Evolutionary medicine

The term 'evolutionary medicine' refers to the study of, teaching of, and application of the concepts, principles and perspectives of evolution and evolutionary biology to the understanding, prevention and management of human disease, both mental and physical, and to the general improvement of human well-being. Advocates of evolutionary medicine regard those projects as stemming from an understanding of human biology as an evolved living system embedded in an environment of diverse other evolved living systems. They implicitly or explicitly subscribe to the proposition of the 20th century's pioneer geneticist and evolutionary biologist, the Russian-American, Theodosius Dobzhansky, to wit: "Nothing in biology makes sense except in the light of evolution."

Often the advocates of evolution-informed medicine see the light of evolution shining from the theories of Charles Darwin, in particular that of adaptation due to natural selection, hence the term 'Darwinian medicine' in alternative use. As we approach the bicentennial of Darwin's birth (February 12, 2009), however, we now know that evolutionary forces comprise a wide range of natural processes in addition to selection. The advance of evolutionary medicine will require an appreciation of all evolutionary processes potentially affecting human health.

Scientists from many different disciplines have contributed in diverse ways to the developing discipline of evolutionary medicine: anthropology cardiovascular medicine, endocrinology, evolutionary biology, exercise physiology, family medicine, genetics, geriatrics, gynecology, immunology, physiology, metabolism, nutrition, obstetrics, oncology, pediatrics, psychiatry, psychology, and systems biology.

Does medicine without evolution make sense?

This section's question header echoes the title of a recent (April, 2007) editorial in the open-access journal, PLoS Biology, by senior editor Catriona J. MacCallum. MacCallum laments the fact that evolution does not figure prominently in the medical community and in the curriculum of medical schools. She notes one of the reasons:

As explained at a meeting on evolution and medicine I recently attended in York, United Kingdom (the Society for the Study of Human Biology and the Biosocial Society’s 2006 symposium, “Medicine and Evolution", medicine is primarily focused on problem-solving and proximate causation, and ultimate explanations can seem irrelevant to clinical practice. Crudely put, does a mechanic need to understand the origins, history, and technological advances that have gone into the modern motor vehicle in order to fix it?

Biology Workgroup Editorial Comment: As if the modern motor vehicle, or any other human artifact for that matter, could match the organizational constitution the most complex living system on Earth.

MacCallum continues to highlight the York meeting mentioned in the previous quote:

Participants at the York meeting discussed:

• not only how vulnerability to cancer is an inevitable but unfortunate consequence of imperfect human engineering and natural selection (Mel Greaves, Institute of Cancer Research, UK),
• but how life history theory can potentially explain patterns of pregnancy loss (Virginia Vitzthum, Indiana University),
• how a comparative approach applied to different human cultures and different primates can improve rates of breastfeeding (Helen Ball, University of Durham),
• whether clinical depression has an adaptive origin (Lewis Wolpert, University College London), and
• if suicide attempts are really just evolutionary bargaining chips in intense social disputes (Ed Hagen, Humboldt University).

MacCallum concludes her editorial:

The time has clearly come for medicine to explicitly integrate evolutionary biology into its theoretical and practical underpinnings. The medical students of Charles Darwin’s day did not have the advantage of such a powerful framework to inform their thinking; we shouldn’t deprive today’s budding medical talent of the potential insights to be gained at the intersection of these two great disciplines.

Selections from the Society for the Study of Human Biology and the Biosocial Society’s 2006 symposium, “Medicine and Evolution”

A selection of titles of, and of Abstract excerpts from, the oral presentations at the Society for the Study of Human Biology and the Biosocial Society’s 2006 symposium, “Medicine and Evolution”), help give a sense of the topics of interest to advocates of evolutionary medicine as a separate discipline:

• Randolph Nesse:  Darwinian medicine is flowering: will it set seed?

"[T]he incorrect metaphor of the body as a machine can now be replaced with an evolutionary view of the body as a bundle of tradeoffs shaped by natural selection to maximize Darwinian fitness. This change in perspective is fundamental. It will lead to advances at all levels of analysis in all fields of medicine."

• Sarah Elton:  Environments, adaptation and evolutionary medicine

"In this presentation I will consider the concept of the 'environment of evolutionary adaptedness' [viz., the Stone Age] and its utility in understanding human evolutionary history and current human health. I will also critically review the importance of the selective pressures of the environment in shaping the health and disease profiles of modern human populations, and argue that although ideas such as 'Stone Age' adaptations to diet and consequent implications for human health in the 21st Century are attractive, they do not necessarily stand up to scrutiny."

