Evolutionary Medicine

Unit 3: Global Health

  • Focus on how evolution predisposes humans to particular diseases, specifically non-communicable diseases (NCDs).

  • Examine how ancestral environments and modern degraded environments contribute to disease susceptibility.

  • Text: Case study of maternal and child health in Mali.

Evolutionary Medicine

  • Examining the question: Why do we get sick?

  • A debate on whether evolutionary theory should be incorporated into medical school curricula.

Levels of Explanation

  1. Bilirubin

  2. Aging

  3. Antibiotic Resistance

  4. Modern Diets

  5. Fever

  6. Public Health

How Do Birds Get Their Colors?

  • Birds acquire colors in their feathers/skin through:

    • Melanins

    • Carotenoids

    • Porphyrins

  • The colors serve important functions:

    • Attracting mates

    • Signaling status

    • Disguising from predators

Causation in Coloration

  • Proximate Causation: Immediate mechanisms explaining how birds achieve their color.

  • Ultimate Causation: The evolutionary reasons why coloration is beneficial.

Tinbergen’s Four Questions

  • Framework to understand behavior and biological functions through different lenses.

Proximate Questions

  1. Mechanism: How does the mechanism work?

  2. Ontogeny: What is the ontogeny of the mechanism?

Evolutionary Questions

  1. Selective Advantage: What is the selective advantage of this mechanism?

  2. Phylogeny: What is the phylogeny of this mechanism?

Key Definitions

  • Ontogeny: Refers to the development of an organism from fertilization to death.

  • Phylogeny: Refers to the evolutionary history of a taxonomic group of organisms. It can be visualized on a phylogenetic tree showing relationships among groups.

  • Tinbergen’s questions are complementary, providing a comprehensive understanding beyond proximate causes alone.

  • Quote by Theodosius Dobzhansky: "Nothing in Biology Makes Sense Except in the Light of Evolution."

Hemoglobin and Bilirubin

  • Hemoglobin in red blood cells breaks down into:

    • Heme

    • Globin

  • Heme yields:

    • Bilirubin (toxic, water insoluble).

Bilirubin and Heme Catabolism

  • Phototherapy is used to reduce elevated bilirubin levels in newborns.

Proximate Question Regarding Bilirubin

  • What is the metabolic pathway via which heme is degraded to bilirubin? (Requires biochemistry knowledge).

Evolutionary Question Regarding Bilirubin

  • Why does the body produce bilirubin, given its toxicity? Why not stop at biliverdin, a nontoxic and water-soluble product?

Answers to Evolutionary Questions

  • Bilirubin acts as an effective antioxidant, protecting against aging caused by oxidative damage.

  • Oxidative damage is linked to atherosclerosis, and bilirubin provides protection against heart attacks.

Implication of Evolutionary Knowledge in Medicine

  • While evolutionary knowledge may not have immediate practical effects on day-to-day medical practice, it informs future research that may lead to changes in clinical practice, akin to how other basic sciences are learned in medical school.

Understanding Aging

Causes of Senescence

  • Factors leading to aging include:
    a. Stronger selection pressure in early life due to a larger number of individuals and their reproductive potential.
    b. Natural deterioration as organisms age and wear out.
    c. Reproductive constituents becoming "disposable."
    d. None are entirely correct, but aspects are valid.

Aging Research Insights

  • Lifespan as a life history trait influenced by natural selection (cited from G.C. Williams).

  • Certain genes can be pleiotropic, carrying age-related disadvantages if they also confer advantages early in life.

  • Thomas Kirkwood’s disposable soma theory states that genetics do not ensure health past the lifespan.

Comparative Longevity

  • Birds and bats generally experience lower predation rates and longer life spans in comparison to rodents, demonstrating divergence in aging processes.

Antibiotic Resistance

  • Development of resistance in bacteria due to antibiotic selection pressures.

  • Economic Impact: Estimated at $80 billion annually in the USA.

  • Recognition Gap: There's a lack of acknowledgment that antibiotic resistance exemplifies natural selection in medical literature (18% in medical journals vs. 79% in biology journals).

  • Misconceptions exist around antibiotic discovery vs. the co-evolution of bacteria and fungi, which are parts of an ongoing arms race.

Hygiene Hypothesis

  • Relationship between the incidence of autoimmune disorders and helminth infestation illustrates:

    • High, Moderate, Low incidences.

    • Public health methods like vaccination and sanitation have substantially reduced mortality in the last century but also shifted the immune environment significantly.

    • Absence of helminth parasites potentially links to increased rates of autoimmune diseases, asthma, type-1 diabetes, and childhood leukemias.

    • Clinical trials indicate that treatment with pig whipworm eggs saw a 70% remission rate in Crohn’s disease patients.

Modern Diets

Dietary Preferences

  • Humans tend to overconsume fats, sugars, and salt due to historically scarce availability of these nutrients.

Obesity and Dieting Implications

  • Yo-yo dieting leads to weight gain as the body may perceive inadequate food supply and adapt by increasing fat storage in anticipation of scarcity.

The Role of Fever in Infection

  • Fever occurs in response to infection through endogenous pyrogens (e.g., prostaglandins and cytokines) acting on the hypothalamus, elevating body temperature.

Purpose of Fever

  • A protective, evolved defense mechanism, where suppression (unless severe) has adverse effects. Benefits include:

    • Enhanced immune activation (neutrophil migration, lymphocyte proliferation).

    • Inhibition of bacterial growth and viral replication via reduced plasma iron levels.

    • Suppressing fever can elevate mortality rates, notably in cases like flu and malaria.

Hypotheses Regarding Fever Management

  1. Metabolic costs of fever exceed physiologic benefits.

  2. Fever acts as a protective adaptive response; suppression is often more harmful.

  • Reference: Ray and Schulman 2015 survey on fever management.

Public Health and Evolutionary Considerations

  • Findings from Gibson and Mace (2006), highlighting a development project in rural Ethiopia where women, previously carrying water over 30 km/day, showed increases in maternal fertility and childhood malnutrition due to accessibility of water without accompanying family planning measures.

Concluding Thoughts on Evolutionary Medicine

  • Evolutionary medicine investigates the reasons behind natural selection's failure to eliminate vulnerabilities to disease and seeks to answer the following:

    1. Mismatch: Between ancestral and modern environments (e.g., changing diets).

    2. Pathogen Coevolution: Continual coadaptation with hosts hinders host adaptation.

    3. Selection Constraints: Constraints on evolutionary changes over time.

    4. Trade-offs: Balances between survival, reproduction, and health.

    5. Maximized Reproduction: Natural selection prioritizes reproduction over health consequences.

    6. Defenses' Nature: Many physiological defenses (e.g. pain, fever) may cause suffering, yet serve protective roles.

Call to Action

  • There is strong advocacy for the inclusion of evolutionary theory in medical school curricula, affirming its critical role in understanding health and disease dynamics!