In this article, let’s see how the genetics of skin color together with the influence of the environment can lead to the evolution of human skin color.
According to Nina Jablonski, the human brains are gray while the human blood is red, and our bones are off-white. Well, it doesn’t matter where you were born or where you are born or to whom you are born. But the human skin is different. Some of us do have rich, dark brown skin. Meanwhile, some of us have pinkish white skin while most of us are somewhere in between.
For the longest time now, the reason this variation exists was a real scientific mystery that has opened the door for some to invest in this biological trait with moral value, and then use that to justify the suffering of others. But biological traits aren’t just good or bad.
What are they then?
Biological traits are simply features that have evolved with time because they enhance an organism’s odds of surviving and passing onto its genes. Like some other animal traits, the sepia rainbow of human skin color evolved through natural selection.
Now, thanks for the advancement in anthropology and genetics, exactly how and why it did is no longer a mystery.
Biological anthropologists spend their lives studying how humans evolved and why we differ physically from one another. Our skin provides one of the most visible markers of human variability. It’s something that sets us apart from our closest animal relatives.
Under their dark fur, chimpanzees have pale skin. And millions of years ago, that was probably also the case for the primates that were our common ancestors.
So, Where Did Humanity’s Range Of Skin Colors Come From?
Taking from physics, we all know that the color of any object comes from the wavelengths of light that it reflects back to an observer’s eye.
We see leaves as green because they reflect back to our eyes a green wavelength which is what our eyes see as green, absorbing the wavelengths we see as other colors, like blue or red. In humans, different wavelengths of light are reflected or absorbed by a pigment in the top layer of our skin.
Our individual genetic inheritance determines the type of melanin inside our melanosomes. The red-yellow pheomelanin is abundant in lightly pigmented people.
The darker pigmented people have more of the brown-black eumelanin. And the more eumelanin, the darker the skin. Melanin colors human as well, and animal hair and the feathers of many birds.
Interestingly, the wavelength of light that melanin reflects are far less important, biologically, than the ones it actually absorbs. And of the ones that absorb, the ones that are the most important are those that we can’t even see.
Much of the light given off by the Sun is invisible to our eyes. Some of that is what’s called ultraviolet radiation, which is highly energetic. So much so it can actually penetrate living cells. When this happens, it can actually wreak havoc within them. (Oh Yes). It can even cause mutations in skin cell DNA.
Benefits Of Melanin
Fortunately for us, the melanin in our skin is what is actually standing between us and the threat you just read above. Now according to Dr. Zalfa Abdel-Malik , The melanin is like the sensor, it’s more like a guardian molecule whose main job is to protect. It forms what are called supranuclear caps and absorbs UV which is a process that protects the skin cell DNA.
They’re like little parasols around the nucleus. And the UV cannot penetrate these to go and attack the DNA. That’s just one of the most remarkable things about melanin according to molecular biologist Dr. Zalfa Abdel-Malek.
Another benefit of melanin is, it provides a diverse collection of species. We all know that melanin in the lower vertebrates is regulating body temperature. It can also give animals camouflage and allows them to recognize other members of the species.
One of the functions of melanin in humans is to protect the cells from UV damage. The more we evolve, we lost hair and increase melanin production in our skin.
Let’s Look At The Connection Between The Intensity Of UV Radiation and The Human Skin Color
I the early 2000s, I was so fascinated with UV and the human skin color. But as I searched about the global distribution of solar UV, I discovered the available data was in fact quite spotty.
In the early 80s, there were concerns about the number of risks that the depletion of UV-blocking atmospheric ozone posed to human health. This concern led NASA into action and NASA took millions of UV measurements from space.
Nina Jablonski was able to find a way to turn all those data points into a map that ended up showing exactly how UV exposure varies throughout the world.
Most striking was the clear gradient between the equator and the poles, which was interrupted only in places where altitude increased UV exposure specifically in the Tibetan Plateau and persistent cloud cover decreased it.
The Congo basin is full of humidity and moisture, which is blocking the UV. Solar energy is a fundamental attribute of any environment. And it’s a well-established fact that organisms living at different latitudes adapt in some way to their local solar conditions.
To see how closely humans skin color correlates with UV exposure, she collected skin pigment measurements made by anthropologists studying indigenous peoples.
For many years, anthropologists have faced the challenge of how to accurately measure skin color. We now use this little device called a reflectometer. Basically, it sends out light of specific colors and then measures the amount of light that is reflected back.
This tells us what color a person’s skin is, and we can then compare this to people all over the world. A second map was then created using measured skin colors and environmental data.
It showed that the UV intensity does indeed predict skin color. Wherever UV is strong, skin is dark, like it is near the equator or at high altitude. At the poles, the skin of indigenous people is almost always lighter.
That suggests that variation in human skin melanin production arose as different populations adapted biologically to different solar conditions around the world. As we’ve noted, our early ancestors probably had full body hair covering pale skin, just like other primates.
So When Did The Darker Shades Of Human Skin Begin To Evolve?
DNA sequencing has made it possible to find evidence that can help answer that question. Rick Kittles is a geneticist who’s skilled at deciphering such clues. According to him, whenever a species undergoes some form of selection, some form of natural selection, evidence of that selection is found in the genome.
