Skin colour varies according to the latitude and, therefore, according to the intensity of incident ultraviolet light: individuals living at low latitudes have darker skin, whereas those living at high latitudes have pale pigmentation.
Skin colour varies according to the latitude and, therefore, according to the intensity of incident ultraviolet light: individuals living at low latitudes have darker skin, whereas those living at high latitudes have pale pigmentation. Saioa López, a researcher at the UPV/EHU, has studied why this depigmentation has taken place. In her research she concluded that evolution is furthering mutations that are lightening the skin, probably owing to the need to synthesise vitamin D at latitudes where there is reduced solar irradiation (compared with Africa) although, in turn, this increases the probability of developing melanoma or skin cancer. Specifically, she has identified two mutations that cause skin to become paler. And she has also identified new genes and genetic variants that could be linked to the differences in pigmentation in our population.
The first hominids that appeared in Africa probably had pale skin covered with hair, like other primates. They are thought to have lost their hair when they became bipedal, and that natural selection subsequently furthered darker skins in Africa as they protect against ultraviolet (UV)light. However, when humans left Africa (about 100,000 years ago) and headed for Asia or Europe, where UV intensity was lower, they once again acquired a less pigmented skin colour. What caused the depigmentation of these populations is not clear, and two hypotheses have in fact been put forward: firstly, it could be due to a relaxing in natural selection that keeps the skin dark in Africa, since when leaving Africa, UV levels are lower; secondly, it could be due to the fact that natural selection furthers certain mutations so that individuals can have paler skin, since at these latitudes having a skin with a dark pigmentation prevents the synthesis of adequate levels of vitamin D, essential for our survival.
Saioa López, a researcher at the Department of Genetics, Physical Anthropology and Animal Physiology of the UPV/EHU’s Faculty of Science and Technology, has carried out research into this subject. The main aims of her research work were to identify the selective pressures that have guided the evolution of this trait, as well as identify new genes and genetic variants responsible for pigmentary differences between individuals. More specifically, the aim was to show that depigmentation has been an adaptive process furthered by natural selection. The methodology used covered a whole host of techniques, including those relating to molecular and cell biology and bio-computing.
Through all this two mutations that functionally cause the lightening of the skin in our population were identified. Highly significant evidence was found to suggest that natural selection is positively furthering these mutations, which are being maintained in the population to produce a paler skin colour. What is more, melanoma samples were analysed and it was observed that these same mutations lead to an increase in susceptibility towards developing melanoma, in other words, the most aggressive, deadly skin cancer.
Why does natural selection further a mutation that causes cancer?
“If natural selection is furthering these mutations that lighten the skin,” explained López, “it is because there has to be some advantage for the individuals, probably enhanced synthesis of vitamin D.” Vitamin D can be obtained through the diet but also in an indirect way through a process in the skin in which UV light intervenes. Dark skins contain a pigment (melanin) that acts as a barrier and prevents UV rays from penetrating. At high latitudes where the intensity of UV light is very low, this is a problem, as insufficient quantities of vitamin D are synthesised.
Vitamin D is essential for skeletal mineralisation and development, and the lack of this vitamin can lead to various problems in children’s health. Yet melanoma is a cancer that tends to appear in adult life following the reproductive phase. From the evolutionary point of view, as these adult individuals have by now produced offspring, they are no longer important in evolution.
The above story is based on materials provided by University of the Basque Country. Note: Materials may be edited for content and length.
- S. López, O. García, I. Yurrebaso, C. Flores, M. Acosta-Herrera, H. Chen, J. Gardeazabal, J. María Careaga, MD Boyano, A. Sánchez, JA Ratón-Nieto, A. Sevilla, I. Smith-Zubiaga, A. García de Galdeano, C. Martinez-Cadenas, N. Izagirre, S. de la Rúa C and Alonso. The Interplay between Natural Selection and Susceptibility to Melanoma on Allele 374F of SLC45A2 Gene in a South European Population”. PLoS ONE, 2014; 9: e104367