Adaptation to solar radiation

Humoral Theory & Linnaean Classification

• Humoral theory explained human variation (including skin color) as the result of climate altering bodily fluids.

• Linnaean classification categorized humans into racial types based on physical traits, including skin color.

• Both treated skin color as superficial and typological, rather than adaptive or evolutionary.

Buffon & Kant’s Treatment of Skin Colour

• Both viewed skin color as a climatic effect, not a fixed biological difference.

• Buffon emphasized environmental degeneration from an original form.

• Kant argued skin color reflected inherited climatic effects, laying groundwork for later racial typologies.

• Neither used evolutionary mechanisms.

Ultraviolet Radiation

• Electromagnetic radiation from the sun, divided into UVA, UVB, and UVC.

• Most UVC and UVB are filtered by the atmosphere; UVA largely reaches Earth.

• UVR is both biologically necessary and biologically damaging.

Physiology and Roles of the Skin

• Skin functions include protection, thermoregulation, immunity, and vitamin D synthesis.

• Melanin, produced by melanocytes, absorbs and scatters UVR.

• Skin pigmentation balances protection from UV damage with metabolic needs.

Geographic & Experimental Method

• Geographic method identifies spatial patterns (e.g., skin color correlates with UV intensity).

• Experimental method tests biological mechanisms (e.g., UV effects on folate or vitamin D).

• Together, they link pattern to process.

Insolation

• The amount of solar radiation reaching Earth’s surface.

• Determined by:

  • Solar zenith angle (latitude),

  • Seasonality,

  • Atmospheric thickness and opacity.

• Insolation directly affects UV exposure.

Function & Sources of Vitamin D

• Vitamin D is essential for:

  • Bone mineralization,

  • Immune function,

  • Calcium metabolism.

• Produced in the skin via UVB exposure and obtained from diet (e.g., fatty fish).

Storage of Vitamin D

• Vitamin D is fat-soluble and stored in body fat.

• Storage allows short-term buffering but does not prevent deficiency in low-UV environments.

Dietary Adaptations to Low UVB

• Populations in low-UV regions rely on dietary vitamin D (e.g., marine foods).

• Cultural practices buffer biological constraints imposed by environment.

Hypotheses Explaining Skin Color Variation

• Skin color variation reflects adaptation to solar radiation.

• High melanin protects:

  • DNA,

  • Skin and eyes,

  • Folate.

• Low melanin facilitates sufficient vitamin D synthesis in low-UV environments.

Relevant Data Supporting Skin Color Adaptation

• Strong correlation between UV intensity and pigmentation.

• Experimental evidence of folate photodegradation by UVR.

• Clinical evidence linking vitamin D deficiency to rickets.

Molecular Clock

• Method for estimating the timing of evolutionary events using rates of genetic change.

• Used to date mutations related to skin pigmentation genes (e.g., MC1R).

Evolution of Skin Colour: Hominin Origins to Present

• Early hominins likely had light skin under body hair.

• Hair loss (~1.5–2.0 MYA) increased UV exposure.

• Dark pigmentation evolved ~1.2 MYA with Homo erectus.

• Later depigmentation evolved independently in low-UV regions.

Selective Sweep

• A process where strong selection rapidly increases the frequency of a beneficial allele.

• Reduced genetic variation around pigmentation genes reflects past selective sweeps.

Rickets

• A disease caused by vitamin D deficiency.

• Leads to poor bone mineralization and skeletal deformities.

• Demonstrates the fitness costs of insufficient UV exposure.

Evolution of Skin Colour (Synthesis)

• Skin color evolution reflects a trade-off between UV protection and vitamin D production.

• Dark skin is favored in high UV environments; light skin in low UV environments.

• Skin color variation is adaptive, not racial or typological.