In 1871 the naturalist Charles Darwin published The Origin of Man , a work that appeared twelve years after his best-known book, The Origin of Species , which forever changed how we would observe and interpret nature, the world around us. Darwin changed biology, explaining to us the concept of the evolution of species, which constantly adapt to a changing environment that relentlessly selects the best adapted individuals at all times, who are the ones that manage to leave more offspring. And this is how organisms change, evolve.
Naturally, evolution not only affects the finches of the Galapagos Islands but all species, including the human species, us. If today we look around us and contemplate the diversity of appearances that human beings have, the first thing that we will highlight are the different colors and tones of skin (and hair, and eyes) that we are capable of finding. Indeed, the color of our skin is the first thing that identifies us, what is obvious . And this has always been so. Darwin himself was already aware of this and wrote in The Descent of Man the following: “Among all the differences that exist between the human races, the most notorious and the most pronounced is the color of the skin.” Here Darwin tells us about the concept of “race”, very much in vogue at the time, in which thoughtful discussions were held about whether people with black skin and white skin belonged to the same species, or corresponded to different species or subspecies, being the term of race which ended up being consolidated. It is surprising to realize that the absurd discussion about races has remained in the bibliographic record and has reached our days. And even that it continues to be incomprehensibly used in some countries to identify people according to the racial group to which they belong (whites, African-Americans, Hispanics, Asians…). It is therefore important to make it clear from the beginning of this article: human races do not exist.
More similar than it seems
We must not get carried away by appearances, which tend to hide what is really relevant. There is no doubt that a Norwegian with extremely white skin and hair has an external appearance very different from that of an Ethiopian shepherd, with an intensely black skin, extraordinarily dark. But, if we forget for a moment the color of the skin, we will discover that in reality they are two people, two human beings who are much more alike than some people would be willing to recognize . Darwin also noticed. His insight and intuition led him at all times to focus on what is relevant, not what is incidental. And so he told us in The Origin of Man : «Human races, even the most different, have much more similar forms than is believed at first sight». And he added: “Although the existing human races differ from each other in various respects, such as color, hair, shape of skull, proportions of the body, etc., nevertheless, considered in their total structure, they are found to be very much alike in one respect.” endless points.
Darwin could not know it, he did not know the secrets of our DNA, our genome, which we only began to discover after the year 2001, when the first drafts of our genetic information were published. But Darwin was right. Indeed, even at the genetic level, there is much more that unites us to any human being on the planet than what separates us. The differences between two human beings are essentially individual, not group. This means that, beyond a small number of distinctive characters that appear in human populations, an Eskimo and an Australian aborigine look as much alike (and differ) as a Berber and a Hawaiian. All human beings belong to the same species. There are no races that distinguish us, despite the fact that the pigmentation of our skin suggests otherwise. From the analysis of many human genomes we have been able to deduce that any person resembles another approximately 99.9%, in relation to their DNA. It is only 0.1% of our genome that differentiates us from another human being.
There is little doubt about the African origin of man, as a species. The detailed study of the fossil record found indicates that we descend from other earlier primates, which gave rise to both the diversity of current monkeys and men, ours being one of the species that appeared ( Homo sapiens ) and the one that ended up spreading and colonizing all the planet.
Our main defense barrier
The situation of the African continent, with its central part located between the lines that define the two Tropics of Cancer and Capricorn, around the equator, makes it one of the areas of the Earth with the highest solar irradiation, in which the rays of sun daily punish all the organisms that populate it. The sun provides us with the heat and energy we need to live, through the plants and animals that then feed on them and that we also eventually consume. But the sun also holds dangers. The ultraviolet radiation that travels in the rays of our star can damage DNA, can produce mutations that alter skin cells and transform into a tumor that can become malignant and spread throughout the body (metastasis), ending the life of person. That is why it is important to protect yourself from the sun: to avoid developing any of the various types of skin cancer that we know, such as melanoma, the most aggressive and terrible of all, generally with a poor prognosis, if it is not detected in time and the initial injury by surgery.
The skin prevents the entry of viruses, bacteria and parasites, it maintains the water we need in our body, preventing dehydration and, in addition, it contains cells that manufacture a pigment, melanin, which is what protects us from solar radiation. These cells are called melanocytes and they are the main responsible for us being able to defend ourselves against harmful ultraviolet radiation. From all this it can be easily deduced that the more pigment we have in our skin, the better protected we will be against the sun’s rays.
Dark skin, a consequence of the African origin of man
Surely we descend from ancestral primates that inhabited the jungle, the African forest, with many shaded areas, protected from the sun, and whose skin was covered with hair. Somehow similar to modern chimpanzees. However, these primates and our ancestors have whitish, pinkish skin with very little pigment. The thick hair that covers their body gives them the warmth and protection they need. Some time later, those ancestral hominids from whom we descend began to colonize other territories, such as the savannah, where it is warmer and where there is less shade. The hair-covered body was no longer useful in these environmental conditions and, therefore, the hair loss was replaced by an increase in pigmentation and the appearance of many more sweat glands, to better control body temperature with sweat. Hence, the first hominids that appeared in the horn of Africa , to the east, in the territory now occupied by Kenya, Somalia, Ethiopia and neighboring countries, must have evolved, 1-2 million years ago, to have dark skin, to protect yourself from solar radiation. The first ancestors of human beings had dark skin, probably as black as the current inhabitants of that African region. If so, and we are all descended from those early hominids, then why don’t we all have black skin?