And so as a geneticist, they get really excited when they explore the genome for these signatures. One way in which that’s done is by sampling worldwide populations and looking throughout the genome at variation and comparing across populations.
And it’s a very exciting process.
One of the many genes that genetic detectives have linked to human pigmentation is called MC1R. Sampling from around the world indicates that there’s a fair amount of variation in the DNA sequence of that gene, but not from every corner of the globe. When we look at MC1R within African populations, we don’t see a lot of diversity.
And the particular allele that they have in those African populations is the one that codes for darker skin. MC1R codes for a protein which is involved in the switch from the production of pheomelanin to eumelanin. And we know that pheomelanin is the red-yellow colored pigment, and then the eumelanin is the brown-black pigment.
Now, the absence of MC1R diversity in African populations indicates that, in that part of the world, there is strong negative selection against any alleles that would alter dark skin. And how long has this allele been fixed in African populations?
Other genetic studies have calculated that it has been as much as 1.2 million years. Since our species evolved in equatorial Africa, it’s reasonable to conclude that by that time, all humans were dark-skinned.
The fossil record supports what we’ve gleaned from genetic evidence. But here’s where we confront what was, for me, the heart of the mystery. The evolution of dark skin in humans suggests that under strong UV light, that trait provided a survival advantage.
So What Exactly Was That Advantage?
It’s certainly true, UV damages to skin cell DNA can lead to cancer, and skin cancer can be fatal. For a long time, that seemed to be the likeliest explanation. Except skin cancer generally develops after a person’s peak reproductive years.
For that reason, though it might cut your life short, it’s unlikely to affect your ability to pass on your genes. As I was struggling to conceive of an alternative explanation, I happened to attend a lecture on severe birth defects.
That talk was about a research project that had found evidence that certain birth defects are far more common among pregnant women with diets deficient in a B vitamin called folate.
Only weeks before, I realized how strong sunlight breaks down folate circulating in skin blood vessels. Here was a direct link between UV radiation, skin color, and reproductive success. It was a small eureka moment for me. In the years since we’ve learned that folate is not only essential for normal embryonic development, it’s even needed for healthy sperm production in males.
Folate is biological gold. It is an essential nutrient. And it needs to be protected from UV radiation as it circulates in the blood vessels in the skin. That is what melanin does. I felt I was halfway home on my quest to understand human skin color variation.
But The Big Question Remained, Why Aren’t We All Dark Skinned?
It turns out that there’s another side to our relationship with UV light. UV light is not all bad. In fact, a small portion of it known as UVB is critical for the synthesis in our bodies of vitamin D, a process that starts in the skin. Without Vitamin D, humans cannot absorb calcium from our diet to build our bones and for a healthy immune system.
Back when all our ancestors lived close to the equator, there was no problem getting enough UVB through dark skin to make the vitamin D needed. But thereafter, some populations started moving north, where the UV striking Earth’s surface is much weaker.
In northern latitudes, dark skin makes it hard to produce the Vitamin D that human bodies really need. The consequences of vitamin D deficiency include rickets, a bone development disease that can cripple the young.
In higher latitudes with less UV, the selective pressure on MC1R that produced dark skin in our ancient ancestors began to abate. When we look at the early movement out of Africa when that constraint was relaxed, we then see a plethora of variation n European and Asian
populations, geneticists have discovered a greater variation in the MC1R gene, but less variation in several other genes, ones associated with lighter skin types. Different environments lead to other genes being selected for and being important for those populations in terms of skin color. Selection for light skin gene variants occurred multiple times in different groups around the world, some of it in just the last 10,000 years.
Support for the idea that the UV vitamin D connection helped drive the evolution of paler skin comes from the fact that indigenous people with diets rich in this essential vitamin have dark pigmentation.
The tension between these two aspects of our biological inheritance, on the one hand, the need to protect ourselves from most ultraviolet radiation, and on the other, the need to use some ultraviolet radiation for our own benefit, these forces drove the evolution of the wonderful variation in human skin color that we see around us today.
It’s the legacy of an evolutionary balancing act, necessitated by the different environmental conditions people have faced around the globe. The thing is, where once human migrations took many generations, we now move about the planet at the speed of sound.
That means increasing numbers of us have pigmentation that’s not a good match with where we live. People with fair skin and red hair, your phenotype is telling you, that you have a high risk of skin cancer if you’re out in the Sun.
If you’re a dark-skinned individual, for example, in Scandinavia or in Minnesota, you’re not going to have optimal exposure to UV for optimal vitamin D synthesis, and you need to take supplements.
We now know that we need to make cultural adaptations like these to stay healthy. But that’s not all we’ve learned. With the knowledge we now have about evolution, we also know that skin color is a flexible trait that has changed through time as various groups of people moved to sunny or less sunny parts of the world.
And we know that skin color is inherited independently of other traits and is not associated with other aspects of a person’s appearance or behavior. Skin color is a product of evolution and should never have been judged as something good or bad.
We are a very clever and adaptable species. And we are one under the Sun.
I hope this was helpful. if you ever need a hand or have any contribution or a question to ask, do feel free to leave them below. I will be more than glad to help you out.
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