• Stan Ulijaszek:  Human nutrient requirements from a life history perspective

"In this presentation, human life history tradeoffs will be considered in relation to protein requirements, and the evolutionary implications of protein requirements will be considered in relation to the major nutritional transition that took place at the origins of agriculture."

• Tessa Pollard:  Populations with very high rates of type 2 diabetes and cardiovascular disease:  competing explanations

"The finding that people who grow slowly in early life have a greater risk of type 2 diabetes and cardiovascular disease in later life if they are exposed to a western environment, offers a simpler explanation for what has happened in populations that have experienced rapid changes in lifestyle. In addition, these populations are usually exposed to other risk factors such as high rates of infection, racism and poverty. Examination of these different explanations shows how evolutionary theory can be applied in helpful and unhelpful ways by those trying to understand the causation of some of the most important health issues facing humans today."

• Lewis Wolpert:  The evolutionary biology of depression

"It has recently been proposed that depression and suicidality might be bargaining strategies... a hypothesis I test... As predicted by the bargaining hypothesis, in a large subgroup of cases there is clear recognition by all parties involved that suicidality is meant to apply pressure in intense social disputes. In many cases, the strategy works, yielding benefits for the suicidal individual."

• Ed Hagen:  The bargaining model of suicidality

"It has recently been proposed that depression and suicidality might be bargaining strategies... a hypothesis I test... As predicted by the bargaining hypothesis, in a large subgroup of cases there is clear recognition by all parties involved that suicidality is meant to apply pressure in intense social disputes. In many cases, the strategy works, yielding benefits for the suicidal individual."

• Mel Greaves Evolutionary origins of vulnerability to cancer.

"I will outline the argument that vulnerability to cancer, especially in ageing Homo sapiens, can be viewed as unfortunate consequences of several inherent features of evolution by natural selection… As an example of the application of evolutionary logic to the causality of a particular cancer, I will discuss our research on childhood leukaemia. Here a case is made for a mismatch between patterns of common infections during infancy in affluent societies and historical programming of the immune system."

• Virginia Vitzthum:  Evolution and Endocrinology:  the Regulation of Pregnancy Outcomes

"Two central challenges in the study of biological and behavioral variation are distinguishing adaptive and non-adaptive responses, and elucidating the physiological pathways that mediate these responses. Evolutionary endocrinology has emerged as a cross-disciplinary field of research that addresses these and other questions regarding the neurophysiological architecture that drives the implementation of life history strategies (the choices made by an organism regarding the timing and magnitude of investment in somatic demands and reproductive efforts). "

• Laurence Shaw:  Seasonality, food deprivation and PCOS [polycystic ovary syndrome]

"If genetics are at the core of such a common condition [PCOS], the question of whether there is an evolutionary advantage to the possession of the gene or genes is inevitably raised. Three hypotheses are proposed. "

• Jon Laman:  Evolution as a tool to teach immunology to doctors

"I will aim to engage you in a discussion whether the following two premises hold, and can successfully withstand your critical review: (a) Evolutionary thinking provides a valuable, if not the best, teaching paradigm for instructing students and doctors on the immune system. (b) Recent scientific progress in evolution of immunity (e.g. signaling principles retained from plant to mouse to human) is so rapid and exciting, that it provides an excellent framework to train doctors and biomedical investigators in translational medicine, a central concept in current models of medical innovation."

Medicine needs evolution

"Medicine Needs Evolution" introduces another recent (February, 2006) editorial arguing for the need of a discipline of evolutionary medicine, published in the journal Science, by Randolph M. Nesse, professor of Psychiatry and Psychology at the University of Michigan, working in the field of evolution and medicine; Stephen C. Stearns, Edward P. Bass Professor of Ecology and Evolutionary Biology at Yale University, working in the field of evolutionary biology; and, Gilbert S. Omenn, president of AAAS and professor of Medicine and Genetics at the University of Michigan, working in cancer proteomics, computational biology, and science policy. Those researchers argue the value of evolutionary explanations for:

• the narrowness of the birth canal
• the persistence of genes involved in bipolar disease
• the persistence of genes involved in senescence
• the nature of the arms race among bacteria that account for bacterial resistance to natural plant and artifactual antibiotics
• vector-related increases in pathogen virulence
• the biological role of cough, fever, and diarrhea and when to counter them
• why the commonality of low-back problems
• why the body synthesizes bilirubin
• how the modern diet causes diseases by thwarting evolutionary dietary norms
• why the modern high incidence of breast cancer

Neese, Stearns and Omenn end their editorial with the argument that "…both the human body and its pathogens are not perfectly designed machines but evolving biological systems shaped by selection under the constraints of tradeoffs that produce specific compromises and vulnerabilities. Powerful insights from evolutionary biology generate new questions whose answers will help improve human health."

