Evolution of Skin Pigmentation Differences in Humans

Abstract

Human skin pigmentation is a complex trait that evolved as an adaptation to local environmental conditions. The distribution of human skin colour is highly correlated with the intensity of incident ultraviolet radiation. Several hypotheses that assign a key role to natural selection have been proposed to explain the global distribution of human skin pigmentation, such as photoprotection, protection against low vitamin D levels and protection against folate deficiency. The genetics underlying skin colour variability is also being deeply investigated. A handful of genes have already been reported to be associated with the normal pigmentation variability and/or to be under the action of natural selection: TYR, TYRP1, TYRP2, OCA2, SLC45A2, SLC24A5, MC1R, ASIP, MITF, KITLG and, recently, β‐defensins. Genome‐wide scans have suggested many other candidate genes with a role in pigmentation variability. However, functional studies should be carried out in order to ascertain the significance of these findings.

Key Concepts:

  • Human skin colour evolved as an adaptation to local environmental conditions.

  • Dark skin probably originated after the loss of fur by our ancestors in Africa.

  • A highly pigmented skin reduces the damaging effects of UV radiation, such as skin cancer, sunburn or folic acid photolysis.

  • Light skin probably originated after the Out‐of‐Africa exodus, of modern Homo sapiens into Eurasia, approximately 100 000 years ago.

  • Skin depigmentation at higher latitudes occurred independently in Asia and Europe.

  • Lighter skins allow UV radiation to penetrate the skin to catalyse the synthesis of vitamin D.

Keywords: skin pigmentation; evolution; adaptation; depigmentation; natural selection; melanogenesis; photoprotection; vitamin D

Figure 1.

Schematic outline of melanogenesis. UVR stimulates the expression of POMC by keratinocytes. The peptide produced is the precursor of the hormone α‐MSH, that binds to MC1R in melanocytes. This union leads to an increase in cellular cyclic adenosine monophosphate (cAMP) that in turn leads to increased levels of MITF expression, which upregulates the transcription of TYR, TYRP1 and TYRP2, producing brown‐black eumelanin. In the absence of α‐MSH, the antagonist ASIP binds to MC1R, and phaeomelanin is synthetised instead. Melanosomes are then transferred to keratinocytes through the dendrites via a shedding vesicle system proposed by Ando et al. ().

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López, Saioa, and Alonso, Santos(Jun 2014) Evolution of Skin Pigmentation Differences in Humans. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0021001.pub2]