As a population we have had a long time to change, evolve, and adapt to our environments resulting in the variety of shapes, colors, and sizes we see today. Why is it that people evolved to be so different? For example, everyone has skin, so why should skin be so different from one person to another?
If you know about genetics, you might quickly point out that genes are passed on from generation to generation, and that genes that result in traits that make an organism better adapted succeed (think, survival of the fittest). But, it is not that simple, there are plenty other things to take into account that complicate matters immensely.
For example, research suggests that there is a link between autism and schizophrenia. There is no one single gene for autism or schizophrenia then, how are they related? It turns out it may not be exactly a genetic link, at least not in the sense that gene A is passed on and then you have schizophrenia or autism. Let’s leave that for a future post.
Another important question to consider about genetics is the genetic history of a species. In other words, the historical progress of genes from the most ancient species in a lineage to the most recent.
What is the evolutionary history of human skin color?
A matter of Skin
Skin is the outermost protective layer of the human body. Humans are characterized by mostly absent hair (as opposed to fur), although most of the skin has hair follicles.
Evolutionary speaking, skin is the result of modifications to ancient reptilian-like scales. The first fossil evidence of hair is in Castorocauda, small, semi-aquatic mammal, from 164 million years ago.
Let me emphasize that reptilian scales were modified into mammalian hair and avian feathers, and these modifications also lead to the evolution of nude skin. Since the research is still unable to answer the how? question precisely I leave you with a forum question with a brief outline of hair evolution (from Biology Stack Exchange) and if you are truly interested in the question here is a link to an article on a A new scenario for the evolutionary origin of hair, feather, and avian scales.
.The molecule of color: Melanin
Melanin is a dark biological pigment found in skin, hair, feathers, scales, eyes, and some internal membranes. There are many kinds of melanin pigments that help give skin and hair its color, in addition to neuromelanin which is present in some neurons in the brain, but no known function has been identified yet.
Melanin pigments are deposited in skin cells through specialized vesicles formed by melanocytes, pigment producing cells in the skin, eyes, inner ear, meninges, bones, and heart.
The presence of melanin increases in the presence of sunlight capturing most of the UV radiation. It acts like a natural sunscreen and prevents the development of cancer. In other words, tanning is a protective measure of your body against UV radiation.
Human evolution is a messy story, there is no one linear path for our development. The general story is this: Homo sapiens arose in Ethiopia about 195,000 Years Ago. At this point, our ancestors walked upright and had large brains, we then migrated out of Ethiopia to populate the rest of the world. In our evolution, we eventually lost most of the hair that covered our bodies.
The absence of melanin in the skin results in an increased chance of developing skin cancer, that is why research by Professor Melvin (Mel) Greaves at the Institute of Cancer Research in London suggests that melanin production was a selective advantage for individuals in Africa when our ancestors began losing their hair. According to Professor Greaves, humans originally had light colored skin and eventually evolved to produce melanin and lost the trait once they left Africa and went north.
However, other research also suggests that light skin is a mutation that evolved after dark skin. If you consider that blond hair is produced with a small amount of brown eumelanin, one of the many types of melanin, in the absence of other pigments, you would need a mutation that decreased the other pigments, then another that decreased the amount of brown eumelanin. Those are too many changes if light skin had evolved first.
Also, there are other examples in other animals that suggests dark skin or scales are more likely to evolve into light skin or scales, than the other way around.
As the early humans migrated out of Africa, into a colder climate, with less sun exposure, melanin production is less likely to occur and their diets changed. Vitamin D is not produced in many foods, but exposure to sunlight stimulates the skin to produce this vital compund.
Vitamin D deficiency is more likely to occur in dark skin people living in colder climates that receive little sunlight. This occurs because the melanin present in their skin captures the light efficiently leaving little else to stimulate the other skin cells. For this reason, light skin was most likely selected for in areas further away from the equator.
Note that Inuit and other Aboriginals living in northern climates have a diet rich in Vitamin D coming from fish, this would help them maintain production of melanin that would also protect them from light reflected from the snow.
Human skin pigmentation is the combination of diet and the adaptation to the local environment when early populations began settling down. Humans are a group of highly adapted organisms and skin pigmentation is only one part of their historical evolution.