Why are you white and am I black? Where Does My Skin Color Come From?

 

What we perceive as skin color is mainly determined by the amount of a particular pigment produced in our skin—melanin.

Skin pigmentation is not a simple trait. It's a complex trait because more than 100 genes affect skin pigmentation. These multiple genes contribute to skin color, and someone has multiple mutations within each of those genes that can lead to similar effects. We then also have to account for the two copies of the genes and the interactions between genes.

Our skin, the largest organ of our body, is made up of three layers: the epidermis, dermis, and hypodermis. The top layer of our skin, the epidermis, is where the magic happens. Right at the base of the epidermis, we find star-shaped cells called melanocytes.

Melanocytes are the cells that produce melanin, our skin pigment. So how is melanin produced? Melanocytes make little things called melanosomes. These are little melanin-producing factories that get transferred from the melanocyte to surrounding keratinocytes. There, melanin provides protection from UV radiation and determines the color of our skin.

We have the melanin gene. It’s not just one gene, there are many. And we all have the genes for melanin. It's just that we have different variants. Genes are DNA segments containing information about specific traits like eye color, height, and blood type. Genes are made up of molecular building blocks called nucleotides. And the exact sequence of those nucleotides can vary between individuals. Different versions of that genetic sequence can be called genetic variants. When it comes to color, we know quite a few variants that affect pigmentation, and at least 100 have been identified and studied. Some genetic variants only affect eye color or hair color. Others affect the entire melanin synthesis pathway by increasing or decreasing the total amount of melanin that we produce and changing body-wide pigmentation. But things are more complicated than just making more or less melanin because there are two different types of melanin; the reddish-yellow pheomelanin particularly evident in people with red hair, and the brown-black eumelanin, which explains the range of variation from lightly to darkly pigmented skin.

The type of melanin, the amount of melanin, and the number of melanosomes vary from person to person and are dependent on our individual genetic makeup, as well as the environment and our age.

Skin color isn't organized along continental lines. Rather, it maps onto global distributions of UV radiation. People in various regions of the world have skin pigmentation that reflects how their ancestors adapted to their environment. The earliest hominin ancestors would likely have had light skin underneath their dark fur. This is the case for our closest living genetic relatives, Chimpanzees. During the course of human evolution as our hominin ancestors lost their fur, their skin was the last barrier between them in the sun. Losing the protective coverage of our fur meant that our formerly lightly pigmented ancestors had to evolve skin that was rich in melanin to protect themselves from UV radiation, but as those early humans moved into areas with less UV radiation, they faced a trade-off. Melanin limits the amount of radiation that passes through the epidermis, which is great for protecting our DNA from UV damage. Reducing the amount of UV radiation that passes through our skin can have some negative consequences, specifically relating to the production of vitamin D. 

Our bodies can convert a precursor of vitamin D in our skin to its active form using UV radiation, specifically UVB. There's lots of evidence that Vitamin D plays a crucial role in immune function and bone health, among many other things. So as humans moved through regions of the world with varying UVR intensities, natural selection played a careful balancing act of keeping skin melanated enough to protect against the harmful effects of UVR, while keeping it de-pigmented enough to produce sufficient vitamin D.

When we look at the distribution of skin pigmentation around the world, we see that there's quite a bit of variation within the continents. So it doesn't make sense to equate such large regions of the world to particular skin color. We see that there are people across continents that have similar skin pigmentation, and we see people within the same continent that have very different skin pigmentation. An important thing to keep in mind is the distribution of skin color for populations whose ancestors have remained in the same region for long enough to be affected by natural selection.