Unlike conventional medicine, Darwinian medicine asks not how the human body works - or fails to do so - but why it functions or malfunctions in a particular way. The answers it provides can be startling. "While modern medicine has been remarkably successful in explaining the mechanisms that cause disease," Nesse points out, "when we ask why sickness and suffering should exist at all, we are referred out of the clinic to the philosophy department. By asking evolutionary, rather than proximate, questions we may be able to solve many of medicine's mysteries."
A good example of the Darwinian approach is its understanding of fever. Most people regard fever as a nasty by-product of certain illnesses. Darwinian researchers, on the other hand, view it as an adaptive response triggered by bacterial toxins. The body raises its own temperature as a way of fighting off invading micro-organisms. Reduce the fever - by taking aspirin, for instance - and the disease may last longer, as Timothy Doran of Baltimore's John Hopkins University has demonstrated in the case of chickenpox.
Diarrhoea is another good example. Sometimes, diarrhoea occurs as a result of pathogen manipulating the body for its own purposes. Cholera bacteria, for example, induce diarrhoea by producing toxins that make the cells of the intestine leaky. The resultant diarrhoea both flushes competing beneficial bacteria from the gut and gives the cholera bacteria a ride into the outside world, so that they can find another hapless victim.
In other cases, however, many Darwinian doctors believe diarrhoea is not so much a symptom of illness as a defence mechanism mounted by the body. Infection by shigella bacteria causes the muscles surrounding the gut to contract more frequently, apparently in an attempt to flush out the bacteria as quickly as possible. Studies done more than a decade ago showed that the use of certain anti-diarrhoeal drugs may actually prolong infection by blocking the body's own defence.
Darwinian medicine does not go so far as to urge us to throw away the medicine chest and allow nature to its course. But a better understanding of the disease process, its advocates believe, would enable the doctor to know when and if to intervene. "The distinction between defect and defence is not merely of academic interest," observes Randolph Nesse. "For someone who is sick it can be crucial. Correcting a defect is almost always a good thing. But eliminating a defence by blocking it can be catastrophic."
Nesse gives the example of a pneumonia patient whose skin is blue and who has a cough. The bluish skin is a defect, the result of deoxygenated blood. The cough, however, is a defence mechanism, designed to expel foreign material from the respiratory tract. "Turning a pneumonia patient's skin a warm pink can be beneficial," says Nesse. "Blocking his cough could kill him."
Nesse became drawn to an evolutionary approach to medicine through his interest in senescence, or ageing. Senescence has long posed a problem for biology: why should genes which cause the organism to die be selected? A possible answer was outlined back in the fifties by the evolutionary biologist George Williams. Williams suggested that genes that cause ageing may confer benefits earlier in an animal's life. Imagine that there is a gene that changes calcium metabolism so that bone heals faster, but also causes slow steady calcium deposition in the arteries. Such a gene might well be selected, because many individuals will benefit from its advantages in youth while few will survive to experience the disadvantages of arterial disease in old age.
When he came across Williams' original work, Nesse realised that Darwinian theory could illuminate the origins, not simply of senescence, but of many illnesses, diseases and disorders. Nesse and Williams teamed up to investigate this possibility and their collaboration led to 1991 paper that signalled the real birth of Darwinian medicine as a paradigm. In this paper, Nesse and Williams argued that the evolutionary reasons for our vulnerability to diseases fell neatly into just six categories. We get infections because pathogens, having a far shorter life cycle, evolve faster than we do. This allows them to overcome our immunological defences faster than we can evolve new ones.
Atherosclerosis, obesity, cancer, and the bulk of so-called "diseases of civilisation" arise from living in an environment different from the one in which our bodies evolved. "Natural selection," says Nesse, "has not had time to revise our bodies to cope with fatty acids, automobiles, drugs, artificial lights and central heating. From this mismatch between design and environment arises perhaps most preventable modern disease, such as coronaries." Similarly, Boyd Eaton has suggested that breast cancer may be the product of the differences in the reproductive life histories of modern and Stone Age women - in particular the fact that women today reach puberty earlier, the menopause later and have fewer pregnancies.
Thirdly, genetic ailments occur largely because genes that cause disease have benefits that outweigh their costs. Sickle-cell anaemia has survived because the gene for the disease also conferred greater immunity against malaria. The gene for cystic fibrosis is believed to make individuals more resistant to cholera. Genes predisposing individuals to Alzheimer's disease may similarly have a benefit earlier in life.
The fourth category of vulnerability arises from what Nesse and Williams call "design compromises". Humans have not been designed from scratch, but cobbled together during evolution through the process of natural selection. Because every evolutionary adaptation is a compromise between different needs, evolutionary gains often have a negative consequence. Back pain is the price of bipedal posture. The price of effective tissue repair is cancer. A powerful immune system can lead to auto-immune disorders, when the body's own tissues are attacked.
Fifthly, there are many problems, like the blind spot in our visual fields or our tendency to choke, that arise simply because natural selection can never go back and start afresh to correct a fundamentally faulty design. In an ideal world the tubes that convey food to the stomach and air to the lungs would be entirely separate. But thanks to an accident of evolutionary history, the trachea and the oesophagus cross each other, leading to the possibility that food may enter the lung, rather than the stomach, and hence choke us.
