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 because darker skin protect against ultraviolet (UV) light.When humans left Africa about 100,000 years ago and moved toward Asia and Europe where UV light intensity was much lower, they reverted to lighter skin color. What caused this? Scientists now say it was natural selection related to vitamin D levels and have found the gene mutations and other evidence to show it.
Above and above right: worldwide skin color distribution
Natural Selection Is Furthering Mutations That Are Making Skin Paler
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.
Saioa López (Barakaldo, Basque Country 1986) is a graduate in Biology, and has done a master's and a PhD in Molecular Biology and Biomedicine. She wrote up her PhD thesis, Identification of genes and biological pathways involved in human skin pigmentation variability in an evolutionary framework, in the Human Molecular Evolution research group of the Department of Genetics, Physical Anthropology and Animal Physiology of the UPV/EHU’s Faculty of Science and Technology under the Supervision of Santos Alonso and Isabel Smith. Right now, she is a post-PhD researcher at the UCL Genetics Institute (University College London). To conduct her research, she had the collaboration of the UPV/EHU's Faculty of Medicine and Odontology, the Department of Medicine of the University of Castellon, the University Hospital Nuestra Señora de la Candelaria (Tenerife), the Forensic Genetics Department of the Ertzaintza (Police Service of the Basque Autonomous Community) in Bizkaia, and the Dermatology Services of the Hospital de Cruces and Hospital de Basurto and of the Biomedical Research Institute of Valencia (INCLIVA).