On Skin Color: A Discussion of a Human Adaptation with Justification of Far-Reaching Conclusions on the Specifics of the Action of Ecological Factors. Column for Kompyuterra #121
Why is the skin of indigenous Africans darker than the skin of indigenous Europeans? If this question seems too simplistic, one more detail can be added: why is the skin of indigenous inhabitants of Central America dark, but lighter than that of indigenous Africans?
← Dmytro Shabanov → Anomalous frogs and the health of the environment: the search for new approaches at the border of Europe and Asia On skin colour: an attempt to discuss one human adaptation with a substantiation of far-reaching conclusions about the specifics of the action of ecological factors And more on skin colour: why we are white and why we get a tan Column for Computerra #120 Column for Computerra #121 Column for Computerra #122 I discussed and discussed with students at ecology seminars one example of the influence of environmental conditions on the features of organisms, and this example so appealed to me that I decided to set it out here too.
What appealed to me about it is that it lays a foundation under a «moral» — conclusions of a general character deserving special discussion. The «moral» will not fit into this column, and I plan to talk about it next time.
But the problem discussed is in itself very simple: why is the skin of native Africans darker than the skin of native Europeans?
If this question seems too primitive, one can add one more detail: why is the skin of the native inhabitants of Central America dark, but lighter than that of native Africans? This illustration from Wikipedia gives a fairly good idea of the distribution of people with different skin colours over the surface of the Earth. However, some interesting phenomena are not taken into account in it. For example, it does not show that the skin of the Eskimos turns out to be unexpectedly dark for the latitude at which they live. It is clear that the question «why» admits of answers of varying validity. For example, one can say that Africans have darker skin because significantly more melanin (a brown pigment) is synthesized in it than in Europeans. A correct explanation? Correct, but not leading to final clarity. And why is more melanin synthesized in them? Because they have a different activity of the melanocortin receptor of the first type (a protein that, in response to a hormonal signal, triggers the synthesis of pigment in the cells of the skin). And why is this activity different in different people? Because it is provided for by their genetic features. And why are precisely such genes characteristic of them? The traditional answer — as a result of adaptation to environmental conditions. How could such adaptations have developed? Some environmental factors would have had to cause selection favouring the manifestation of the trait that we observe. When did this selection take place — now or in our evolutionary past? Of course, in the past. The results of selection are always anachronistic. The properties of organisms are conditioned by their evolutionary history; present-day selection will become the explanation of what will manifest itself in the future. Obviously, when a species exists in a stable environment, the character of selection too remains unchanged: present-day selection turns out to be the same as that which acted once. But no species can boast such a speed of change of way of life as ours. Our innate features are explained by the way of life led by the scattered tribes of hunter-gatherers, our very distant ancestors. More or less significant corrections to this «hunter-gatherer» basis have managed to be introduced only by the epidemics of infectious diseases that swept across the more civilized part of humanity… So, what factor then determines the colour of our skin? The first assumption, which often comes to the mind of students who are asked about this, turns out to be wrong. No, the temperature does not affect the colour of human skin. Strange, of course, it ought to affect it… In low (near-equatorial) latitudes the angle of incidence of the sun's rays on the earth's surface is closer to a right angle than in temperate and high, circumpolar latitudes. Firstly, at each section of the surface in low latitudes a more intense flux of solar radiation is directed; secondly, this flux passes a shorter path in the atmosphere and turns out to be filtered by it to a lesser degree. It is precisely for this reason that in low latitudes it is much hotter than in high ones. For the inhabitants of low latitudes the struggle against overheating is far more often important, while for the inhabitants of high ones — protection from overcooling. Note: a denser flux of solar radiation falls on the equatorial latitudes, which moreover passes a shorter path through the atmosphere. The temperate and polar latitudes receive less energy, which moreover differs in spectral composition due to atmospheric filtration. …Once, in the days of my studentship, I, together with my coursemates, was sent on a one-day trip to a collective farm. Now this is hard to understand, but under the Soviet Union classes at the university were sometimes