Charles Darwin's daughter Anne, 'the joy of the household' died aged 10 due to tuberculosis.

The bacteria Mycobacterium tuberculosis could kill her as it had evolved to subvert the protection offered by her immune defenses

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. Researchers in the field of evolutionary medicine have suggested that evolutionary biology should not simply be an optional topic in medical school, but instead should be taught as one of the basic medical sciences.^[1]

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 ancestral 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.

[edit] 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.^{[2] p. 510}

George C. Williams was the first to apply evolutionary theory to health in the context of senescence^[3]. 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^[4].

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?^[5]

The paper of Paul Ewald in 1980, 'Evolutionary Biology and the Treatment of Signs and Symptoms of Infectious Disease',^[6] and that of Williams and Nesse in 1991, 'The Dawn of Darwinian Medicine'^[7] were key developments. The latter paper 'draw a favorable reception',^{[8]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.

[edit] 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,^[9] pathogen virulence.^[10] and pathogen subversion of the immune system.^[11]

[edit] 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. It is unclear whether the genetic information responsible for antibiotic resistance typically arises from an actual mutation, or is already present in the gene pool of the population of the organism in question.^{[citation needed]}

For more details on this topic, see antibiotic resistance

[edit] 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 offspring 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

[edit] 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

[edit] Human adaptations

Adaptation works within constraints, makes compromises and tradeoffs, and occurs in the context of different forms of competition.^[12]

[edit] Constraints

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 means that cancer, which is caused by somatic mutations, can never be completely eliminated by natural selection.
• Humans cannot biosynthesize Vitamin C, and so risk scurvy, Vitamin C deficiency disease, if dietary intake of the vitamin is insufficient.
• Retinal neurons and their axon output have evolved to be inside the layer of retinal pigment cells. This creates a constraint on the evolution of the visual system such that the optic nerve is forced to exit the retina through a point called the optic disc. This in turn creates 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.

[edit] 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.

• Running efficiency in women, and birth canal size^[13]
• Encephalization, and gut size ^[14]
• Skin pigmentation protection from UV, and the skin synthesis of Vitamin D
• Speech and its use of a descended larynx, and increased risk of choking^[15]

[edit] Competition effects

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

• mate choice and disease susceptibility ^[16]

• genomic conflict between mother and fetus that results in pre-eclampsia^[17][18]
• Major histocompatibility complex mate choice^[19][20]–Ž
• Maternal-paternal genetic competition that by altering genetic imprinting might underlie autism and schizophrenia^[21]

[edit] Evolved defense mechanisms

Evolution has selected defense mechanisms that protect against injuries and infections.^[8] These include

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

Management

Evolved defense mechanisms can be costly, due to increased energy use (fever increases BMR by 10-15% for each degree rise in body temperature), and due to the risk of damaging the body (vomiting can risk aspiration). A fitness advantage therefore exists in deploying defense mechanisms selectively only when the 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 tradeoffs between costs and benefits that take into account relevant health circumstances. This evolved regulation functions as a health management system.^[22]

[edit] 'Diseases of civilization'

Humans evolved to live as simple hunter-gatherers in small tribal bands, a very different way of life and environment than that faced by contemporary humans ^[23][24]. This change makes present humans vulnerable to a number of health problems, termed diseases of civilization' and 'diseases of affluence'.

[edit] 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.^[25][26][27]

• Trans fat health risks
• Dental caries
• High GI foods
• Modern diet based on "common wisdom" regarding diets in the paleolithic era

[edit] Life expectancy

• Diseases of old age

[edit] Exercise

Contemporary humans engage in little physical exercise compared to the physically active lifestyle engaged in by ancestral hunter-gatherers.^{[28][29][30][31][32]} It has been proposed that since prolonged periods of sedentariness 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.^[33]

[edit] 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.

• autoimmune diseases
• Allergies
• hygiene hypothesis
• Helminthic therapy

[edit] Specific explanations

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

[edit] Life stage related

[edit] Other

[edit] Evolutionary psychiatry

[edit] See also

Evolutionary biology portal

• Evolutionary physiology
• Evolutionary psychology
• Evolutionary neuropathology
• Evolutionary developmental psychopathology
• Evolutionary approaches to depression
• Illness

[edit] References

[edit] Further reading

Books

• Williams, George; Nesse, Randolph M. (1996). Why we get sick: the new science of Darwinian medicine. New York: Vintage Books. ISBN 0-679-74674-9. 
• Stearns SC, Koella JK (2008). Evolution in health and disease (2nd ed.). Oxford [Oxfordshire]: Oxford University Press. ISBN 0-19-920745-3. 
• McKenna, James J.; Trevathan, Wenda; Smith, Euclid O. (2008). Evolutionary medicine and health: new perspectives (2nd ed.). Oxford [Oxfordshire]: Oxford University Press. ISBN 0-19-530706-2. 
• O'Higgins, Paul; Sarah Elton (2008). Medicine and Evolution: Current Applications, Future Prospects (Society for the Study of Human Biology Symposium Series (Sshb)). Boca Raton: CRC. ISBN 1-4200-5134-2. 
• Ewald, P. W. (1996). Evolution of Infectious Disease. Oxford: Oxford University Press. ISBN 0-19-511139-7. 
• Moalem, S.; Prince, J. (2007). Survival of the Sickest. New York: HarperLuxe. ISBN 978-0-06-088965-4. 

Online articles

• Straub, RH.; Besedovsky, HO. (2003). "Integrated evolutionary, immunological, and neuroendocrine framework for the pathogenesis of chronic disabling inflammatory diseases". FASEB J 17 (15): 2176'2183. doi:10.1096/fj.03-0433hyp. PMID 14656978. http://www.fasebj.org/cgi/content/full/17/15/2176. 
• LeGrand, E. K.; Brown C. C. (1 July 2002). "Darwinian medicine: applications of evolutionary biology for veterinarians". Canadian Veterinary Journal 43 (7): 556'9. ISSN 0008-5286. PMID 12125190. PMC 341948. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=12125190. 
• Randolph M. Nesse, Stephen C. Stearns (2008). "The great opportunity: Evolutionary applications to medicine and public health" (PDF). Evolutionary Applications 1 (1): 28'48. doi:10.1111/j.1752-4571.2007.00006.x. http://www.eebweb.arizona.edu/attach/Nesse_Stearns_2008%201.pdf. 
• Naugler, Christopher T. (1 September 2008). "Evolutionary medicine: Update on the relevance to family practice". Canadian Family Physician 54 (9): 1265'9. PMID 18791103. PMC 2553465. http://www.cfp.ca/cgi/content/full/54/9/1265. 
• Childs, B.; Wiener, C.; Valle, D. (2005). "A science of the individual: Implications for a medical school curriculum". Annual Review of Genomics and Human Genetics 6: 313'330. doi:10.1146/annurev.genom.6.080604.162345. PMID 16124864. 
• Stiehm ER (2006). "Disease versus disease: how one disease may ameliorate another". Pediatrics 117 (1): 184'91. doi:10.1542/peds.2004-2773. PMID 16396876. 

[edit] External links

• The Evolution and Medicine Review
• Evolution and Medicine Network
• Randolph M. Nesse Website
• Steve Lewis's new Evolutionary Medicine site
• Special Issue of Evolutionary Applications on Evolutionary Medicine
• Publications in evolutionary medicine and further aspects of human evolution