Evolutionary medicine in action

Why has Nature not designed the human body and mind better, so that it can remain healthy throughout its life span? Professor Randolph Neese, University of Michigan, tries to answer that question with examples. We discuss one of them here:

Obesity

The evolutionary explanation for the high prevalence of obesity in modern times relates to the problems human ancestors had to solve during millions of years human evolution in an East African grasslands with scattered trees. Intermittent periods of food shortage encouraged evolution of body mechanisms that kept appetite high and body weight with some reserve whenever possible. In modern times, where food shortages in many parts of the world do not present a problem, those mechanisms promote overeating. Moreover, those mechanisms favored high appetite for fat and sugar, energy-rich foods often in scarce supply and therefore then unlikely to result in obesity. The agricultural, industrial and fast-food revolutions have eliminated the short supply of those foods in much of the world, and exploited the human appetite with consumer encouragement by separating fats and oils and refining foods to readily absorbable sugars. Voluntarily trying to limit food intake for weight reduction stimulate those evolved appetite-activating mechanisms, acting against the willpower resolve.

Though our human ancestors undoubtedly led a life of high physical activity, hunting, gathering, and often relocating to greener pastures, their body mechanisms evolved to minimize as much as possible wasteful expenditures of energy through physical activity — a tendency to inactivity that can run wild when food supplies do not present a problem and when modern transportation minimizes human locomotion. In combination with the evolved mechanisms for high appetite and food preferences, obesity comes easy in modern times.

One example of the major themes in evolutionary medicine

Many leading themes in evolutionary considerations relates to the history of human evolution, which we can exemplify in the context of human nutrition.

The human lineage extends back as many as 5-7 million years of hominin evolution before scientists can recognize an ancestor that humans have in common with their closest relative, the chimpanzee. The environmental conditions the human lineage survived in during that entire period of evolution undoubtedly contributed importantly to the modern human genetic composition, with eating preferences and patterns counting as major genetic selection determinants of the environment. Paraphrasing the Oxford historian, Felipe Fernandez-Armesto, a species’ most intimate contact with its natural environment occurs when the species eats it.

The nutritional requirements for human survival and reproductive fitness therefore presumably established themselves, at least in part, through the natural selection of genes over millions of years. Homo species first appeared at the beginning of the Stone Age approximately 2 million years ago—the Stone Age [a.k.a., the Paleolithic epoch] extended from ~2 million years ago to the beginnings of agriculture ~10 thousand years ago. During that period ancestral Homo species (Homo habilis, Homo erectus, Homo ergaster, inter alia) adapted to a profile of diets markedly different from that of the diets of contemporary humans.   

When our present Homo species took up agriculture and animal husbandry about 10,000 years ago, Homo sapiens began to forsake their lifestyle as hunter-gatherers—wherein they ate only wild animal and plant foods—and began settling down as farmers and animal husbanders, and, significantly, began introducing foods that they or their hominin ancestors had had no or negligible exposure to: cereal grains, legumes, animal milk and milk-products, and fatty meats.

Evolutionary biologists have argued that the 10,000-year (300 generation) interval between the beginnings of agriculture and the present time offered natural selection too little time to produce the comprehensive restructuring of our physiology and metabolism for optimal functioning in the face of such a major shift in dietary patterns. "Natural selection can never redesign a mechanism. It can only bring about slight quantitative shifts in its parameter.s" An even shorter period has natural selection had to adapt modern humans to dietary novelties since the more recent industrial and fast-food revolutions that further drastically changed the human dietary environment.

With respect to integrated metabolic and physiologic functioning, Homo sapiens’ genome, it is argued, therefore has remained at core unchanged since agriculture and animal husbandry began. Accordingly, students of evolutionary medicine look to the hominin ancestral diets, especially during the Paleolithic, and compare them with modern diets, to try to discover similarities that presumably would favor optimal functionality, or to discover important differences that might render aspects of human metabolism and physiology maladapted. Accumulating evidence suggests that the large-scale mismatch between the modern diet and the nutritional requirements set by the Paleolithic genome contributes importantly in the pathogenesis of obesity, hypertension, diabetes, certain forms of cancer, atherosclerotic cardiovascular disease, kidney stones, age-related muscle wasting, and osteoporosis.