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Evolutionary medicine or Darwinian medicine is the application of modern evolutionary theory to understanding health and disease. It provides a complementary scientific approach to the present mechanistic explanations that dominate medical science, and particularly modern medical education.

Such adaptations concern:

• The evolution of pathogens in terms of their virulence, resistance to antibiotics, and subversion of an individual’s immune system.
• The processes, constraints and trade-offs of human evolution.
• The evolved responses that enable individuals to protect, heal and recuperate themselves from infections and injuries such as immunity, fever, and sickness behavior, and the processes that regulate their deployment to maximize fitness.
• How past adaptation of early humans to their ancestrial environment now affects contemporary humans with their different diet, life expectancy, degree of physical exercise, and hygiene.

Important researchers in evolutionary medicine include: Randolph M. Nesse, George C. Williams, Paul W. Ewald, James McKenna, and Rainer H. Straub.

Contents 1 History 2 Pathogens 2.1 Antibiotic resistance 2.2 Virulence 2.3 Immune evasion 3 Human adaptations 3.1 Constraints 3.2 Trade-offs and conflicts 3.3 Competition effects 4 Evolved self-treatments 5 “Diseases of civilization” 5.1 Diet 5.2 Life expectancy 5.3 Exercise 5.4 Cleanliness 6 Specific explanations 6.1 Life stage related 6.2 Evolutionary psychiatry 6.3 Other 7 See also 8 References 9 Further reading 10 External Links

History

Charles Darwin did not discuss the implications of his work for medicine, though biologists quickly appreciated in the germ theory of disease its implications for understanding the evolution of pathogens, and 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.^{1 p. 510}

George C. Williams was the first to apply evolutionary theory to health in the context of senescence² 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,³

Randolph 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?⁴

The paper of Paul Ewald in 1980, “Evolutionary Biology and the Treatment of Signs and Symptoms of Infectious Disease”,⁵ and that of Williams and Nesse in 1991, “The Dawn of Darwinian Medicine”⁶ were key developments. The latter paper “draw a favorable reception”,^{7page 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.

Pathogens

The adaptive evolution of bacteria, viruses, other microbes and parasites plays a central role in medicine since this process is needed to understand issues such as antibiotic resistance,⁸ pathogen virulence.⁹ and pathogen subversion of the immune system.¹⁰

Antibiotic resistance

Microorganisms evolve resistance through natural selection acting upon random mutation. Once a gene conferring resistance arises to counteract an antibiotic, not only can that bacteria thrive, but it can spread that gene to other types of bacteria through horizontal gene transfer of genetic information by plasmid exchange.

For more details on this topic, see antibiotic resistance

Virulence

The effect of organisms upon their host can vary from being symbiotic commensals that are beneficial, to pathogens that reduce fitness. Many pathogens produce virulence factors that directly cause disease, or manipulate their host to allow them to thrive and spread. Since a pathogen’s fitness is determined by its success in transmitting offsprings to other hosts, it was thought at one time, that virulence moderated and it evolved toward commensality. However, this view is now questioned by Ewald.

For more details on this topic, see virulence, virulence factors and optimal virulence

Immune evasion

The success of any pathogen depends upon its ability to evade host immunity. Therefore, pathogens evolve methods that enable them to infect a host, and then evade detection and destruction by its immune system. These include hiding within host cells, within a protective capsule (as with M. tuberculosis), secreting compounds that misdirect the host's immune response, binding its antibodies, rapidly changing surface markers, or masking them with the host’s own molecules.

For more details on this topic, see manipulation of the immune system by pathogens, and evasion of the innate immune system

Human adaptations

Adaptation works within constraints, makes trade-off compromises, and occurs in the context of different forms of competition.¹¹

Constraints

Adaptation can only occur if they are evolvable. Some adaptation which would prevent ill health are not possible.

• DNA cannot be totally prevented from undergoing somatic replication corruption, and so creates the possible existence of cancers
• Humans (due to earlier ancestral loss in the primate lineage) cannot biosynthesize Vitamin C, and so risks if diet is inadequate in an external source, scurvy.
• Retinal neurons and their axon output has evolved to be inside the layer of retinal pigment cells. This creates the constraint upon evolution that the optic nerve is forced to exit the retina through a point called the optic disc. This create a blind spot. More importantly, it makes vision vulnerable to increased pressure within the eye (glaucoma) since this cups and damages the optic nerve at this point resulting in impaired vision.

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 benefit cost trade off.

• Running efficiency in women, and birth canal size¹²
• Encephalization, and gut size ¹³
• Skin pigmentation protection from UV, and the skin synthesis of Vitamin D
• Speech and its use of a descended larynx, and increased risk of choking¹⁴

Competition effects

Different forms of competition exist and these can shape the processes of gene change.

• mate choice and disease susceptibility ¹⁵

• genomic conflict between mother and fetus that results in pre-eclampsia¹⁶¹⁷
• MHC mate choice¹⁸¹⁹‎

• Maternal-paternal genetic competition that by altering genetic imprinting might underlie autism and schizophrenia²⁰

Evolved self-treatments

Evolution has selected self-treatments (vis medicatrix naturae, or “healing powers of nature” ) that protect, heal, or restore injures, infections and disrupted homeostasis. They include

• Sickness behavior (Lethargy, Depression, Anorexia, Sleepiness, reduction in grooming, and failure to concentrate)
• Expulsions: Sneezing, Vomiting, Coughing, Diarrhea

The deployment of such evolved self-treatments is regulated.

Precautionary

The deployment of evolved self-treatments can be used on a precautionary basis.⁷ As Nesse notes: "Vomiting, for example, may cost only a few hundred calories and a few minutes, whereas not vomiting may result in a 5% chance of death" page 77⁷.

Management

On the other hand, evolved self-treatments are costly both in using energy fever increases BMR by 10-15% for each degree rise in body temperature, and in their risk of damaging the body (vomiting can risk asspiration). A fitness advantage therefore exists in deploying self-treatments selectively only when potential benefits outweigh such costs. Their deployment is controlled at several levels including through biomolecular pathways using factors such as proinflammatory cytokines, and through higher neural top down processes in cerebral cortex areas such as the insular cortex. Neural control provides advantages in that deployment can be based on trade-offs between costs and benefits that take into account health relevant circumstances. This evolved regulation functions as a health management system.²¹

“Diseases of civilization”

Humans evolved to be adapted to live as simple hunter-gatherers in small bands, a very different way of life and environment than that faced by contemporary humans. This change makes present humans vulnerable to a number of health problems, “diseases of civilization” and “diseases of affluence”.

Diet

In contrast to the diet of early hunter-gatherers, the modern one contains high quantities of fat, salt, and refined sugars. These create health problems.²²²³²⁴

• Trans fat health risks
• Dental caries
• High GI foods
• Diet informed by Paleolithic foods

Life expectancy

• Diseases of old age

Exercise

Contemporary humans engage in little physical exercise compared to the physically active lifestyle engaged in by ancestral hunter-gatherers.^2526272829

Cleanliness

Contemporary humans due to medical treatment, frequent body and cloth washing, and sanitation are mostly free of parasites, particularly intestinal ones. This causes problems in the proper development of the immune system.

• autoimmune diseases
• Allergies
• hygiene hypothesis
• Helminthic therapy)

Specific explanations

This is a partial list: all links here go to a section describing or debating its evolutionary origin.

Life stage related

Evolutionary psychiatry

Other

See also

• Evolutionary physiology
• Evolutionary psychology
• Illness

References