cancelled for a while (the very classes for the sake of which, generally speaking, students were students and even sometimes received some stipend), the students were loaded into buses, taken to a field together with supervisor-teachers and collective-farm «specialists», and then the «city» dwellers were compelled to put in a certain number of hours of low-efficiency labour. This was called «the help of the city to the countryside» and was supposed to compensate for the crying inefficiency of the kolkhoz and sovkhoz organization of agriculture. Together with the Soviet students, foreign students from the «fraternal countries» were also directed to this absurdity. So, on the occasion that I am recalling, one black man was riding with us. Before this trip gloomy and cool weather had lasted a long time, and here suddenly the day turned out to be sunny and hot. We were without headgear, in dense clothing, which we immediately began to take off. There was practically no drinking water (no one had foreseen it), and we suffered from the heat. We, the European-type students, were surprised that the negroid suffered from it practically the same as we did. He allowed us to touch his skin: it was much hotter than, say, mine. However, the dense head of small curly hairs well protected his head from overheating, and this too could be determined even by touch. But even among «our» students not everyone suffered to the same degree. Among us there was one blond, and his head remained cool even under the rays of the sun… As black (and dark) we see surfaces that absorb the overwhelming part of the light rays falling on them; as white (and light) — those that reflect the greater part of the light radiation falling on them. That is why black surfaces heat up in the light more strongly than white ones. If the colour of the skin were determined by selection for the effectiveness of thermoregulation, in low latitudes people with light skin would live, and in high ones — with dark. We see the opposite picture. Why? Apparently, there exists some factor whose action is more important than the influence of temperature. What is it? The influence of ultraviolet radiation. Before discussing the action of ultraviolet, I want to remind you of its probable consequences. After that trip to the collective farm many of us got sunburned. In many respects a burn from UV radiation resembles a burn from elevated temperatures. In both one and the other case cells die, blisters filled with lymph form (and sometimes burst), inflammation develops (accompanied by a rise in the temperature of the affected area of skin). Why high temperatures cause a burn is relatively clear: there the cause of the destruction of the tissues is precisely their heating. But an ultraviolet burn can develop even after such an irradiation of the skin as practically did not change its temperature! The cells of a multicellular organism demonstrate two fundamentally different modes of death, which differ roughly the same way as murder and suicide. Murder corresponds to necrosis. This is a process not controlled by the cell, triggered by the action of external factors. During apoptosis, on the contrary, the cell itself launches its own dismantling. The cell dies in both one and the other case, but, for example, the release of toxic substances capable of poisoning other cells turns out to be far smaller in the second case. Apoptosis is a necessary part of the normal process of development. Our hand is formed as a paddle; for it to be divided into fingers, apoptosis is needed of those cells that ended up in the middle between the axes corresponding to the future fingers. So, in a thermal burn the cells die as a result of necrosis, and in an ultraviolet one — as a result of apoptosis. The death of skin cells, their destruction, the influx of tissue fluid, the detachment of the epithelium — the «initiative» of the organism itself! What is its sense? Quanta of ultraviolet possess an energy corresponding to the activation energy of many chemical reactions. Irradiation with such quanta significantly raises the rate of the course of these reactions. For our discussion the most important is the photodimerization of DNA nucleotides. DNA is a chain consisting of four types of letters — the nucleotide residues A, T, G, C. The pyrimidine bases in their composition, thymine and cytosine, corresponding to the letters T and C, upon the use of the energy of ultraviolet merge in pairs into dimers (in any combinations: TT, TC, CT, and CC). The normal spatial arrangement of the DNA molecule is disrupted. A complex of specific proteins repairs the damage (restores the original state). The proteins recognize such an anomaly, cut out the damaged fragment and its surroundings (by several hundred nucleotides in both directions), and then complete one link after another. All would be well, but in the course of this process the proportion of errors — mutations — turns out to be very high. In any case the frequency of mutations as a result of the work of the repair system is lower than when repair is disrupted. A disruption of the work of this system leads to a severe hereditary disease — xeroderma pigmentosum. The skin of patients suffering from it