the history of the earth, but it nonetheless can reach firm conclusions about how basins and ranges got that way. Like evolutionary hypotheses, geological hypotheses are tested by explaining available evidence and by predicting new findings that have not yet been sought in the existing record.

Finally, medicine, like other branches of science, is especially wary of ideas that in any way resemble recently overcome mistakes. Medicine fought for years to exclude vitalism, the idea that organisms were imbued with a mysterious “life force,” so it continues to attack anything that is even vaguely similar. Likewise,
teleology
of a naive and erroneous sort keeps reappearing and must be expelled. Many people recollect from freshman philosophy class that teleology is the mistake of trying to explain something on the basis of its purpose or goal. This admonition is wise if it establishes an awareness that future conditions cannot influence the present. It is unwise if it also implies that present plans for the future cannot affect present processes and, through them, future conditions. Present
plans may include printed recipes for baking cakes or the information in the DNA of bird’s eggs. Functional explanations in biology imply not future influences on the present but a prolonged cycling of reproduction and selection. A bird embryo develops wing rudiments in the egg because earlier individuals that failed to do so left no descendants. Adult birds lay eggs in which embryos develop wing rudiments for the same reason. In this sense, a bird’s wing rudiments are preparation for its future but are caused by its past history. Evolutionary explanations based on a trait’s function do not imply that evolution involves any consciousness, active planning, or goal-directedness. While medicine is wise to be on guard against sliding back into discredited teleological reasoning, this wariness has prevented it from taking full advantage of the solid advances in mainstream evolutionary science. Through its efforts to keep from being dragged back, medicine has, paradoxically, been left behind.

M
edical education is similarly in trouble because of trying to guard against the old mistakes. The origins of its current quandary lie in the solution
to
a previous one. Early in this century, the Carnegie Foundation sponsored an extensive investigation of medical education by Abraham Flexner. In his cross-country travels, he reported a haphazard system of medical apprenticeship in which physicians, good and bad, took on assistants who, one way or another, learned something about medicine. Doctors’ formal study of basic science was sporadic, and even their knowledge of basic anatomy and physiology was inconsistent. The Flexner report, published in 1910, formed the basis of new accreditation standards that required medical schools to provide future physicians with a foundation in basic science.

On this count, medical schools have far exceeded Flexner’s hopes. In fact, one wonders what Flexner would say if he could see today’s medical curricula. Now medical students are not only exposed to basic sciences, they are inundated with the latest advances by teachers who are subspecialist basic science researchers. At curriculum meetings in every medical school there are battles for students’ time
and minds. The microbiologists want more lab time, the anatomists want more too. The pathologists feel they cannot possibly fit their material into a mere forty hours of lecture. The pharmacologists say they will continue flunking 30 percent of the class until they get enough time to cover all the new drugs. The epidemiologists and biochemists and physiologists and psychiatrists and neuroscience experts all want more time, and certainly the students must keep up with the latest advances in genetics. Then they need to learn enough statistics and scientific methodology to be able to read the research literature. And they must somehow learn, before they start their work on the wards, how to talk with patients, how to do a physical exam, how to write up a patient report, how to draw blood, do a culture, a spinal tap, a Pap smear, measure eyeball pressure, examine urine and blood, and, and,… The amounts of knowledge and the lists of tasks are overwhelming, but all must be completed in the first two years of medical school.

How is all this possible? It isn’t. Why set impossible expectations? In part because we naively want our physicians to know everything. Another reason, however, is that no one person is in charge. When a committee decides on the class schedule and every basic science wants more time, the solution is to go on increasing the total amount of class time. Thirty or more hours each week in class is not unusual. After that, the students go home to study their textbooks and notes.

One might think that students’ complaints would lead to reform, but decades of polite complaints changed little. It was technology that finally precipitated some change, technology in the form of the photocopy machine. Instead of going to class, students hire one person to take notes for each lecture, then all of them receive copies. It turns out to be a better survival strategy to stay home and study the notes than to go to class. When only twenty students attend a class for two hundred, professors hit the roof and curriculum reform is born. New attempts are being made, under the strong leadership of some deans, to cut back on the hours, reduce the amount of material, find new ways to transmit it. If these efforts succeed, it will be wonderful indeed.

Such efforts might even make room for Darwinian medicine, except that there are no Departments of Evolutionary Medicine to advocate inclusion of this material and few medical faculty members who know the material and want to teach it. It will take time and further
leadership from medical school deans to make room in the medical curriculum for an introduction to the basic science of evolution and its applications in medicine. When evolution is included, it will give students not only a new perspective on disease but also an integrating framework on which to hang a million otherwise arbitrary facts. Darwinian medicine could bring intellectual coherence to the chaotic enterprise of medical education.

W
hile many clinical implications of an evolutionary view await future research, others can immediately transform the way patients and doctors see disease. Let us listen in as first a pre-Darwinian and then a post-Darwinian physician talk to a patient about gout.

“So, Doctor, it is gout that has my big toe flaming, is it? What causes gout?”

“Gout is caused by crystals of uric acid in the joint fluid. I expect you can imagine only too well how some gritty crystals could make a joint painful.”

“So why do I have it and you don’t?”

“Some people have high levels of uric acid in their systems, probably because of some combination of genes and diet.”

“So why isn’t the body designed better? You would think there would be some system to keep uric acid levels lower.”

“Well, we can’t expect the body to be perfect, now, can we?”

At this point our pre-Darwinian physician gives up on science and dodges the question, implying that such “why” questions need not be taken seriously. Most likely, he or she doesn’t recognize the distinction between proximate and evolutionary explanations, to say nothing of the importance and legitimacy of evolutionary explanations for disease.

The Darwinian physician gives a different answer, one closer to what the patient wanted and was entitled to.

“That’s a good question. It turns out that human uric acid levels are much higher than those of other primates and that uric acid levels
in a species are correlated with its life span. The longer-lived the species, the higher the uric acid levels. It seems that uric acid protects our cells against damage from oxidation, one of the causes of aging. So natural selection probably selected for higher uric acid levels in our ancestors, even though some people end up getting gout, because those higher levels are especially useful in a species that lives as long as we do.”

“So high levels of uric acid prevent aging?”

“Basically, that seems to be right. So far, however, there is no evidence that individuals with high uric acid levels live an especially long time, and in any case you don’t want your toe to stay like that, so we are going to go ahead and get your uric acid levels down to the normal range to get the gout under control.”

“Sounds sensible to me, Doc.”

This is not an isolated example. A Darwinian perspective can already assist in the management of many medical conditions. Take strep throat:

“Well, it’s strep all right, so you will need to take some penicillin for seven days,” says the Darwinian physician.

“That will make me better faster, right?” the patient says hoarsely.

“Probably, and it will also make it less likely that you will develop diseases like rheumatic fever because of your body making immune substances that attack the bacteria.”

“But why doesn’t my body know better than to make substances that will attack my own heart?”

“Well, the streptococcus has evolved along with humans for millions of years, and its trick is to imitate the codes of human cells. So when we make antibodies that attack the strep bacteria, those antibodies are prone to attack our own tissues as well. We are in a contest with the strep organism, but we can’t win because the strep evolves much faster than we do. It has a new generation every hour or so, while we take twenty years. Thank goodness we can still kill it with antibiotics, although this may be a temporary blessing. You will do yourself and the rest of the world a favor by taking your antibiotics even after you feel better, because otherwise you may be giving a lift to those variants that can survive short exposures to antibiotics, and those antibiotic-resistant organisms make life difficult for us all.”