Photograph: Txabi Gaztelu.
Biological anthropologists are interested in human diversity, of any type. But unlike naturalists of previous centuries, we can now do more than just describing it. Advances in molecular and bioinformatics techniques allow us to investigate how human diversity arose and which were the molecular, genetic and evolutionary mechanisms that shaped it. In particular, genetic diversity is of paramount importance because it constitutes the raw material for evolution. Without diversity we could not evolve, we could not adapt to a fluctuating environment. A species without genetic diversity is doomed to extinction, as one thing is certain, the environment will always be changing.
Usually evolution is understood as a process that leads to better adapted individuals, i.e., individuals with a better rate of survival and reproduction in a given environment. But it is important to remark that evolution is not an engineer: evolution lacks foresight and consequently, does not plan in advance. This is what the Nobel Prize Jacques Monod called “Evolution by tinkering” (Jacob F, 1977). Instead, when facing an environmental challenge, evolution does what it can with the ‘material’ available at the moment. This ‘material’ consists of the repertoire of genetic variants in the population, our genetic diversity, which has been produced by random mutation, (and recombination to a certain extent). But Natural Selection never induces these changes to happen. Natural Selection just sieves those haphazard changes at the DNA level that improve adaptation from those which do not, although it is also important to have in mind that sometimes evolution randomly favors some changes over others. This random effect is normally inversely correlated with the population size, and thus selection is more effective in big populations. In any case, tinkering implies that the solutions found by Natural Selection may not be optimal, but rather, just enough to keep going. As a consequence many adaptations lead to evolutionary trade-offs that convey adverse consequences for our health at a post-reproductive age, which is also a topic of interest and active research (evolutionary medicine).
In this regard, one of the aspects in which we humans are conspicuously diverse is in the color of our skin. Skin color has been (and still is) at the base of much suffering and pain along human history because it has been used to justify wrong ideas about supremacy of one group of people over another. Of course, other non-biological traits, like wealth, religion or political ideas have also been excuse to the violation of human rights, but unlike these, only skin color is susceptible of objective evolutionary analysis.
Skin color is a continuous trait, which means that watertight classification clusters like ‘white’ or ‘black’ or ‘yellow’ are not applicable to skin color. I recommend a visit to Angelica Dass’ “humanae-project” web page (https://www.angelicadass.com/humanae-project/), where we can see that humans show a rich palette of skin colors. Although it is true we can recognize some people as “white” or “black”, we may nevertheless find difficulty in categorically classifying many others. The boundaries between the colors are blurred and arbitrarily set.
Thus, in the following I am going to try to argue that due to the African origin of our species, we ultimately are all Africans under our skin, and that, despite our skin color, our blood is equally red, our flesh is equally pinkish and our skeleton equally white. Consequently, we must consider skin color simply as the result of an adaptation to the environment.
The cellular basis of skin pigmentation
The pigmentary molecule responsible for our skin color is called melanin, which is in fact a complex mix of polymers. Melanin is produced in the external layer of our skin, the epidermis, which is mostly composed of cells called keratinocytes, but also includes a small proportion of another cell type, melanocytes, which are the cells that produce melanin within specific organelles called melanosomes. Melanocytes are dendritic cells, meaning that they show many ‘arms’, which they need in order to distribute melanosomes to a set of several tens of surrounding keratinocytes. Within keratinocytes, melanosomes distribute like an umbrella around the external face of the nuclei, maximizing thus the protective effect on DNA, which resides in the nucleus. We might then expect that the differences in skin pigmentation between different people are due to a different number of melanocytes on the skin. But this is not so. In fact, although different areas of the skin may have different densities of melanocytes, the same skin area in people of light and dark skin has the same density of melanocytes. The difference comes instead from the types of melanin synthesized. Thus, while dark skins have a greater proportion of dark melanin (eumelanin), light skins have a greater proportion of light melanin (pheomelanin, which is browner, or reddish). Besides, eumelanosomes tend to be single entities, while melanosomes consisting of pheomelanin are clustered in groups of several smaller membrane-bound groups of melanosomes called ‘melanosomal complexes’.
How we got a colored skin
Many of you may have a dog, or a friend who has a dog. You may have noticed then that if you delve into the thick layer of hair, their skin is usually pink or white, i.e., lacks color. Similarly, we also were furry animals once, with an unpigmented skin under our layer of hair. Thus, we can say that at the beginning of the process of becoming humans our skin was unpigmented. But this was to change soon after we started to walk on two feet. This is thought to have happened about 6-7 million of years ago (mya). Specimens like Sahelanthropus tchadensis (~7mya), Orrorin tugenensis (~6mya) or Ardipithecus ramidus (4.2mya) show already morphological traits indicative of an incipient bipedalism, i.e. they possibly could combine dwelling on the trees and walking on the ground, even though inefficiently. We had to wait until ~2mya to develop full bipedalism, and then another advance, endurance running. This means that we can run long distances at a moderate speed. This is a demanding exercise that has a side effect: it increases our body temperature. To cool off our body, evolution favored two complementary strategies: on the one hand, we developed a huge number of sweating glands (eccrine glands to be precise), which can produce up to 3L of water per hour. All this water goes to the surface of our body where it evaporates. Evaporation is an endothermic process, which by using the heat of the body can transform liquid water into water vapor, cooling of the skin as a result. On the other hand, an efficient evaporation of sweat demands a skin without hairs, and consequently evolution further favored a naked skin. But losing our protective layer of hair means we are now naked against the noxious effects of sunlight, in particular of ultraviolet (UV) radiation. And this alternative strategy consisted on darkening our skin.
