Evolutionary Medicine Perspectives on Alzheimer’s Disease – Comprehensive Study Notes

Context & Scope

  • Author: Molly Fox, Ph.D. (Psychiatry, Biobehavioral Sciences & Anthropology, UCLA)
  • Publication: Ageing Research Reviews, Vol. 47 (2018), pp. 140–148
  • Central Question: Why does Alzheimer’s Disease (AD) – a highly deleterious, partly‐genetic disorder – persist and even increase in prevalence despite natural selection, which should remove maladaptive traits?
  • Method: Review & critical appraisal of 8 previously-proposed evolutionary explanations, followed by proposal of a new “environmental mismatch” hypothesis.

Core Terminology & Concepts

  • Evolutionary (Darwinian) Medicine – applying evolutionary theory to understand health & disease.
  • Mismatch / Diseases of Civilization – disorders driven or exacerbated by recent environmental changes that diverge from the conditions under which human biology evolved.
  • Antagonistic Pleiotropy – one gene has opposite fitness effects at different life stages or in different contexts.
  • Inclusive Fitness – an individual’s genetic success through reproduction plus aiding relatives’ survival/reproduction.
  • APOE (Apolipoprotein-E) Alleles (ε2, ε3, ε4) – genetic variants strongly modulating AD risk; ε4 is ancestral & highest risk.
  • Amyloid-β (Aβ) – peptide forming AD plaques; also exhibits antimicrobial activity.
  • Key temporal markers
    • Industrialization ≈ 200 y200\,\text{y} ago
    • Agricultural period: 12.5 ky→200 y12.5\,\text{ky} \rightarrow 200\,\text{y} ago
    • Foraging lifeways dominate ∼93.4%\sim93.4\% of human history.

Synopsis of 8 Earlier Evolutionary Hypotheses

1. Novel Extension of the Lifespan

  • Claim: Pre-modern humans rarely lived past the 40–50s; AD simply never had time to manifest.
  • Rebuttal
    • Hunter–gatherer data: Modal age at adult death >7070 y; 23\tfrac23 of adults surviving to puberty lived to ≥70\ge70 y.
    • !Kung & Hadza: ~10%10\% of population >60 y.
    • Therefore, geriatric traits were exposed to selection.
    • AD is pathological, not inevitable senescence (biochemical cascade vs. mere aging).

2. Antagonistic Pleiotropy

  • Idea: Genes boosting early-life fitness may cause late-life AD.
  • Candidate mechanisms
    • Enhanced neuroplasticity → higher learning → later vulnerability (no empirical support; prodromal cognition actually lower).
    • APOE-ε4 benefits: protection vs. hepatitis-C liver damage, malaria, diarrheal mortality, miscarriage reduction, fertility in high-pathogen Ghana.
    • Aβ: antimicrobial (influenza-A, HSV-1, yeast); oligomerization/fibrilization needed for defense but seeds plaques.
    • Immune response cytokines – acute protection vs. chronic neuroinflammation.
  • Take-home: Balancing selection may retain risk alleles.

3. Rapid Brain Evolution (“Phylogenetic Over-Shoot”)

  • Brains expanded from ∼400 cm3\sim400\,\text{cm}^3 (Australopithecus afarensis) → ∼1450 cm3\sim1450\,\text{cm}^3 (Homo sapiens) over ≈3 My\approx3\,\text{My}.
  • Regions enlarged most (frontal, parietal, temporal cortices; hippocampus) are those hit hardest by AD.
  • Hypothesis: Genes (e.g., NOTCH2NL, HARE5/Fzd8) driving encephalization also inadvertently increase regional AD susceptibility.
  • Requires linking the same alleles to both phenotypes; evidence pending.

4. Post-Reproductive Relaxed Selection

  • View: AD emerges after reproduction; selection is weak.
  • Counter-arguments
    • Early/mid-life cognitive deficits & cardiovascular comorbidities in future AD patients imply fitness costs before menopause.
    • Long dependency of AD patients burdens kin → reduces inclusive fitness.
    • Grandmothers enhance descendant survival; losing them early is costly.

5. Delayed Neuropathy via Selection for Grandmothering

  • Sapolsky & Finch: Lifespan extension + maternal/allo-maternal care selected for delayed AD onset until grandchild independence.
  • Mechanistic suggestion: Estradiol during reproductive years postpones pathology.
  • Critiques
    • Human AD onset (even in ε4 homozygotes) still ≈20 y after last pregnancy – a gap unaccounted for.
    • Conflates infant altriciality with compulsory grandmaternal care.

6. Selection of Novel APOE Isoforms (ε3, ε2) to Offset AD

  • Timeline: ε3 arises ∼300 ky\sim300\,\text{ky} BP; ε2 ∼200 ky\sim200\,\text{ky} BP.
  • Frequencies: ≈95%\approx95\% of humans carry ≥1 ε3; ε4 homozygosity ≈1−2%1{-}2\%.
  • ε3/ε2 delay AD onset vs. ε4.
  • Nevertheless, ε4 homozygous humans onset ≈66.466.4 y, still >20 y later than non-human primates, challenging the simple estradiol-delay model.

