And More on Skin Color: Why We Are White and Why We Tan. Column for Компьютерры #122

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Dmytro Shabanov
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← Dmytro Shabanov → On skin color: an experience of discussing one human adaptation with justification of far-reaching conclusions about the specificity of ecological factors' action And more on skin color: why we are white and why we tan Three conclusions that can be made after discussing the reasons for differences between people in their skin color

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In the previous column we established why the skin of the indigenous population of Africa (the continent where our species originated) has a more or less black color. Although the needs of thermoregulation should have contributed to the lightening of African skin, the action of this factor was overcome by an even more powerful vector of selection—the need to reduce the probability of tumor occurrence. Somatic mutagenesis, which can cause malignant tumors, is not the only adverse effect of UV rays. Additionally, ultraviolet destroys folate (folic acid) in the skin—one of the vitamins, an important regulator of our reproductive activity. Prolonged exposure to the sun reduces human fertility, but increased melanin content in the skin reduces this effect. By the way, folate deficiency is the most common type of vitamin deficiency. Both for protection against tumors and for preserving folate, people benefit from having dark skin rich in melanin. Europeans descended from Africans—their skin lightened over time. This indicates the presence of some even more powerful factor determining skin color. This factor is related to vitamin D, extremely important for regulating calcium and phosphorus metabolism. Actually, the regulator of calcium metabolism is formed in the kidneys. For this, its precursor, bound to a transport protein, arrives there from the liver. What happens in the skin is an even earlier stage of vitamin D metabolism. [IMG_1] Vitamin D metabolism according to the classic textbook by A. Lehninger (1985). The substance whose name is highlighted in red regulates calcium exchange. The right side shows the pathway for obtaining artificial provitamin Honestly, I don't understand why a substance that forms in all organisms through a photochemical reaction—that is, by a relatively "unreliable" path—became the regulator of calcium ion content (an extremely important indicator). Why wasn't some simpler path realized? But one way or another, this method of calcium balance regulation is much older than our species, our genus, and even our class. When our dark-skinned ancestors began colonizing the temperate latitudes of Eurasia, they received less ultraviolet light and didn't get enough vitamin D. Such a deficiency leads to nothing good. In adults, it causes osteomalacia (softening of the bones); in children—rickets. Do you think skin cancer is worse than rickets? Don't jump to conclusions. Cancer affects people at various ages, but most often mature ones (the accumulation of somatic mutations leading to malignant growth can be a fairly long process). Many people who die from cancer manage to leave offspring and pass their genes to their children. Almost all children develop rickets under unsuitable conditions. Weak rachitic children develop into weak adults with deformed skeletons and a whole bunch of associated abnormalities. Women who suffered from this disease in childhood in serious form will, without adequate medical assistance, most likely die during their first childbirth (now their lives and their children's lives are saved through cesarean section). Apparently, hunters who received significant amounts of vitamin D from the liver of their prey were more resistant to rickets than farmers, but selection for rickets resistance must have been occurring among them as well. Those people who (possibly due to their lighter skin) will suffer from rickets to a lesser extent or will be able to avoid it altogether will leave many more descendants. So, although cancer leads to rapid death, and rickets does not, selection for preventing rickets can be stronger! Now you understand why our skin is white? It remains to explain why we tan! Tanning is a specific reaction of people with light (but not excessively light!) skin. Negroids don't tan: melanin synthesis in their skin does not depend on ultraviolet radiation. Brunettes, who themselves have swarthy skin, tan most easily; it is most difficult to tan and avoid sunburn for blonds with very light skin. Negroids have skin with phototype IV according to Thomas Fitzpatrick's classification; light-skinned people who cannot tan are characterized by phototype I; most readers of this column belong to one of the intermediate types. I will temporarily pause the discussion about skin color to turn to one of the controversial issues of modern evolutionary biology. In school, you learned that organism traits are divided into hereditary and non-hereditary. The diversity