Study finds evidence of strong positive selection for skin, eye and hair pigmentation over last 5,000 years.
Why do people living in the tropics have dark skin whereas those living in higher latitudes have lighter skin? The traditional explanation is that is an evolutionary balancing trick between protection from skin cancer on one hand and the synthesis of Vitamin D by skin cells on the other. Dark skin results from higher levels of the pigment melanin: for those living nearer the equator, higher melanin levels provides a better protection from the sun’s more intense UV radiation; conversely, for those living at latitudes where UV radiation is weaker, the protection is not required and high melanin levels would block the production of Vitamin D.
However, things may not be quite so straightforward. A study carried out in 2012 at the University of Porto in Portugal considered alleles (variants) of four genes known to be associated with pigmentation, using samples taken from present-day Portuguese and sub-Saharan Africans. The evolutionary history of the four genes was estimated using a statistical model (Monte Carlo) to simulate the effects of genetic drift, natural selection and mutation. It was found that three of the alleles linked to lighter pigmentation did not start to sweep through European populations until around 11,000 to 19,000 years ago – at least 40,000 years after modern humans left Africa (Beleza, et al., 2012).
Two recently-published studies have investigated ancient DNA extracted from prehistoric human remains in Europe. The first study, published in the journal Nature, considered the pigment genes of DNA obtained from human remains found at the Mesolithic cave site of La Braña-Arintero, near León, Spain. The remains were identified as those of a male hunter-gatherer, who lived 7,000 years ago. He was found to have been dark-skinned and blue-eyed. Although present-day Spaniards are darker-skinned than northern Europeans, they are far paler than Africans (Olalde, et al., 2014). The result again suggests that paler skin colour was a fairly late development.
The second study, published in the journal PNAS, considered the pigment genes TYR, HERC2 and SLC45A2. TYR produces the enzyme tyrosinase, which is used as a catalyst in the production of melanin. HERC2 is responsible for determining eye colour; and SLC45A2 is involved in the distribution and processing of tyrosinase and other pigment-producing enzymes. The various alleles of these genes are responsible for different colours of skin, hair and eyes. Researchers extracted ancient DNA from 63 Chalcolithic (6500 to 5000 years old) and Bronze Age (5000 to 4000 years old) individuals from Ukrainian sites on the Pontic-Caspian steppe. 43 individuals yielded DNA from which the pigment genes could be sequenced, and these were compared with those of present-day Ukrainians.
The researchers found that the pigmentation of the prehistoric population differed from that of the present-day Ukrainians. The latter have 8.5 times as many alleles of TYR related to light skin colour as did their prehistoric forbears. Alleles of HERC2 related to blue eye colour were also far more common in the present-day population. However, none of these lighter pigmentation alleles are present in African populations. Thus it seems that the shift to lighter pigmentation was underway in the Chalcolithic and Bronze Ages, but it was at that stage incomplete – despite the immense passage of time since modern humans had left Africa. Computer simulations showed that these effects could not be explained by genetic drift alone, and that natural selection must have been a factor (Wilde, et al., 2014).
The team speculated that selection for lighter skin colour was related to the change in diet following the arrival of agriculture. The diet of hunter-gatherers was more likely than that of the farmers to include items rich in Vitamin D, such as fish and liver. To make up the difference, individuals needed to be able to synthesise it more efficiently – hence a lighter skin colour. At the same time, the trend to lighter hair and eye colour may have been the result of sexual selection: the initially-unusual colouring might have been more attractive to the opposite sex (the researchers noted that this phenomenon has been documented in guppies).
- Beleza, S. et al., The timing of pigmentation lightening in Europeans. Molecular Biology and Evolution 30 (1), 24-35 (2012).
- Olalde, I. et al., Derived immune and ancestral pigmentation alleles in a 7,000-year-old Mesolithic European. Nature 507, 225-228 (2014).
- Wilde, S. et al., Direct evidence for positive selection of skin, hair, and eye pigmentation in Europeans during the last 5,000 y. PNAS (Early Edition) (2014).