7. Cardiovascular By-Product Hypothesis

  • Finch & Stanford: Meat eating → higher cholesterol → atherosclerosis; selection favored ε3 to mitigate cardiovascular risk; AD delay a side-effect.
  • Issues
    • ε3 preceded agriculture by ≈288 ky\approx288\,\text{ky}.
    • Hunter-gatherers eat more meat than early farmers but show higher ε4 frequency – opposite of prediction.
    • Ancient mummy CTs reveal atherosclerosis in foragers & farmers alike, muddling selection gradients.

8. Thrifty Genotype / Energy-Saving Senescence

  • Reser: Mid-life caloric stress in foragers favors neural down-sizing (pre-clinical AD) to save energy for survival.
  • Conflicts with
    • Importance of cognition for foraging success & social life.
    • Empirical hunter–gatherer mortality schedules & brain-energy prioritization.

Author’s New Hypothesis: Environmental Mismatch

  • Premise: Modern (post-industrial) conditions amplify AD risk factors; earlier humans with identical genotypes faced lower age-matched risk.
  • If AD onset historically occurred nearer to life’s natural end, its fitness impact – and thus selective removal – would be minimal.

Major Mismatch Pathways

1. Insulin Resistance (“Type-3 Diabetes”)
  • AD brains show impaired glucose metabolism.
  • Modern diet: ∼20%\sim20\% calories from simple sugars & lower daily expenditure (≈2000 kcal2000\,\text{kcal}) vs. ancestral ∼1.5%\sim1.5\% sugar & 3000 kcal3000\,\text{kcal} output.
  • Epidemics of obesity, metabolic syndrome, type-2 diabetes coincide with AD surge.
2. Estrogenic Neuroprotection
  • Estrogens inhibit Aβ production, promote clearance, reduce tau phosphorylation & oxidative stress.
  • Pre-industrial women:
    • Later menarche (≈1717 y vs. modern 1212 y).
    • Lower baseline estrogen but higher cumulative pregnancy estrogen (median hunter–gatherer lifetime: ∼70.8\sim70.8 mo pregnant vs. modern British 2121 mo).
  • Study: More cumulative pregnancy months → lower AD risk.
3. Immune Education & Inflammation (“Old Friends”)
  • Modern hygiene and low pathogen exposure ↓ T-regulatory cell development → chronic low-grade inflammation → AD facilitation.
  • Cross-national study: Low childhood pathogen proxies (clean water, sanitation) correlate with higher age-adjusted AD prevalence.
  • Chronic infections (H. pylori, C. pneumoniae, HSV-1) may further inflame CNS; damage worse when immunoregulation is weak.
4. Industrial & Occupational Toxins
  • Novel exposures: air pollution, lead, mercury, cadmium, pesticides; some linked to AD pathology via neuroinflammation/oxidative stress.
  • Historical absence of these agents implies lower ancestral burden.
5. Context-Dependent APOE Effects
  • ε4 heightens AD risk in industrialized settings but shows no association in multiple non-industrialized cohorts (Ibadan, Kenyan, Tanzanian, Arab Israeli, Bantu, Nilotic, Khoi San).
  • Hypothesis: Robust immunoregulation in high-pathogen environments neutralizes ε4’s pro-inflammatory liability.

Integrative Conclusions & Implications

  • Evidence undermines ideas that AD was invisible to selection; instead, changing environments likely shifted gene × environment interactions.
  • Mismatch view lessens the need to explain ε4 retention; allele may have been neutral/beneficial historically.
  • Practical Insights
    • Distinguish adaptive vs. pathological amyloid and immune responses when designing therapies.
    • Target modern risk factors (metabolic dysregulation, chronic inflammation, environmental toxins) for prevention.
    • Consider evolutionary life-history (grandmothering, inclusive fitness) when assessing societal burden.
  • Future Directions
    • Empirical testing of mismatch pathways using longitudinal, cross-cultural, and mechanistic studies.
    • Genetic analyses linking encephalization alleles with AD vulnerability.
    • Reconstruct past AD incidence via paleopathology & demographic modeling.

Quick Reference: Numerical & Statistical Nuggets

  • ∼93.4%\sim93.4\% of human history = foraging lifestyle.
  • Agricultural era: 12,500 y12{,}500\,\text{y} to 200 y200\,\text{y} BP.
  • ε4 carriers: 28%28\% (≥1 copy); homozygotes 1−2%1{-}2\%.
  • Average AD survival post-diagnosis: 7−107{-}10 y.
  • ε4 homozygote mean onset: 66.466.4 y.
  • Hunter–gatherer cumulative pregnancy: ≈70.8\approx70.8 mo vs. modern British 2121 mo.