Epidermal pigmentation in the human lineage

Epidermal pigmentation in the human lineage is an adaptation to ultraviolet radiation

Nina G Jablonski & George Chaplin 2103
Journal of Human Evolution 2013 Nov;65(5):671-5. doi: 10.1016/j.jhevol.2013.06.004.

Source - http://dx.doi.org/10.1016/j.jhevol.2013.06.004

Marc Verhaegen

Source - http://groups.yahoo.com/neo/groups/AAT/conversations/messages/61219



Elias cs (2009, 2010, 2013) hypothesized that
a) epidermal pigmentation evolved primarily to enhance the barrier functions of the epidermis:increased melaninization of the skin evolved to provide "a competentpermeability barrier, [as] a requirement for life in a desiccating terrestrial environment".
They cite evidence that
- darkly pigmented skin possesses a more competent skin barrier than lightly pigmented skin (Gunathilake cs 2009),
- it prevents trans-epidermal water loss (TEWL) when humans are traveling over long distances.
b) darkly pigmented skin is more resistant to infections, because it is
- drier,
- more acidic, 
- capable of producing more anti-microbial peptides (Wassermann 1965,Mackintosh 2001). 
c) reduced epidermal pigmentation at higher latitudes was determined by natural selection for reduced levels of the protein filaggrin (FLG) in the stratum corneum, leading to enhanced cutaneous synthesis of pre-vitamin D3 in extreme northern latitudes.
d) a 2d driver of depigmentation was the "ever-present imperative to conserve energy": retention of genes for eumelanin production was no longer necessary under low UVR conditions, and mutations to "weed out energy-consuming processes [became] beneficial, and favored by natural selection."

The barrier properties of darkly pigmented skin

Their central claim is
- darkly pigmented skin possesses a more competent permeability barrier than lightly pigmented skin,
- evolution of genes promoting dark pigmentation occurred under xeric conditions & the threat of high TEWL.
The proximate mechanism for the superior barrier functions of darker skin is proposed to be acidification of the stratum corneum (SC) by transfer of more eumelanin-containing melanosomes from the dendrites of darkly pigmented melanocytes into the outer epidermis (Gunathilake cs 2009, Elias & 2013). 
One of the most consistent findings to emerge from recent comparative studies of the barrier functions of the human SC is that darkly pigmented African or African-American skin exhibits the highest rate of TEWL among modern humans (Reed cs 1995, Wesley & 2003, Rawlings 2006, Muizzuddin cs 2010). 
It is not less "leaky", especially under dry conditions (Elias & 2013:688). 
What comparative studies (incl.many by Elias) consistently have revealed is that the skin of darkly pigmented Africans & African Americans has a more compact SC with significantly thicker corneocyte envelopes
(Gunathilake cs 2009, Muizzuddin cs 2010).
These properties have nothing to do with pigmentation per se, but with the high amount of covalently bonded proteins & the enhanced rigidity of the corneocyte envelope (Muizzuddin cs 2010).

Skin pigmentation is most strongly correlated with UVR, not aridity