. Author manuscript; available in PMC: 2010 Mar 22.

Published in final edited form as: Pigment Cell Melanoma Res. 2009 Jun 5;22(4):420–434. doi: 10.1111/j.1755-148X.2009.00588.x

Table 1.

Hypotheses commonly advanced to explain latitude-dependent increase in pigmentation

Pigment evolved	Arguments for / against
To prevent	Against: ↑ photoisomerization to inactive isomers with ↑ UV-B; 1,25(OH₂) Vit D-generation downregulated as serum Ca²⁺ increases
Vitamin D intoxication	Absence of molecular genetic correlates
Photodegradation of folic acid	Against: congenital neural tube defects too rare to influence reproduction rates
Skin cancers	For: melanin forms ‘caps’ over epidermal nuclei. Against: occurs too late to influence reproductive success
To improve	Against
Antioxidant defense	Melanin is a free radical absorber, but synthetic intermediates are themselves free radicals
Camouflage	No evidence for or against
Sexual display	No evidence for or against
Innate immunity	For Consistent with present hypothesis
Barrier function	Present hypothesis
Pigment de-evolved	Arguments for / against

To promote	Against
Cutaneous Vitamin D synthesis	No fossil evidence of rickets in early Homo (Industrial Age phenomenon) Clothing blocks more UV than pigment Sufficient vitamin D synthesis occurs in dark skin
Sexual selection	Against Reflects possible cultural bias
Metabolic cost of melanogenesis	For: increased polymorphisms in pigment-related genes in light-pigmented populations