On Skin Colors

Approximately 1.5 million years ago, the earth endured a megadrought that drove hominids from lush rainforests into arid, open landscapes. This, coupled with the loss of dense body hair, caused early human skin to endure excess UV-B radiation and xeric stress (Elias et al., 2010). Rogers et al. (2004) performed an examination of the variation in MC1R nucleotide sequences for people of different ancestry and compared the sequences of chimpanzees and humans from various regions of the Earth. Rogers concluded that roughly five million years ago, at the time of the evolutionary separation of chimpanzees and humans, the common ancestors of all humans had light skin that was covered by dark hair. Additionally, our closest extant relative, the chimpanzee, has light skin covered by thick body hair. Over time human hair disappeared to allow better heat dissipation through sweating (Jablonski, 2000).

When it comes to skin tones, the social significance of differences in skin color has varied across cultures and over time. Von Luschan's chromatic scale is a method of classifying skin colors. It consists of 36 opaque glass tiles which were compared to the subject's skin, ideally in a place which would not be exposed to the sun (such as under the arm). The scale was used extensively throughout the first half of the 20th century in race studies and anthropometry. However, it was considered problematic, even by its practitioners, because it was very inconsistent. In many instances, different investigators would give different readings of the same person.

The scale was also used to establish racial classifications of entire cognate population according to skin color.

Recent study investigated by Quillen et al (2011) examined signatures of selection at 76 pigmentation candidate genes that may contribute to skin pigmentation differences between Indigenous Americans and Europeans. Analysis was performed on two samples of Indigenous Americans genotyped on genome-wide SNP arrays. Using four tests for natural selection-locus-specific branch length (LSBL), ratio of heterozygosities (lnRH), Tajima's D difference, and extended haplotype homozygosity (EHH)-we identified 14 selection-nominated candidate genes (SNCGs). SNPs in each of the SNCGs were tested for association with skin pigmentation in 515 admixed Indigenous American and European individuals from regions of the Americas with high ground-level ultraviolet radiation. In addition to SLC24A5 and SLC45A2, genes previously associated with European/non-European differences in skin pigmentation, OPRM1 and EGFR were associated with variation in skin pigmentation in New World populations for the first time.

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Ellia, P.M. et al. (2010). Barrier requirements as the evolutionary "driver' of epidermal pigmentation in humans. American Journal of Human Biology 22 (4): 526-537. DOI: 10.1002/ajhb.21043. 

Jablonski, N.G., et al. (2000). The evolution of human skin coloration. Journal of Human Evolution 39 (1): 57-106. DOI: 10.1006/jhev.2000.0403

Quillen E.E., et al. (2011). OPRM1 and EGFR contribute to skin pigmentation differences between Indigenous Americans and Europeans. Human genetics. PMID 22198722

Rogers, A.R., et al. (2004). Genetic variation at the MC1R locus and the time since loss of the human body hair. Current Anthropology 45: 105-8. DOI: 10.1086/381006