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The application of modern evolutionary theory to understanding health and disease
Utilizing the Delphi method, 56 experts from a variety of disciplines, including anthropology, medicine, and biology agreed upon 14 core principles intrinsic to the education and practice of evolutionary medicine. These 14 principles can be further grouped into five general categories: question framing, evolution I and II (with II involving a higher level of complexity), evolutionary trade-offs, reasons for vulnerability, and culture. Additional information regarding these principles may be found in the table below.
Environmental factors can shift developmental trajectories in ways that influence health and the plasticity of these trajectories can be the product of evolved adaptive mechanisms.
Defenses (reasons for vulnerability)
Many signs and symptoms of disease (e.g. fever) are useful defenses, which can be pathological if dysregulated.
Mismatch (reasons for vulnerability)
Disease risks can be altered for organisms living in environments that differ from those in which their ancestors evolved.
Cultural practices (culture)
Cultural practices can influence the evolution of humans and other species (including pathogens), in ways that can affect health and disease (e.g. anti-biotic use, birth practices, diet, etc.).
Adaptation works within constraints, makes compromises and trade-offs, and occurs in the context of different forms of competition.
Adaptations can only occur if they are evolvable. Some adaptations which would prevent ill health are therefore not possible.
DNA cannot be totally prevented from undergoing somatic replication corruption; this has meant that cancer, which is caused by somatic mutations, has not (so far) been completely eliminated by natural selection.
Humans evolved to live as simple hunter-gatherers in small tribal bands. Contemporary humans now have a very different environment and way of life. This change makes present humans vulnerable to a number of health problems, termed "diseases of civilization" and "diseases of affluence". Stone-age humans evolved to live off the land, taking advantage of the resources that were readily available to them. Evolution is slow, and the rapid change from stone-age environments and practices to the world of today is problematic because we are still adapted to stone-age circumstances that no longer apply. This misfit has serious implications for our health. "Modern environments may cause many diseases such as deficiency syndromes like scurvy and rickets".)
In contrast to the diet of early hunter-gatherers, the modern Western diet often contains high quantities of fat, salt, and simple carbohydrates, such as refined sugars and flours. These relatively sudden dietary changes create health problems.
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%.
Many contemporary humans engage in little physical exercise compared to the physically active lifestyles of ancestral hunter-gatherers. Prolonged periods of inactivity may have only occurred in early humans following illness or injury, so a modern sedentary lifestyle may continuously cue the body to trigger life preserving metabolic and stress-related responses such as inflammation, and some theorize that this causes chronic diseases.
Contemporary humans in developed countries are mostly free of parasites, particularly intestinal ones. This is largely due to frequent washing of clothing and the body, and improved sanitation. Although such hygiene can be very important when it comes to maintaining good health, it can be problematic for the proper development of the immune system. The hygiene hypothesis is that humans evolved to be dependent on certain microorganisms that help establish the immune system, and modern hygiene practices can prevent necessary exposure to these microorganisms. "Microorganisms and macroorganisms such as helminths from mud, animals, and feces play a critical role in driving immunoregulation" (Rook, 2012). Essential microorganisms play a crucial role in building and training immune functions that fight off and repel some diseases, and protect against excessive inflammation, which has been implicated in several diseases. For instance, recent studies have found evidence supporting inflammation as a contributing factor in Alzheimer's Disease.
This is a partial list: all links here go to a section describing or debating its evolutionary origin.
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.
Possible Causes of Psychological 'Abnormalities' from an Adaptationist Perspective
Summary based on information in Buss (2011), Gaulin & McBurney (2004), Workman & Reader (2004)
Functioning adaptation (adaptive defense)
Fever / Vomiting (functional responses to infection or ingestion of toxins)
Mild depression or anxiety (functional responses to mild loss or stress)
By-product of an adaptation(s)
Intestinal gas (byproduct of digestion of fiber)
Sexual fetishes (?) (possible byproduct of normal sexual arousal adaptations that have 'imprinted' on unusual objects or situations)
Adaptations with multiple effects
Gene for malaria resistance, in homozygous form, causes sickle cell anemia
Adaptation(s) for high levels of creativity may also predispose schizophrenia or bi-polar disorder (adaptations with both positive and negative effects, perhaps dependent on alternate developmental trajectories)
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
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.
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