A brief overview of: Why We Get Sick: The New Science of Darwinian Medicine by Randolph Nesse and George Williams, Vintage Books, 1996. Published in the United Kingdom as Evolution and Healing: The New Science of Darwinian Medicine, Orion Press, 1996.
Darwinian medicine is the enterprise of trying to find evolutionary explanations for vulnerabilities to disease. Every trait needs an evolutionary as well as a proximate explanation. Disease, not being a product of selection, would seem to be excluded. This is one reason why doctors have not realized that evolution might be useful. Another reason is that medical research looks for differences between individuals in order to explain why one person gets sick while another stays healthy. But Darwinian Medicine does not seek evolutionary explanations for disease itself, and does not usually try to understand why one individual becomes ill when another does not. Instead, it tries to understand why all humans are vulnerable to each disease. It asks how it is possible that natural selection can shape the eye or heart or brain but cannot eliminate our vulnerabilities to nearsightedness, atherosclerosis, depression, or cancer. Darwinian Medicine applies the advances that have revolutionized evolutionary biology to the problems of medicine and tries to provide, for each disease, an explanation for why the body isn't better. These evolutionary explanations for disease fit nicely into just a few categories: defenses, infection, novel environments, genes, design compromises, and evolutionary legacies.
Defenses are often confused with diseases. Knowing the difference is crucial, because interfering with a defense is often unwise. Pain is a defense against tissue damage; people who lack this defense usually die by age thirty. Fever is a defense that protects against infection. The low iron levels associated with infection are the body's way of keeping iron away from invading bacteria. Nausea and vomiting and diarrhea are useful ways to rid the body of infection and toxins. The nausea that accompanies pregnancy discourages the mother from eating toxic substances that may harm her baby. Even anxiety and sadness can be useful. As for the runny nose that accompanies colds, we don't yet know if it benefits us or viruses, but we certainly need to know in order to decide if nose sprays will help or harm us. Much of clinical medicine relieves people's discomfort by blocking defenses like fever, pain, nausea and diarrhea. How can this be safe? Just as smoke detectors are designed to give many annoying but inexpensive false alarms so that they are sure to warn about any actual fire, the mechanisms that regulate the body's defenses have evolved to express defenses whenever they are possibly useful, thus causing much unnecessary suffering.
Infections are neither a divine punishment nor an arbitrary failing, but merely a contest between our bodies and smaller organisms that want to eat us. Because these viruses and bacteria reproduce so rapidly, they evolve faster than we can, so we cannot escape them. We can and do, however, evolve weapons of ever more subtle destruction. They, in turn, evolve ever more sophisticated ways to escape our defenses. Is evolution moving towards some happy accommodation? Not at all. This is war to the death, except, that is, when death is not in the pathogen's interests.
Genes that cause disease usually turn out not to be simple mistakes. Many offer a benefit, like the sickle cell gene that protects against malaria, or a tendency to gout that may delay aging. The gene that causes cystic fibrosis may protect against death from dehydration. Others, like most genes that cause nearsightedness and heart attacks, are harmless quirks for people who live in the natural environment; they cause problems only when they interact with novel aspects of the modern world, like learning to read or a high fat diet. Some "outlaw" genes even manage, by intracellular warfare with other genes, to get themselves transmitted even though they harm the carrier. Even the genes that cause aging are not just accidents; many of them seem to have been selected because they offer benefits early in life. Before we use our new technologies to eliminate genes that cause diseases, we must consider the possibility that they also offer benefits. Environmental factors that cause disease are mostly novel, new in the past 10,000 years, that is. Intermittent exposure to sunlight is novel and results in cancer. Suntan creams may, paradoxically, cause even more cancer. We crave fat salt and sugar because they were in short supply in the paleolithic. Now these appetites prove more powerful than our willpower and cause epidemics of obesity and atherosclerosis. We are well protected against plant toxins, but cannot reliably detoxify novel substances. The differences between the setting we evolved in and the setting we live in is vast and often dangerous.
Design compromises account for much disease. Just as there are costs associated with many genes that offer an overall benefit, there are costs associated with every major structural change preserved by natural selection.
Walking upright gives us the ability to carry food and babies, but it predisposes us to back problems. Many of the body's apparent design flaws aren't simply mistakes, they are just compromises. To better understand disease, we need to understand the hidden benefits of apparent mistakes in design.
Finally, evolution is an incremental process without the possibility of fresh starts. Our food passes through a tube in front of the windpipe, and must cross it to get to the stomach, thus exposing us to the danger of choking. It would be sensible to relocate the nostrils to somewhere on the neck, but that will never happen.
In summary, Darwinian Medicine proposes that descriptions of disease in current medical textbooks omit a crucial section - an evolutionary explanation for why humans are vulnerable to this disease. Finding these explanations will have immediate practical benefits for medical practice. General physicians still don't think of fever as useful and they still give iron supplements to patients with chronic infections. Infectious disease specialists still think that pathogens evolve to benign co-existence. Psychiatrists still act as if all anxiety, sadness, and jealousy is abnormal and they don't yet look for the selective advantages of genes that predispose to schizophrenia and bipolar disorder. Rheumatologists don't know that the high uric acid levels of gout may have been selected to slow aging and they prescribe anti-inflammatory agents that may hasten hip degeneration. Obstetricians have not considered the possibility that nausea of pregnancy may be a defense against toxins. The foundations for a new field of inquiry are now being laid by many people including Ewald, Profet, Rose, Konner, Eaton, Buss, Cosmides, Tooby, Diamond, Trivers, Durham, Austad, Ames, Daly, Wilson, and many others. It will not be easy to explain all this to doctors. They will, like the reporters, imagine that we are saying that disease is useful, or that we should let nature take her course or any number of other misunderstandings that will no doubt rain on our heads for the next few years. George Williams and I will be criticized for writing for a general audience, for daring to speculate about the causes of disease before the research is done. But our goal is not to prove any specific hypothesis, but to highlight a set of questions that are, we think, genuinely new. As people take them seriously, the resulting research should prove profoundly useful as well as most interesting.
Randolph M. Nesse, M.D. University of Michigan, Ann Arbor, MI 48109-0704 (313) 936-8269 Fax. 936-9761 nesse@umich.edu
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