reacts with inflammation literally to every direct ray of sunlight. Usually such people do not live long and die from regularly arising cancerous tumours. Have you ever wondered why for microorganisms ultraviolet radiation is lethal, while for macroorganisms (like you and me) — it is not? In infectious-disease hospitals premises are disinfected with the help of ultraviolet lamps. During such a procedure people leave. But even if they do not leave, they will only receive burns that are not lethal, while various pathogens of diseases will perish. The point is that ultraviolet «cuts through» microorganisms (and destroys the DNA critical for their existence), while in macroorganisms it is retained in the surface tissues (above all in the skin), where its effect is less serious. We more often recall mutations in connection with cells of the germ line. According to widespread notions, these mutations are the main material for evolution. There are also other points of view, but for the topic discussed this is not very fundamental. In any case, an evolutionary prospect is had precisely by those changes that occurred in the precursor cells of the sex cells — and hence, potentially, of other organisms. In the case of a human, mutations occur in the cells of the mortal body — the soma. Our skin consists of the epidermis (the outer layer, which originates in the process of embryonic development from the outer germ layer, the ectoderm) and the corium (the inner layer, which develops from the mesoderm, the middle germ layer). The epidermis on the open areas of skin itself consists of five layers: the basal, the spinous, the granular, the lucid, and the horny. The reproduction of cells takes place in the basal layer; it is precisely here that the cambial cells of the skin are located (the cambium is in fact the name given to the layer of dividing cells that provides for the growth of trees in thickness). These cells constantly divide. One of the daughter cells remains cambial, while the other specializes, passing in time into the higher layers of the epidermis. The cells of the horny layer are already dead and have in fact turned into horny scales. In time they flake off, constituting a significant component of house dust. Somatic mutations of the cells of the upper layers of the epidermis are insignificant: they will not have time to change anything in the fate of the future horny scales. Far more dangerous is the accumulation of mutations in the cells of the basal layer. The damage induced in them can put out of action many regulatory mechanisms of the cells. Therefore, in those cases when cells have received a sufficiently high dose of ultraviolet, the mechanism of apoptosis is triggered in them: just in case, whatever happens. And what can happen? Sometimes in cells with a disrupted apoptosis system the mechanism of stopping division is also switched off. They divide, transmitting the acquired defects to the daughter cells. A benign tumour arises. During its growth competition takes place between different cell lines. Selection favours an increase in the proportion of rapidly dividing cells and, accordingly, an increase in the rate of growth of the tumour. The disruptions that cause a tumour are diverse. Some lead to the formation of tumours with a limited growth potential; others, on the contrary, induce the development of neoplasms with reduced connections between cells. Such tumours are called malignant (cancerous); the cells can leave them, spreading by the blood (or spreading themselves) to other parts of the body and to other tissues and forming daughter tumours — metastases. Moreover, benign tumours can in time become malignant. A typical variety of skin cancer — melanoma — is one of the most rapidly growing forms of cancer. The process of malignant growth ends with the death of the organism (and of the tumour cells along with it). And in order to protect the organism from such a development of events, the cells that have received a dose of ultraviolet commit suicide. However, among the multitude of correctly self-eliminated cells, there may be found one in which the mechanism of self-elimination does not work (possibly as a result of an ultraviolet-induced somatic mutation). How is the probability of such a development of events to be reduced? To place above the basal layer of the epidermis (and the deeper layers of the skin) a sufficient quantity of molecules that intercept the quanta of ultraviolet. That is why native Africans are black!
Selection for resistance to skin cancer outweighs selection for the effectiveness of thermoregulation. …A trifle remains — to explain the white colour of the skin of Europeans.
Do not hasten to assert that there are no interesting turns of the topic here.
But about them — next time. ← Dmytro Shabanov → Anomalous frogs and the health of the environment: the search for new approaches at the border of Europe and Asia On skin colour: an attempt to discuss one human adaptation with a substantiation of far-reaching conclusions about the specifics of the action of ecological factors And more on skin colour: why we are white and why we get a tan Column for Computerra #120 Column for Computerra #121 Column for Computerra #122