The idea of having a different skin colour to someone else is often used to differentiate between people and separate people into groups. Our brains aim to find a difference between people and find ‘our own people’. However, skin colour actually indicates what geographical region our ancestors lived in rather than our behaviour. It’s just a result of an evolutionary process – like how some people have free earlobes and other have attached earlobes.
Melanin is a black pigment that is made from dopamine and tyrosine with the help of enzymes.
Photo Credit: The Cosmetic Chemist
Melanin is found in melanocytes (specialised cells) in the hair, skin, ears and iris. These cells produce melanin and contain it into melanosomes. This process is called melanogenesis.
Photo Credit: Science Direct
Once the melanosomes are mature they are transferred to other cells called keratinocytes via the dendrites. Keratinocytes are found in the epidermis of the skin. The exchange regulates the production of melanin. Keratinocytes ensure there is a correct pH and supply nutrients for melanocytes to function.
There are also two different types of melanin found in the skin:
- Eumelanin which creates brown skin tones
- Pheomelanin which creates reddish hair and fairer skin tones
There is another found in the brain called neuromelanin too.
The type of melanin produced depends on the enzymes that are involved in the process. Tyrosinase is an enzyme that controls melanin production. In plants, such as potatoes, it causes them to turn black when exposed to air. Lighter skinned melanocytes are more acidic and have lower activity of tyrosinase. Tyrosinase works better at a more neutral pH so in darker skinned individuals there is a neutral pH and higher tyrosinase activity. Higher tyrosinase activity means more melanin production.
Sunlight plays a large role in this. A greater exposure to UV light will trigger light sensitive receptors called rhodopsin to signal the production of melanin. Rhodopsin will convert the light into an electrical impulse. As a result, those with a greater exposure to the sun (such as in Africa, Asia and South America) will produce far more melanin, causing more pigment.
Evolution of skin pigmentation
What is even the point of melanin? Well having melanin whilst living in hotter countries will give you an evolutionary advantage. When evolution was taking place, humans did not have sunscreen to protect them from the sun. Lack of protection could lead to sunburn, skin cancer and other complications. Melanin acts as a ‘built in’ sunscreen to protect you by absorbing or deflecting UV rays. As humans lost fur (due to evolution) they got darker in order to accommodate for the increasing UV ray exposure.
All humans originated in Africa but some humans migrated to the North which is where the differentiation takes place. In Northern regions people have lighter skin. This is also an evolutionary advantage. Melanin will ‘block’ the suns rays. The sun does give us some benefits. We need the sun in order to make Vitamin D3 from a chemical called 7-dehydrocholesterol. D3 allows us to absorb vital minerals for our bones and without it we could develop rickets (a severe weakening of bones). So in Northern regions, there was less exposure to sun. Hence, it seemed counter productive to produce melanin to block the sun when there was little of it. There is a lowered risk of developing skin cancer but a higher risk of developing rickets. As a result, the body evolved to produce less melanin to allow greater exposure to the sun and Vitamin D.
You can read Part 2/2 here: https://glamsci.blog/2021/02/26/the-sun-and-skin-part-2-2-sunburn/