Dark skin provided an effective defense against the sun, but it also fulfilled other functions that today we believe were equally or more relevant, such as providing protection to skin cells to prevent them from becoming carcinogenic. In effect, ultraviolet radiation breaks down folic acid (also called vitamin B9) that we need to survive. A deficiency of this vitamin causes serious defects during embryogenesis, which usually end in serious congenital anomalies or spontaneous abortions. So those early hominids had an excellent reason to pigment themselves. And natural selection did the rest. Those who were able to increase their pigmentation levels were able to better protect this compound, survived and were able to leave more offspring. On the contrary, those that did not manage to pigment or did not manage to do it with sufficient intensity faced extinction, suffering serious reproduction problems. Human evolution in its purest form.
We now know why dark pigmentation appeared in Africa and what function it served. We also know that the first human beings gradually extended their territory and began to colonize other regions, located further north, where there was not so much sun, where solar radiation was much less, where it was no longer necessary to protect oneself from ultraviolet radiation and where those individuals who, due to spontaneous mutations, accumulated less pigment in their skin also had a chance of surviving. Hence, human beings with lighter skin began to appear. But again another vitamin we believe played an essential role in whitening the skin of humans displaced to more northern regions of the planet.
The evolution towards white skin
The sun can be harmful, but it is also necessary for life. In particular, we know that one of the steps in the biosynthesis of vitamin D , which we need to survive, must occur thanks to the intervention of solar ultraviolet radiation. Our skin accumulates a compound called 7-dehydrocholesterol which, due to the action of ultraviolet solar radiation, is converted into another, known as pre-vitamin D3, which will in turn end up transforming into the active form of vitamin D, after passing through the liver and the kidney. Of course, today we can make up for this shortfall in sunbathing (in the Nordic countries, for people who never leave the house for various reasons, etc.) through diet, acquiring vitamin D precursors through the consumption of the relatively few foods rich in this vitamin, mainly of animal origin (fatty fish, milk and dairy products, eggs, etc.), or simply through tablets that contain it.
But, hundreds of thousands of years ago, when the dark-skinned hominids populated the northern lands, they had to face a new handicap. The dark pigmentation that had protected them from solar radiation in Africa was now inconvenient, even dangerous. The presence of excessive pigment in the skin filtered the little solar radiation that these ancient humans who inhabited the cold and dark lands of the north could take advantage of. And, again, evolution and natural selection once again played a fundamental role. As Theodosius Dobzhansky said “nothing in biology makes sense if it is not in the light of evolution”. Indeed, those individuals who, through mutations in certain genes, managed to reduce the pigmentation of their skin, were the ones who managed to take advantage of the meager solar radiation to manufacture the minimum amount of vitamin D they needed to survive, and were the ones who managed to leave more offspring. While those who remained dark-skinned faced a deficit of vitamin D, essential for absorbing the calcium that builds our bones. A deficit leads to a loss of bone density, to bone fractures —as they become more fragile—, to rickets. Likewise, it has an important role in the correct functioning of the nervous, muscular and immune systems. Hence, its deficit compromised the survival of the first settlers of northern regions, and those who managed to survive were those who lost or significantly reduced the ability to pigment.
From all of the above, it can be deduced that with changes in relatively few genes we were able to increase or reduce our body pigmentation, as we evolved and adapted to the environment at each moment of our recent history, whether in regions with greater or lesser solar radiation. And it also follows that there are few genetic differences that separate a person with white skin from another with black skin, once again dismantling any hint of truth in the idea of race, which must be discarded from our minds.
The genes that modulate our pigmentation
In the pigment cells, in the melanocytes, it is where the synthesis of melanin takes place. However, there are two main types of melanin: a darker, blackish one called eumelanin, and a much lighter, yellowish-reddish one called pheomelanin. The final proportion of each of these melanins, together with many other variables of pigmentation, is what explains the great diversity of skin, hair and eye colors that human beings have. The switch that regulates what type of melanin the melanocyte is going to make is a protein that is in the cell membrane of these cells, called melanocortin receptor type 1 (MC1R). This receptor can be activated by a hormone (a-MSH) that causes the synthesis of eumelanin. And it can also be inhibited in the absence of the hormone or in the presence of blocking molecules that deactivate it (such as ASP). When the receptor is deactivated, the melanocyte is only capable of manufacturing pheomelanin, which accumulates a different pigment that makes the skin and hair much lighter. There are people who have mutated the gene that encodes this MC1R receptor, and then it is unable to be activated. These individuals will only make yellowish-reddish pheomelanin throughout their lives. They are red-haired people.
Therefore, it is possible to enhance eumelanin synthesis by activating a limited number of genes, resulting in a darkening of the skin, and in turn it is equally possible to reduce eumelanin synthesis, increasing pheomelanin synthesis, inactivating only one gene, that of the MC1R receptor , among other possible solutions. The latter, the inactivation of MC1R, is what happened at various times in the evolution of human beings and that is why the majority of red-haired people appeared and continue to be in northern countries, mainly Ireland and the United Kingdom, but also in Russia and other northern countries. While the proportion of redheads in countries like ours is 2%, in those further north they can have up to 30%. The red-haired character, with its whitish skin, made it possible to take advantage of the little solar radiation to manufacture the minimum amount of vitamin D necessary to survive. And it was those first light-skinned Europeans who ended up colonizing a large part of the continent and later emigrated to other territories, such as America, Australia or South Africa, which is where we find people with white skin today. That is why not all human beings have black skin.
Lluís Montoliu works at the National Center for Biotechnology (CNB-CSIC) and the Center for Biomedical Research in the Rare Diseases Network (CIBERER-ISCIII).
