Seems like a pretty simple question. However, it is a question that is not particularly simple to answer. Particularly because there are a lot of small points that must be covered for a complete picture to be painted.
In the most basic terms, melanin is a pigment that is found in nearly every living organism, from turtles to birds to lions to people. Its function is to protect organisms from dangerous levels of ultraviolet radiation from sunlight and other UV sources. Its properties make it an excellent photoprotectant, and through the process of ultrafast internal conversion it is able to dissipate more than 99.9% of otherwise damaging radiation as heat.
Melanin is produced by cells called melanocytes, which are found in the layer of skin just below that of melanin, the stratum basale of the epidermis. Two subtypes of melanin determine skin tone, eumelanin and pheomelanin. Eumelanin is more common and consists of a brown-black polymer, while the genes required for pheomelanin are rarer and are responsible for a dark red color.
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Causes of Different Skin Tones
While the skin tones of an individual at birth is determined by genetics, over the course of their life they will alter the levels of the different types of melanin and end up with a distinctly different skin color than that with which they were born. Children born with light hair often end up several shades darker in their first few years, as ultraviolet light absorbed from the sun causes more melanin to be produced, which in turn cause higher levels of dark pigment to be built up in the hair cells.
Those who spend time in the sun developing their tan have actually caused mild damage to the structure of their DNA. Over exposure to the suns harmful UV rays stimulates a process called melanogenesis, in which the body produces more melanin in order to protect itself from further damage.
Melanogenesis is believed to be connected to the darker skin of equatorial races and the lighter skin of more northerly tribes.
Melanin is created by cells with the help of the amino acid tyrosine, which is found most commonly in high protein foods. Tyrosine enzyme is also responsible for photosynthesis in plants, which is interesting considering the photosensitive cells it is responsible for in animals. While it has been found in all vertebrate species, it has not been found in spiders, and its existence in the bacterial families is still being debated.
Eumelanin and Phenomelanin
Melanin is an aggregate of smaller component molecules. Because that means nothing to most people, a slightly more tangible answer is that it consists of two types of melanin: eumelanin and phenomelanin. The two types are generally found together, although not in the same concentrations.
Eumelanin is more pervasive and consists of black and brown variations. Darker skinned races have more black eumelanin while lighter skinned races have more brown eumelanin. It is responsible for brown, black, blonde, and grey hair, and is found in all hair, skin, and areola cells. Gray hair results when this is the only pigment found in the hair cells, and is linked to the melanin producing cells melanocytes shutting down as we age.
Phenomelanin is redder in color and is much rarer than eumelanin, generally collecting in the lips and nipples, those fun parts everyone likes. Phenomelanin in large quantities is responsible for red hair and freckles, but unfortunately can become carcinogenic if exposed to too much ultraviolet radiation.
Because this type of melanin is most common in lighter skin, these organisms are more susceptible to sunburn and its many possible complications such as skin cancer, especially as they age.
A Brief History of Skin Color Variations
Skin tone colors and their wide variety are thought to be caused by a combination of migratory patterns and the adoption of agriculture around six to ten thousand years ago. Because more melanin is needed to protect from particularly high levels of ultraviolet radiation, as people moved north and were exposed to less sunlight, their skin began to lighten.
At the same time, while transitioning from the hunter gatherer lifestyle to the non-migratory agricultural system, some foods that were particularly high in vitamin D were removed from our diets. Because a certain amount of ultraviolet light must be absorbed for the body to make vitamin D efficiently, even less melanin was produced in order to absorb sufficient sunlight to offset the change in our diets.
Because this transition generally took place at higher latitudes after an extended period of migration, today we see darker skinned people near the equator, and lighter skinned people further away from it.
However, the common link that controls the pigment of our entire species is clear in that both light and dark skinned races are able to be affected by albinism, due to the melanin in charge of pigment production being controlled by the same gene and amino acid.
Back to the Original Question
So, getting back to the original question: “What is Melanin?” A simple answer may be that it is the product of cells that play a large part in determining what color our skin is and how well we are protected from ultraviolet radiation.
Because its genetic component and influence over a major structure of the body, its importance is matched by the difficulty of predicting possible issues an imbalance or defect may cause.
Albinism, for example, is caused by a recessive gene which causes melanin to produce little pigment in the case of albinids and none at all in the case of albinos.
The gene that is most likely to cause this has been identified, but because of the long list of other traits that are also dependent on that amino acid, it can be difficult to predict if an organism will display these traits, or if the gene will simply sit in the DNA and do nothing.