Evolution and health by Stephen Boyden

I n our attempts to view the human situation in meaningful perspective, it is essential that we take account not only of our cultural background, but also of our biological history. For the human species evolved in an environment very different from that to which most of us are exposed today, and it was this evolutionary environment which determined, through natural selection, the innate characteristics of the species. While this fact has important implications for many of the major problems facing human societies today, this paper concentrates for the most part on a single aspect, namely, the biological history of Homo sapiens as i t relates to problems of human health and well-being in the modern world. I t concludes, however, by suggesting that this evolutionary approach to the health requirements of the human organism can lead to insights of relevance not only to public health, but also to the major social problems of our times, including the ecological predicament.

For the purposes of this article, I define "biological determinants of optimal health" as those various conditions which tend to promote or permit optimal physiological, mental and social performance in an animal in its "natural" or evolutionary environment. They may also be defined as those various conditions which tend to promote the kind of functioning of

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mental and physiological processes most likely to ensure the survival and successful reproduction of the individual in the natural environment. Whereas, in the case of human beings, this degree of health is certainly not necessary for survival and reproduction under modern urban conditions, I contend that it is nevertheless a most desirable objective in terms of life enjoyment.

The Darwinian theory of evolution through natural selection assumes that populations of animals are heterogeneous, in the sense that there are inheritable differences between individuals, and that, because of these differences, in any given set of circumstances certain individuals are more likely than others to survive, reproduce and successfully rear young. The individuals which are better suited to the prevailing conditions tend to contribute more to the genetic constitution of subsequent generations than do those which are less well suited, so that eventually the population as a whole tends to become better suited to these conditions. From time to time, changes spontaneously occur in the genetic material (due to recombination, mutation, etc.). I f an individual carrying such new genetic characteristics is, as a result of the change, less well suited to the prevailing conditions and consequently less likely to survive and reproduce, then the new genetic material will, before long, be eliminated from the population. If, on the other hand, the individual with the new genetic characteristics turns out to be better suited than its fellows to prevailing conditions, it will tend to contribute more offspring to the next generation, so that eventually the new characteristics will spread in the population, ultimately replacing the less advantageous characteristics. Thus, over many generations, populations tend to become better and better suited to the environment in which they live.

Phylogenetic maladjustment The important corollary to Darwinian theory that I wish to stress has not been given a name, but I shall refer to it here as the "principle of phylogenetic maladjustment". According to this principle, if the conditions of life of an animal deviate from those which prevailed in the environment in which the species evolved, the likelihood is that the animal will be less well suited to the new conditions than to those to which it has become genetically adapted through natural selection and consequently some signs of maladjustment may be anticipated. Obvious though this principle is, and obvious though it's important, it is seldom referred to in the literature, and consequently its significance seems to have been largely overlooked. Although highly pertinent to the study of health and disease in mankind, I have not seen the principle mentioned in any of the standard textbooks of medicine. I have come across only two clear statements of the principle in the medical literature—one in the introduction to The Saccharine Disease by Cleave and Campbell (1966) and the other in Mans Presumptuous Brain by Simeons (1960).

The term "phylogenetic maladjustment",* then, specifically refers to disorders which represent the reactions of organisms to conditions of life which differ from those to which the species has become genetically adapted in evolution through the processes of natural selection. This principle relates not only to environmental changes of a physicochemical or material nature, such as changes in the quality of food or air, but also to various non-material environmental influences, such as certain social pressures which may affect behaviour. Furthermore, signs of phylogenetic maladjustment may be

* The maladjustment is phylogenetic because i t represents a characteristic response o f the species t o the changed environmental circumstances. /