Melanin and human evolution
Humans have evolved over time to successfully live in their surroundings and environment. The great variability in human skin colour has been attributed to evolutionary need, with humans evolving to adapt to the different climates and levels of ultraviolet (UV) exposure across the globe. The most important factor in determining skin colour is the pigment called melanin.
Melanin is a naturally produced polymer found in a multitude of locations in the body, in particular the eyes, ear, brain, hair and skin. There are three main forms of melanin in humans: eumelanin, pheomelanin, and neuromelanin. While neuromelanin is located in the brain, eumelanin and pheomelanin are found in the skin.
Melanin, the pigment contributing to skin colour
Figure 1. Cross section of human skin illustrating the epidermis with the basement membrane, the dermis, and main cellular components
In humans, eumelanin and pheomelanin are the main pigments responsible for different skin tones and, when not distributed evenly, are responsible for ephelides, commonly known as freckles. Eumelanin is a brown-black pigment, whereas pheomelanin is a yellow-reddish pigment.
Skin colour is both determined by the genetic background of an individual (constitutive skin colour) as well as environmental factors (e.g. exposure to UV and visible light; known as facultative skin colour), and varies according to:
- The ratio of eumelanin to pheomelanin; higher levels of eumelanin results in darker skin tone, while higher levels of pheomelanin results in lighter/fairer skin tone;
- The absolute amount and concentration of melanin in the skin; and
- The size of the packets that melanin travels in (melanosomes), the number of melanosomes, and how they are distributed within skin cells.
Figure 2: Melanin synthesis in the epidermal melanin unit
Melanin is produced in specialised skin cells known as melanocytes, located at the base of the epidermis (top layer of skin), through a process called melanogenesis, as illustrated in Figure 2. UV radiation, emitted either artificially (i.e. sunbeds) or naturally (sunlight), is one of the factors that regulates melanin synthesis, where upon exposure and cellular skin damage, melanocytes start producing melanin as a defence mechanism against further exposure. This reaction is commonly known as the tanning response. Cellular UV damage results in the release of alpha-melanocyte stimulating hormone (?-MSH), which binds to the melanocortin 1 receptor (MC1R) located on the surface of melanocytes, inducing the production of melanin within melanosomes which are transferred to the neighbouring keratinocytes. Although the number of melanocytes tends not to vary between individuals, the number of melanosomes, and subsequently melanin density, can vary dramatically, in particular according to the individual’s genetic background.
Eumelanin, the photoprotective pigment of the skin
Melanin is a biological chromophore, i.e. capable of selective light absorption. Eumelanin and pheomelanin have different properties; while eumelanin is a photoprotective pigment, providing protection against UV radiation and visible light, pheomelanin, due to its weak absorptive features, has photosensitising properties, potentially leading to greater sun/UV sensitivity and skin ageing.
Eumelanin and melanosomes provide skin photoprotection through several mechanisms, including the following:
- Melanin acts as a physical barrier or filter, where it is able to absorb incoming UV radiation, shielding the epidermis and dermis from potential damage.
- Melanosomes, which are the large organelles that contain melanin, are able to absorb and scatter incoming UV radiation, depending on their size. Melanosomes of a smaller size are able to scatter better, while absorption is favoured by a large surface area, i.e. larger melanosomes.
- Melanin is capable of dissipating UV radiation energy as heat.
- Melanin is capable of scavenging toxic molecules, called free radicals, that are generated by UV radiation, hence protecting the cells from being damaged by these compounds.
As explained above, the amount of UV radiation that is absorbed or scattered is determined by a number of factors which include the size, shape, and distribution of melanosomes, as well as the wavelength of the incident ray. The absorption spectrum of eumelanin is presented in Figure 3.
Figure 3. Absorption spectrum of eumelanin against specific wavelengths of incident radiation.
Eumelanin is best suited to absorb radiation in the UVA/B region, with peak range at approximately 340nm.
When melanin absorbs energy, electrons are raised to excited states and, in turn, pass the energy on to the cell in which the electrons are situated. Simon (2000) reported that most (99%) of the energy absorbed is dissipated ‘non-radiatively’ within a nanosecond of excitation. The cell is then assumed to use this energy to regulate its conditions and drive chemical reactions, a role similar to that of chlorophyll in photosynthesis.
References & further reading
- Thingnes J, Lavelle TJ, Hovig E, Omholt SW (2012). Understanding the Melanocyte Distribution in Human Epidermis: An Agent-Based Computational Model Approach. PLoS ONE 7(7): e40377. doi:10.1371/journal.pone.0040377
- Wasmeier, C., Hume, A.N., Bolasco, G. & Seabra, M.C. Melanosomes at a glance. J. Cell Sci. 121, 3995–3999 (2008).
- Yamaguchi Y et al. (2007). The regulation of skin pigmentation. J Biol Chem. Vol 282 (no 32). 27557-27561.
- Meredith, P and Sarna, T (2006). “The physical and chemical properties of eumelanin”, Pigment Cell Research. Vol 19, pp572-594.
- Pathak MA. Functions of melanin and protection by melanin. In: Zeise L, Chedekel MR, Fitzpatrick TB, editors. Melanin: Its Role in Human Photoprotection. Overland Park: Valdenmar Publ; 1995. pp. 125–134.
- Chedekel, Zeise and Fitzpatrick (1994). Melanin: Its role in Human Photoprotection. Valdenmar Publishing Company.
- Darwin, C (1859). On the Origin of Species. John Murray, Britain.