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Evolutionary medicine or Darwinian medicine is the application of modern evolutionary theory to understanding health and disease. Modern medical research and practice has focused on the molecular and physiological mechanisms underlying health and disease, while evolutionary medicine focuses on the question of why evolution has shaped these mechanisms in ways that may leave us susceptible to disease. The evolutionary approach has driven important advances in our understanding of cancer, autoimmune disease, and anatomy. Medical schools have been slower to integrate evolutionary approaches because of limitations on what can be added to existing medical curricula.
Contents
Human adaptations
Adaptation works within constraints, makes compromises and tradeoffs, and occurs in the context of different forms of competition.
Constraints
Adaptations can only occur if they are evolvable. Some adaptations which would prevent ill health are therefore not possible.
Other constraints occur as the byproduct of adaptive innovations.
Trade-offs and conflicts
One constraint upon selection is that different adaptations can conflict, which requires a compromise between them to ensure an optimal cost-benefit tradeoff.
Competition effects
Different forms of competition exist and these can shape the processes of genetic change.
"Diseases of civilization"
Humans evolved to live as simple hunter-gatherers in small tribal bands, a very different way of life and environment compared to that faced by contemporary humans. This change makes present humans vulnerable to a number of health problems, termed "diseases of civilization" and "diseases of affluence". Humans evolved to live off of the land, and take advantage of the resources that were readily available to them. They evolved for the stone-age, and the environments of today bring about many disease causing ailments, that may or may not be deadly. "Modern environments may cause many diseases-for example, deficiency syndromes such as scurvy and rickets" (Williams, 1991)
Diet
In contrast to the diet of early hunter-gatherers, the modern Western diet often contains high quantities of fat, salt, and simple carbohydrates, which include refined sugars and flours. These create health problems.
Life expectancy
Examples of aging-associated diseases are atherosclerosis and cardiovascular disease, cancer, arthritis, cataracts, osteoporosis, type 2 diabetes, hypertension and Alzheimer's disease. The incidence of all of these diseases increases rapidly with aging (increases exponentially with age, in the case of cancer).
Of the roughly 150,000 people who die each day across the globe, about two thirds—100,000 per day—die of age-related causes. In industrialized nations, the proportion is much higher, reaching 90%.
Exercise
Many contemporary humans engage in little physical exercise compared to the physically active lifestyles ancestral hunter-gatherers. It has been proposed that since prolonged periods of inactivity would have only occurred in early humans following illness or injury that it provides a cue for the body to engage in life-preserving metabolic and stress related responses such as inflammation that are now the cause of many chronic diseases.
Cleanliness
Contemporary humans - due to medical treatment, frequent washing of clothing and the body, and improved sanitation - are mostly free of parasites, particularly intestinal ones. This causes problems in the proper development of the immune system although hygiene can be very important when it comes to maintaining good health. The hygiene hypothesis says that many modern humans are not exposed to microorganisms that have evolved in establishing the immune system as they should be. "Microorganisms and macroorganisms such as helminths from mud, animals, and feces play a critical role in driving immunoregulation" (Rook, 2012). They play a crucial role in building and training immune functions to fight off and repel some diseases, and protect against excessive inflammation which has been implicated in several diseases (such as recent evidence for Alzheimer's Disease).
Specific explanations
This is a partial list: all links here go to a section describing or debating its evolutionary origin.
Evolutionary psychology
As noted in the table below, adaptationist hypotheses regarding the etiology of psychological disorders are often based on analogies with evolutionary perspectives on medicine and physiological dysfunctions (see in particular, Randy Nesse and George C. Williams' book Why We Get Sick). Evolutionary psychiatrists and psychologists suggest that some mental disorders likely have multiple causes.
See several topic areas, and the associated references, below.
History
Charles Darwin did not discuss the implications of his work for medicine, though biologists quickly appreciated the germ theory of disease and its implications for understanding the evolution of pathogens, as well as an organism’s need to defend against them.
Medicine, in turn, ignored evolution, and instead focused (as done in the hard sciences) upon proximate mechanical causes.
medicine has modelled itself after a mechanical physics, deriving from Galileo, Newton, and Descartes.... As a result of assuming this model, medicine is mechanistic, materialistic, reductionistic, linear-causal, and deterministic (capable of precise predictions) in its concepts. It seeks explanations for diseases, or their symptoms, signs, and cause in single, materialistic— i.e., anatomical or structural (e.g., in genes and their products)— changes within the body, wrought directly (linearly), for example, by infectious, toxic, or traumatic agents. p. 510
George C. Williams was the first to apply evolutionary theory to health in the context of senescence. Also in the 1950s, John Bowlby approached the problem of disturbed child development from an evolutionary perspective upon attachment.
An important theoretical development was Nikolaas Tinbergen’s distinction made originally in ethology between evolutionary and proximate mechanisms.
Randolph M. Nesse summarizes its relevance to medicine:
all biological traits need two kinds of explanation, both proximate and evolutionary. The proximate explanation for a disease describes what is wrong in the bodily mechanism of individuals affected by it. An evolutionary explanation is completely different. Instead of explaining why people are different, it explains why we are all the same in ways that leave us vulnerable to disease. Why do we all have wisdom teeth, an appendix, and cells that can divide out of control?
The paper of Paul Ewald in 1980, “Evolutionary Biology and the Treatment of Signs and Symptoms of Infectious Disease”, and that of Williams and Nesse in 1991, “The Dawn of Darwinian Medicine” were key developments. The latter paper “draw a favorable reception”,page x and led to a book, Why We Get Sick (published as Evolution and healing in the UK). In 2008, an online journal started: Evolution and Medicine Review.
Current activity in the field
Evolutionary medicine as a field began in the early 1990s, but has grown dramatically in recent years. These developments include the creation of the online publication, The Evolution & Medicine Review, which has served as a clearinghouse for important information in the field, two peer-reviewed journals (Evolution, Medicine and Public Health and Journal of Evolutionary Medicine), the founding of two evolution and cancer centers (The Center for Evolution and Cancer at UCSF and The Darwinian Evolution of Cancer Consortium in Montpellier) and The Center for Infectious Disease Dynamics at Penn State. There is now a national working group on evolutionary medicine education at the NSF sponsored National Evolutionary Synthesis Center, Infusing Medical Education with Evolutionary Thinking. Evolutionary Medicine programs have been established at a growing number of Universities, including UCLA, Arizona State University and Durham University in the UK.