Finally, argued Nesse and Williams, many of our defence systems appears as diseases or disorders because they are unpleasant - high temperature, diarrhoea, coughing, panic attacks. By understanding these six categories, they claimed, we can establish an evolutionary history for all medical problems, and have a better understanding of how to treat them. Moreover, such evolutionary logic can be applied not just to physical ailments but to emotional and psychiatric problems too. "Our emotions," says Nesse, "are adaptations shaped by natural selection, just as the physical parts of our bodies are. Unpleasant emotions can be thought of as defences akin to pain or vomiting. Just as the capacity for physical pain has evolved to protect us from immediate and future tissue damage, the capacity for anxiety has evolved to protect us against future dangers and other kinds of threats. Just as the capacity for experiencing fatigue has evolved to protect us from over-exertion, the capacity for sadness may have evolved to prevent additional losses."
An evolutionary perspective, Nesse believes, puts paid to the idea that a normal life should be free from emotional pain. "Emotional pain is not only unavoidable, it is normal and can be useful." But just as the immune system can go awry, so can our emotions. "We need to know when our emotions are useful, and when they are not - when they are acting as defences, and when they are symptoms without any benefits. Once we can distinguish, we can know when to block inappropriate responses with psychotropic drugs, and when it would be dangerous to do so."
When Nesse and Williams' seminal paper came out in 1991, only a handful of medical researchers called themselves Darwinian. Today, Darwinism has become an accepted paradigm among researchers and practitioners in a range of disciplines, helped, no doubt, by the increasing fashion for evolutionary explanations for the human condition. "Buy two copies and give one to your doctor," advised Richard Dawkins on the jacket cover of Nesse and Williams' best-seller, Evolution and Healing.
Others, however, dismiss Darwinian arguments as mere speculation. "Darwinian medicine provides us with interesting, attractive stories - but no data to back it up," argues Oxford biologist Mark Pagel. "I think it's a useful framework within which to consider human diseases, and it would be wonderful if natural selection had evolved us in this fashion. But no reason to suppose that it has. It simply has not been tested."
Michael Fitzpatrick, an author and researcher as well as a GP, goes further still. He believes evolutionary explanations are largely of academic or historical interest, while proximate explanations are much more useful in the clinic. "I am struck by the banality of the examples of clinical applications for conventional approaches to the treatment of gout, sore throats and coronary heart disease. Much labour to produce such a mouse!"
Other critics, such as nutritionist Judith Brown, are dismayed at the tendency of Darwinian medicine to treat every function as an adaptive trait. Brown, a researcher at the University of Minnesota's School of Public Health, believes that evolutionary explanations are overstated, with their presumption that everything in life has a biological purpose. "A number of things are terrible and uncomfortable," she points out, "but that doesn't mean they are good for you."
Brown is particularly critical of Darwinian explanations of morning sickness in pregnant women. Popularised by the biologist Margie Profet in her book Protecting Your Baby To Be, the theory claims that the nausea or food aversions which many women experience early in pregnancy are adaptations designed to protect embryos. Profet argues that some toxins in plants (for instance, allyl isothiocyanate, a carcinogen found in cabbage, cauliflower and Brussels sprouts) evolved to ward off herbivores, and that some of these compounds could, even in tiny amounts, cause embryo defects. Morning sickness, she believes, is an evolved response that leads women to avoid foods that could harm their unborn child. Profet has advised pregnant women to avoid herbs and vegetables, especially bitter-tasting ones.
Profet agrees that her theory is not backed up by evidence, but is simply an "educated guess". Nevertheless, her claims have won the support of many Darwinian physicians. "Perhaps anything that suppresses morning sickness may cause birth defects indirectly, by encouraging harmful dietary choices," Nesse and Williams write in Evolution and Healing. Brown believes not only that Profet's hypothesis is wrong, but that her advice is downright unwise. "She's wastefully scared a lot of people about these vegetables," says Judith Brown. At the doses found in most vegetables "it's not even clear these are toxins at all."
In a study of diet and pregnancy, Brown and her colleagues have noted that high maternal consumption of fruits and vegetables is linked to higher birth-weights in babies, generally considered a good sign. Brown believes that morning sickness may have nothing to do with a need to avoid certain foods but may be just a side effect of hormone changes. Mark Pagel adds that there is no reason to suppose that nausea is an evolved response: "Given our great capacity to learn, it could well be cultural response."
Pagel is also sceptical of claims that a mismatch between our genetic heritage and the environment in which we live is responsible for the majority of today's disorders. "Just because we evolved in the Stone Age world," he observes, "doesn't mean that our behaviours are conditioned by that period."
Michael Fitzpatrick points out a more fundamental objection. "The idea that we live in an 'abnormal' environment suggests that there must be a 'normal' environment for human beings. This is simply not so. Part of what makes us human is our ability to conquer virtually any environment. The Stone Age was no more normal for us than contemporary society. Perhaps Stone Age man did not suffer from cancer or a sweet tooth, but he suffered from many more ailments than we do today."
Nesse accepts that much of Darwinian medicine remains speculation. But, he argues, this is only because the failure of medicine to adopt Darwinism has meant little research into the evolutionary basis of disease. "I'd love to see a dozen proper studies of whether it is wise to bring down fever when someone has influenza," he says. "But it's never been done, and it's astounding that it's never been done. Our speculation will be worthwhile if it forces researchers to address such problems." !Reuse content