It is surprising that although Charles Darwin published his theory of evolution over a hundred years ago, and in so doing precipitated what many people think is the greatest intellectual revolution of all time, the application of these ideas to medicine in any systematic way began a mere ten years or so ago. Even today, if you ask most doctors what they understand by Darwinian medicine they are likely to respond in a fairly puzzled way. Why is this so? Nesse and Williams suggest that one reason "is surely the pervasive neglect of [evolutionary biology] at all educational levels. Religious and other sorts of opposition have minimized the impact on general education of Darwin's contributions to our understanding of ourselves and the world we live in. There has also been a peculiar neglect of evolution in the training of physicians and medical researchers … Still another reason is that many of the evolutionary ideas of greatest bearing on medicine have only been formulated in recent years … Their recognition and the appreciation of their importance have come only in the past few years, far behind the development and application of many really complex and subtle branches of physical science and molecular biology. Exactly why the application of evolutionary biology to medicine and other aspects of human life has advanced so slowly after its magnificent inception in 1859 is a question that ought to be getting major attention from historians of science."
This book is written in a popular style, almost like a Reader's Digest article; in fact, I did wonder if it has received the attention of a ghost writer or at least has been heavily edited to make it accessible to a non-professional audience. That said, however, it does contain plenty of facts and arguments. It is also fairly tough-minded, as one might expect, for Williams is the biologist and evolutionary theorist who was the chief proponent of the 'selfish gene' theory later famously championed by Richard Dawkins.
The book starts by explaining the essentials of Darwinian natural selection and then embarks on showing how these ideas illuminate the reasons why we become ill. Probably the most important notion here is that of the perpetual war that goes on between host and parasites. Like other organisms, we are constantly under attack from bacteria and viruses; we have evolved complex defence systems against them and they, in their turn, have evolved ever more sophisticated weapons of offence. A continual evolutionary arms race is under way. It is our genes which enable us to respond in this way, but there is an inevitable cost; the defence mechanisms can cause disease as well as prevent it. Autoimmunity, in which the body's defence mechanisms are turned against its own tissues, is an example of this. Another may be allergy, for it is thought that allergy may be part of the defence mechanism against cancer. (Cancer cells can be thought of as parasites that are produced internally by the body instead of invading from outside.) Psychological disorders may also have an evolutionary origin: anxiety, for example, may be an exaggeration of the fear mechanism that protects us from dangerous situations. Aging occurs because natural selection has no need for us once we have passed reproductive age, so mechanisms for very prolonged survival have not evolved.
This way of looking at disease has important philosophical implications. Disease doesn't result from random causes or malevolent forces, it arises ultimately from past natural selection. As Nesse and Williams say, this makes disease at once more and less meaningful. Science does not offer us a metaphysical explanation for the existence of disease—if we want that we must look elsewhere—but it does offer us an intellectually satisfying account. And the evolutionary approach to medicine suggests numerous new directions for research.
This is an important book that deserves to be widely read, by both doctors and patients. I am sure that the authors are right in holding that disease cannot be understood in depth unless it is seen in a Darwinian perspective.