What evidence do we have that a dark skin is photo-protective? And photo-protective against what, exactly?
There are several hypotheses that attempt to explain against what is skin color protecting us. One of them claims that melanin, by acting as a shield against the damaging spectrum of sunlight (UV radiation), prevents the degradation of important circulating molecules, like folate, which is essential for normal development of the embryo (Jablonski and Chaplin, 2000). Without excluding this possibility, my favored hypothesis is based on the protection that skin color offers against skin cancer (see Greaves 2014; but see also Jablonski and Chaplin 2014). What evidence do we have for it? Well, if we compare the incidence of skin cancer on individuals of different skin color but, importantly, living in the same environment, like North Americans of European origin (light skinned; EUR-US) and North Americans of African origin (dark skinned; AFR-US), we can see that the incidence in non-melanoma skin cancer is 50 times higher in EUR-US. The incidence of melanoma skin-cancer is also 10 times higher. Similarly, Australians have one the highest incidence of skin cancer in the world. But I am not referring to Australian aborigines, who have a dark skin, but rather to the Australian descendants of the European/British settlers, who originally have light skin as a result of adaptation to Northern latitudes (and low sun radiation).
All this evidence proves that pigmentation protects us from disease, and hence, has biomedical value. But, has it also an evolutionary value? In other words, has dark pigmentation offered some reproductive advantage along our evolutionary history? Because, if a disease tends to occur in our old age it may kill us but it is not significantly affecting our reproductive success, as we already are past our reproductive age. Only if a disease dwindles our ability to produce descendants, then it has selective value, because the ones than are more protected against it will leave more descendants, and with them, their ability (genetic variants) to overcome the effects of that disease. In this regard, skin cancer is considered to occur typically in our old age, which would apparently imply that dark pigmentation has no selective value. However, when talking about evolution we need to consider long times, perhaps thousands or tens of thousands of years back into our history. We must imagine ourselves, light skinned Europeans, in equatorial Africa, under intense solar radiation and without the benefits of an advanced health institutions, facilities and professionals. Perhaps, a good proxy for that scenario could be to observe and compare what is the situation of people with albinism in some African present-day countries. In Africa, with a ten times more intense solar radiation than Europe, 100% of people with albinism less than 20 years old (yo) have pre-cancerous lesions, and 90% of them die by the age of 30yo due to skin cancer lesions (Marçon and Maia, 2019). Consequently, it seems just logical to conclude that dark skin has evolutionary value.
Then, how come we are not all dark skinned?
Paleontological and genetical evidence indicate that all non-African are the result of a migration out-of-Africa that took place some ~100kya. Early Homo sapiens settlements outside Africa, although ephemeral, can be found 100-90kya at the sites of Qfzeh and Skhul, in Israel, and perhaps even earlier, as suggested by the remains of Apidima Cave, in Greece, dated ~200ky (Harvati et al, 2019). However, the full colonization of Europe may have started later, 40-50kya. This migration implied that humans adapted to equatorial Africa started to settle in northern latitudes, where sun radiation, in addition to being much less intense, also suffers seasonal variation (summer/winter). It turns out that under these environmental conditions a dark skin poses some challenges.
These challenges stem from the ability of our skin to synthesize the precursors of vitamin D. This is relevant, because as Holick and Chen (2008) warn us, Vitamin D deficiency is at the base of many health issues, like rickets in children, osteopenia, osteoporosis, fractures in adults, increased risk of common cancers, autoimmune diseases, hypertension, and infectious diseases. But vitamin D synthesis depends on enough UVB entering the skin to ignite the metabolic processes that lead to biologically active vitamin D. However, in regions of low sun radiation, a dark skin will prevent the already scant UVB from entering the skin, which will substantially reduce the amount of vitamin D that will be synthesized. To get an idea of this effect, in the United States scientists (Kumar et al. 2009) have shown that dark-skinned children have about ten times higher risk of vitamin D deficiency than light skinned kids. This increased risk may have driven selection to favor skin depigmentation in northern latitudes. And in fact, population genetic analyses have shown that variants in pigmentation genes that lead to light skin have actually been under positive (Darwinian) selection. And, interestingly enough, these same genetic variants are found also to be associated to an increased risk of melanoma in genetic epidemiological studies (López et al, 2014). So that, in short, in northern latitudes there was an evolutionary pressure for skin depigmentation, most likely in order to synthesize the right levels of vitamin D, the benefits of which outcompete the negative effects of an increased risk of dying from skin cancer.
