The world famous french businesswoman Coco Chanel had once said, ‘In order to be irreplaceable one must always be different.’ And not just in the philosophical sense, but also amongst the evolutionary biology, we can agree that in a way she was correct.
Variations in the gene pool of a species allows it to survive in a wide variety of conditions by enabling some of the indivisuals to adapt to the enviroment. Genetic variation is also an important factor of evolution that allows natural selection to increase or decrease the frequency of alleles in the population.
And we humans, the most dominant species of earth, like all other species here, have a wide variety of variations amongst our gene pool. One of the most conspicuous of which is our skin color. A trait that varies from dark ebony to the lightest of hues based on one’s parentage or the exposure to the sun, or both.
So, through this article let us understand the science behind the variety of skin colors found in diffrent races of human species and how this difference in the pigmentation occured.
The ancient skin tone:
Around 5-7 million years ago, the evolution of bipedalism in Hominids bought along another vital change in this species –‘the loss of body hair.‘
An adaptation that facilitated perspiration through increased number of sweat glands and helped keep the body cool under the direct sunrays. But this hair-less skin also posed a problem – exposing the early hominids to the strong and direct UV radiation of the sun, that fell on the equator where these hominds lived.
Thus, around 1.2 million years ago, emergence of dark skin pigmentations occured in order to protect the skin from damaging rays.
Role of vitamins in the evolution of skin pigmentation:
An important theory for the evolution of various skin tones in humans was based on the conflicting need of two vitamins: Vitamin D and folate.
Folate is a chemical compound easily destroyed under the Sun’s UV radiation, while the same radiation stimulates the production of vitamin D. So, the sun rays must reach us in moderate amount, that won’t destroy the folate and at the same time also be enough to produce vitamin D. Now, we could not control the intensity or the amount of UV light reaching us for obvious reasons, but through the pigmentation of melanin, our body is adapted to control the amount of sunlight that would penetrate our skin. So, the higher will be the melanin deposits on our skin, the darker it will appear and the lesser UV-B will be able to penetrate it.
Why is folate important?
Folate is an essential vitamin that plays an important role in DNA activities. In case of folate deficiences in pregnant mothers, it can cause fatal disorders like spina bifida, where the vertebrae do not fuse around the spinal cord. In other words it can be said that folate plays an important role in evolutionary mechanisms – where it effects one’s ability to survive and reproduce in the next generation.
Through decades of research, it has now been proved that folate breaks down in blood plasma under sunlight. However, it doesn’t happen this easily in darker skinned people, because of the presence of a pigment melanin that absobs UV rays.
Melanin and pigmentation:
Melanin in humans, is a pigment produced by melanocytes and then stored in melanosomes, present in the basal layer of epidermis, which acts as the primary determinant of skin and hair color. It’s mainly of two types-
- The eumelanin, which is the cause of a range of brown skin tones and black, brown and blond hair.
- The pheomelanin, which imparts reddish pigments as seen in lips and is also responsible for the freckles and red hair.
In response to both – UVA and UVB melanocytes show distinct response by triggering pigmentation involved in melanin production. This is done by DNA damage repair pathway where due to DNA damage, P53 tumor supressor protein gets activated and stabilised in keratinocytes. This leads to the production of MSH (melanocytes stimulating hormone). MSH hormone then binds to MC1R receptors present on the melanocyte cells producing a cyclic AMP response. cAMP stimulates protein kinase A, which then phosphorylates CREB (cAMP response element transcription factor), resulting in the upregulation of enzymes like tyrosinase that converts tyrosine into melanin. Tyrosinase enzyme first converts tyrosine into a compound called Dopaquinne which can either be converted into eumelanin or into cysteinyl-dopa, which is then converted into pheomelanin.
The reflex response of the skin to produce melanin on encountering UVR helps protect the skin from burning, DNA damage or even fatal consequence like skin cancer, because melanin absorbs and reflects UV-R and also acts as an anti-oxidant against reactive oxygen species.
The great human migration that led to different skin tones:
Some 2 million years ago with the great human migration, the early hominids migrated out of Africa and dispersed throughout the globe. Settling down in both, higher and lower latitudes of tropics, places with less sunlight than what was encountered in Africa. Which, therefore caused a big problem for vitamin D synthesis in our dark skinned ancestors that ventured out into such areas.
Vitamin D just like folate is an important compond for the survival and reproductive ability of an individual since it plays important role in Calcium absorption, formation of healthy bones and immunity. Thus, in the absence of enough UV-R that could penetrate the dark skin to stimulate production of vitamin D – a range of skin colors that we see today, evolved at different times throughout history and in different lineages as the humans settled in different parts of the world, in varying eco-geographical conditions.
While the Africans and others living near the equator adapted to higher melanin and thus got a dark skin, the caucasians over time adapted to low amounts of melanin in their skin, since they lived near the pole where intensity of sunlight is low and the asians adapted a range of skin tones between the two.
Conclusion:
In the last few years with the invent of new technologies skin lightening therapies have suddenly caught on, people with darker skin specially in southern asiatic countries are going through painful laser (like ablative and Q switched ruby laser) treatments that dissolves the upper layer of the skin, every alternative months just to appear light skinned. Which only goes on to show the current mentality of our society. Skin color is one of most superficial ways of differentiating humans and somehow over time it has became a reason for discrimination against us. Erecting walls where there shouldn’t be one! And now more than ever it’s important to remember that at the end it’s just a trait, like any other, not a reflection of character or ability.
References:
- Brenner M, Hearing VJ. The protective role of melanin against UV damage in human skin. Photochem Photobiol. 2008;84(3):539-549. doi:10.1111/j.1751-1097.2007.00226.x
- https://www.evo-ed.org/Pages/Skin/genesproteins.html