of hereditary traits reflects mutational variability, which is significant for evolution; non-hereditary traits are called "modifications" and are mostly evolutionarily unpromising. This approach is characteristic of the Modern Synthesis, the "synthetic theory of evolution." Within the framework of the Modern Synthesis approach that I like, it can be considered as the II evolutionary synthesis, achieved about 70 years ago. I consider the ETE, the epigenetic theory of evolution, to be the optimal basis for the III evolutionary synthesis. From the ETE perspective, traits are divided not into hereditary (mutational) and non-hereditary (modificational), but into those stable in their realization and unstable. "The concepts of 'mutation' and 'modification' are actually completely incomparable, since the first relates to comparison of individuals, and the second—to comparison of developmental possibilities of the same individual" (Shishkin, 1987). Each organism possesses its genotype, that is, a complex of innate predispositions. All traits of an organism arise during ontogeny, individual development, through the interaction of the developing system with the environment. Development is a choice of a certain state from a set of possible ones. Each genotype is characterized by a certain field of possible states of each trait, realized under different conditions with different probabilities. Stable traits will predictably manifest in various environmental conditions. Unstable ones will be realized only in the case of specific environmental influences. Any, even traumatic "modifications" turn out to be part of ontogenetic trajectories accessible to an organism with a given genotype in a given environment. The difference I described seems insignificant to you, and the very dispute about trait classification—scholastic? You underestimate how many consequences follow from this difference between the Modern Synthesis and the Epigenetic Theory... So, in the establishment of relatively light skin, not only did some quantitative trait (melanin content in the skin) change. One stably realized norm "split" into two, the switching between which depends on environmental influences. However, to be precise, we see two extreme variants with a full spectrum of transitions between them. Moreover, the transition from one state to another can occur many times during a lifetime! When we talk about tanning, it seems to us that sunlight "makes" our skin dark (as, for example, infrared rays in an oven create a crispy crust on a chicken being baked). No! Our skin itself becomes darker or lighter depending on the nature of the radiation received. Little ultraviolet—the more acute danger is the risk of rickets—the skin will be transparent to photochemically active rays. The organism has received sufficient UV irradiation—the danger of rickets has receded, but the risk of malignant growths has increased—a temporary melanin "shield" forms in the skin. In my opinion, this is a beautiful adaptation. Do you remember, in the previous column I drew attention to the fact that the skin of indigenous inhabitants of Africa is darker than that of indigenous inhabitants of Central America? Don't look for the answer in the specifics of the lives of Africans and Americans; if there is one, it's not the reason. The reason is different—in the prehistory (and this is quite typical). The prehistory of skin color must begin with why our ancestors lost their fur covering. Two explanations known to me are not alternative but complementary, and they have already been mentioned in my columns. First, relatively bare skin with an abundance of sweat glands is an effective cooling mechanism. Bipedal body position, bare sweating skin, use of cutting and piercing tools, and possibly the ability to carry drinking water made man a super-predator. Do you remember the video clip in which an African runner catches an antelope after an eight-hour chase under the savanna sun? It was for this that we lost fur and transferred the function of heat preservation to the subcutaneous fat layer. Second, the sparse hairs covering our body turned out to be very convenient for detecting external parasites. Some of them, like fleas and bedbugs, threaten only animals using permanent dens and dwellings; the role of such parasites in our evolution increased over time. The skin of our furless ancestors was light. For the reasons discussed in the previous column, it began to darken. What exact tone it was at each particular stage, we do not yet know with certainty, although we can make well-founded assumptions about this. In the figure below, I have collected three different images of Homo erectus, "Pithecanthropus." All these are modern reconstructions, where this ancestor of ours is—quite reasonably—similar to us. I don't want to discuss the old terrible pictures in which Pithecanthropus were depicted as hunched hairy monsters. [IMG_2] Three reconstructions of Homo erectus. Sources: first, second, third On which reconstruction is the skin color shown most plausibly? Not on the first