Other types of skin color variation
There are some other pigmentary conditions that lay outside of what is considered the normal range of skin pigmentation variation. Among these, vitiligo, which affects ~0.5-1% of the world’s population, causes a patchy loss of skin pigmentation, which is due to the attack of the individuals’ immune system to their skin melanocytes for reasons that are not clear, but may be both environmental (some type of stress) and genetic. Vitiligo does not normally affect the health of people having this condition, unless it goes accompanied by other autoimmune reactions, but may nevertheless pose esthetical and social issues.
Autoimmunity also can cause Addison disease, affecting ~1 per 100,000 people in the world. Contrary to vitiligo, Addison disease can be life-threatening and may also result in hyper-pigmented areas on the skin. In this case, the immune system of the individual attacks not the pigmentary cells, but the adrenal glands, disrupting so the production of several hormones, including cortisol. Cortisol is a hormone that is released in response to stress and low blood-glucose concentration. If the pituitary perceives a drop in cortisol levels it will attempt to stimulate the adrenal cortex by producing adrenocorticotropic hormone (ACTH). ACTH derives from the cleavage of a precursor molecule PMOC (pro-opiomelanocortin). But POMC can be alternatively cleaved to produce alfa-melanocyte stimulating hormone (aMSH) which, as the name suggests, is a molecule that stimulates melanocytes, hence the possible dark color in the skin.
Finally, we will dedicate some words in this shortlist to albinism. It affects about 1 in 17,000 people in Europe, but seems to be more frequent in some sub-Saharan countries, like Tanzania, Burundi and Malawi. Albinism is caused by any of hundreds of different mutations in a total of ~20 different genes (Martínez‐García and Montoliu, 2013). Thus, albinism is a heterogeneous group of congenital conditions. In addition, it constitutes an example of how a single mutation can produce a diverse set of consequences in human phenotype and health. On the one hand, affected individuals typically have very fair skin and white or light-colored hair. In people with albinism living in regions of high solar radiation and low protection, long-term sun exposure produces sunburns and greatly increases the risk of skin damage and skin cancers, including melanoma, as we have mentioned above. On the other hand, people with albinism also tend to have eye abnormalities as a consequence of a lack of a well-developed fovea (a small pit in the macula, in the retina of the eye), which is rich in photoreceptor cells called cones. Photoreceptors are cell types able to transform light into information to the brain, and can be of two types, cones and rods. Cones respond to bright light and rods are instead responsible for vision in dim light. People with albinism also have a deficit of rods; they also show reduced pigmentation of retinal pigment epithelium cells, misrouting of the optic nerves at the chiasm, reduced pigmentation in the iris, problems with focusing, and depth perception, photophobia and nystagmus (involuntary eye movement). Interestingly, all these visual defects and lack of pigmentation have a common origin in the inability to metabolize properly the aminoacid tyrosine, which is a compound at the beginning of the metabolic route leading to melanin. It has been shown that it is not melanin itself which cause visual impairment in people with albinism, but an intermediate compound called L-DOPA (although lack of melanin still remains the factor responsible for the lack of pigmentation in albinism). There is nevertheless some hope for people with albinism, as in transgenic mice models it has been shown that L-DOPA is enough to restore the visual function in mice with albinism of type OCA1A (Montoliu, 2019).
However, perhaps a more devastating effect of albinism in human health comes from the attacks that people with albinism suffer in some countries of Africa. In these countries, body-parts of people with albinism are considered ‘lucky charms’ (like a horseshoe, rabbit foot…) and consequently they are chased and amputated alive, or their tombs desecrated, to trade with their body parts, even by their own family; or they are raped, in the believe that it protects against AIDS. The European Parliament has strongly condemned this type of indiscriminate crime against persons with albinism, as these attacks are also crimes against humanity (see the European Parliament resolution on Situation of people with albinism in Malawi and other African countries (2017/2868(RSP) https://www.europarl.europa.eu/doceo/document/B-8-2017-0544_EN.html)
Evolution is a constant race in the hamster-wheel that is the ever-fluctuating environment. In this context, skin pigmentation should be understood as one of the many strategies that have allowed Homo sapiens to be a successful species and spread all over the world. Pigmentation is thus an adaptation, and as such it is not a biologically valid trait to classify humans in closed boxes, and it is not biologically associated to intellectual ability or morality. We must remember that we are all descendants of a species that originated in Africa, and as a consequence, all non-Africans are just African migrants. In Nina Jablonski’s words, pigmentation is a clear evidence of evolution by natural selection, right on our bodies (https://www.ted.com/talks/nina_jablonski_skin_color_is_an_illusion).
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