How hunter-gatherers could really ruin your sex life

Though recently honoured by the title "Darwinian medicine", the notion that many illnesses result from the interplay between our hostile environment and a genetic make-up that is still adapted to the life of our hunter-gatherer forebears is not new. In a neglected book, Inborn Factors in Disease , published in 1931, Archibald Garrod developed the theme that individual variability in response to pathogens and other environmental hazards reflects biochemical diversity that must be related to evolutionary adaptation.

Garrod's idea was revisited in the 1960s when, following the observation that there was a dramatic increase in the prevalence of diabetes in contemporary human populations, the United States geneticist James Neel suggested that this might be due to dietary plenty imposed on what he called a thrifty genotype, a biochemical constitution that had been selected to take advantage of sporadic food availability. This concept was later extended to explain what became known as the New World syndrome: the extraordinarily high incidence of diabetes and obesity in certain Amerindian populations. It was suggested that it would have been advantageous for the early settlers of North America, as they crossed the Bering Strait and made their way through the ice-bound American continent, to have a fast storage, or thrifty, metabolic genotype; their present ailments may result from exposure to excessive diets.

Although much of the thinking in this field is speculative, it does have the advantage of offering a conceptual framework on which to investigate some of our most intractable diseases. There has been a rapid change in our environments over a very short period, and, at least in our most advanced countries, the opportunities for natural selection to act are limited. Added to this, the rapidly increasing longevity of our populations, with their increasing inability to protect their DNA from noxious agents that emanate from the by-products of their body's chemistry and the environment, may render them prone to diseases that result from damage to their genes.

Amalgamating what is known about environmental factors that may be involved in causing cancer with an up-to-the-minute account of developments in our understanding of the disease at the cellular and molecular level, Mel Greaves places the second most common cause of death in western society fair and square into the category of Darwinian medicine. The fact that he has been able to attempt a synthesis of this type, and to tell a reasonably convincing story, is a tribute to the extraordinary progress that has been made over recent years towards an understanding of what goes wrong in a cancer cell.

Greaves discusses the evolutionary model of cancer at different levels. It is well established that many cancers result from mutations of cellular oncogenes - that is, genes that control the housekeeping activities of cells with respect to how they divide, mature and interact with their fellows. A cancer develops when a particular clone of cells with several damaged oncogenes attains a selective advantage over its normal fellows. This allows it to divide in an uncontrolled way and ultimately to leave its normal habitat and set up house elsewhere in the body.

The reason this does not happen all the time is that we have evolved a series of fail-safe mechanisms; Darwinian medicine would have it that many cancers are the result of the exposure of these safety devices to new environmental insults.

The plot thickens, and incidentally becomes considerably more depressing, when Greaves turns his attention to the cancers of old age, which, in evolutionary terms, means any time after we have finished reproducing our kind. Prostate cancer is a good example. The human male can boast a bigger prostate than any other mammal, including the bull, presumably as an evolutionary adaptation towards more effective reproduction. Appar-ently, however, there is a potential price to be paid for our impressive prostates. Unlike other mammals, some of us continue to indulge in sexual activity as we get older, and hence bombard our prostates with hormones that may evoke the proliferative response that may make malignant change more likely.

Greaves even hints that the blame for President Mitterrand's prostate cancer should be laid fair and square on his young mistress. It should be pointed out, however, that the epidemiological evidence that relates sexual activity to prostate cancer is extremely thin. We can only hope that things will remain this way. Modern epidemiology has already removed the joys of smoking and drinking to modest excess, and most of the pleasures of the table. If it is soon to advocate sexual abstinence after the age of 40, with nothing more titillating to occupy the mind than The THES , an early demise will become an increasingly attractive option.

Like many of those who look to an evolutionary explanation for the human condition, particularly evolutionary psychologists, Greaves occasionally extends his arguments beyond the bounds of evidence, or even credulity. And in parts, his forced user-friendly language to guide the general reader is irritating. But these are small carps and do not detract from the overall excellence of this crystal-clear account of the biology of cancer. Unlike the writers of many popular books on cancer, he does not shy away from its complexity and, by trying to place it in an evolutionary context, he goes a long way to answer the question asked by every cancer sufferer: why me?

Sir David Weatherall is emeritus regius professor of medicine, University of Oxford.