one. Even if it depicts an Asian, not an African erectus, his skin should be much darker. After all, this is already a true representative of the genus Homo, behind whom is a long evolution without a dense fur covering. I also don't fully agree with the third picture. The expressive man shown on it has ebony skin, almost as dark as the skin of the darkest modern Africans. After all, a lot of time has passed since Homo erectus, and selection against skin cancer should have made it significantly darker. So, we choose the second reconstruction! By the way, it seems to me the most plausible also in other details of the appearance of our close relative. Probably, our ancestors were like that. In their descendants, living in Africa, the skin continued to darken. And then in those of them who settled Eurasia, it began to lighten. In the majority (belonging to phototypes II, III, and IV) two significantly different temporary norms arose, the switching between which depends on ultraviolet insolation. Perhaps some role in this process was also played by interbreeding with Neanderthals, who had a much longer history of life in Eurasia and should have had lighter skin. [IMG_3] "Mitochondrial" (based on the analysis of the evolution of the genome of intracellular symbionts—mitochondria) map of Homo sapiens dispersal across the planet. Designations in white circles—mitochondrial lineages; colored arcs and circles—estimation of time elapsed since the settlement of these territories. Source—Wikipedia When settling Asia, the advanced detachments moved along the coast of the Indian Ocean. They fairly quickly settled the islands of Southeast Asia and Oceania, and then—Australia. They retained the original dark skin color of our species. Those migrants who moved northward were forced to lighten. Selection for reducing the amount of melanin in the skin affected northerners the most. However, the peoples of the Far North, who live by fishing and often eat the liver of marine mammals, remained unexpectedly dark-skinned. They have no need to lighten, as their food contains a lot of vitamin D. During glaciations, a significant mass of water was bound in glaciers. The water level in the oceans dropped, and a landmass was exposed at the site of the present-day Bering Strait—Beringia. It was sometimes covered with ice, sometimes largely freed from it. Through the mammoth steppes (a special, now extinct biocenosis) of Beringia, the fauna of Northeast Asia, including people, penetrated into North America. Before them opened a landmass stretching from Alaska to Patagonia. As they dispersed in the temperate latitudes of North America, their skin continued to lighten. In the inhabitants of low latitudes, the skin began to darken, but it never had time to become as dark as that of indigenous Africans. In the inhabitants of the temperate latitudes of South America, the skin began to lighten again... The factors discussed here were superimposed on the complex processes of ethnogenesis, wars, and migrations of peoples. The result was the mosaic of skin colors that we observe. And then the lifestyle of most people on our planet changed dramatically. It came to include clothing and tanning beds, life indoors and the use of creams for and against tanning, oncological centers and vitamin prophylaxis. The vectors of selection acting on us began to change sharply. The regular, climate-dependent pattern of skin color change in representatives of global humanity will never be achieved again. Do you remember I mentioned the concept of adaptive compromise proposed by Alexander Pavlovich Ratsnitsyn? Skin color is a good example of such a compromise. This trait does not have a single cause; it is the result of balancing oppositely directed evolutionary trends. If, when reading this and the previous column, it seemed to you that skin color is regulated more complexly than our other characteristics, you probably missed something. We simply haven't discussed the conflicting complexes of requirements that selection imposes on all our (and not only our) other traits. A decent volume of the column ran out earlier than the general ecological conclusions that I would like to discuss in it. Well, perhaps I will continue this conversation in time... ← Dmytro Shabanov → On skin color: an experience of discussing one human adaptation with justification of far-reaching conclusions about the specificity of ecological factors' action And more on skin color: why we are white and why we tan Three conclusions that can be made after discussing the reasons for differences between people in their skin color

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Dmytro Shabanov
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Про колір шкіри: досвід обговорення однієї адаптації людини з обґрунтуванням далекоідних висновків про специфіку дії екологічних факторів
І ще про колір шкіри: чому ми білі і чому ми загоряємося
Три висновки, які можна зробити після обговорення причин різниць між людьми за кольором їх шкіри

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