By saying that someone is white or black, we may think that it belongs to a biological category defined by its color. Many people believe that the pigmentation of the skin reflects the belonging to a race, “each of the groups in which some biological species are subdivided and whose differential characters are perpetuated by inheritance”. That notion, in the case of our species, has no meaning. From a biological point of view, human races do not exist.
In the skin there are melanocytes, cells that produce and contain pigments. There are two types of pigments, called melanin: one is brownish brown (eumelanin) and the other, yellowish red (pheomelanin). The color of the skin depends on the amount and proportion of both. This depends on different genes: some affect the amount of pigment in melanocytes and others on the ratio between the two types of melanin. Therefore, very similar colors can be the result of different combinations and obey different genetic configurations.
Africans, in general, are dark skinned. Dinka, from East Africa, have it very dark; the san, from the south of the continent, clearer. The natives of southern India, New Guinea and Australia are also dark-skinned. In the center of Asia and the Far East, as well as in Europe, the skins are, in general, clear. Native Americans have different colors, although not as dark as Africans.
If we stick to the color of the hide hidden under the thick fur of the chimpanzees, it is likely that our hominin ancestors had it clear. About two million years ago the members of our lineage reduced the thickness and consistency of the coat, which became a thin layer of hair. This transformation exposed the skin to solar ultraviolet radiation, which can cause cancer and, in addition, eliminate a substance of great physiological importance, folic acid. Surely for that reason genetic variants were selected that darkened the skin, because melanin protects it from such damage.
Human beings have reached almost all latitudes. Our skin has been exposed to different radiation conditions. Just as an excess of ultraviolet rays can be very harmful, so is their defect. Without this radiation you can not synthesize vitamin D, whose deficit causes rickets and other health problems. For that reason, without discarding other possibilities such as sexual selection in favor of lighter skins, human skin has been clarified in some geographical areas by natural selection.
In addition, population movements have led to the mixture of lineages, each with their genetic traits and pigmentary characteristics, to give rise to multiple configurations. The color of current human beings is the result of a complex sequence of biological and demographic events. It is not possible to delimit groups and others biologically according to that feature.
Genetic diversity exists
The foregoing is not intended to deny genetic diversity in the human species. There is diversity, of course.
There are populations with numerous copies of the α-amylase gene and others in which there are very few.
The Inuits tolerate the cold better than other human beings and have desaturasas that allow them to feed themselves with an exclusively carnivorous diet without it causing them the problems that it would cause to other human beings.
African pygmies present genetic variants related to the immune system. A mutation in the PDE10A gene – which encodes a phosphodiestearase – allows the bajau laut (the so-called “nomads of the sea”) to remain submerged in apnea for up to thirteen minutes.
The majority of Europeans and descendants of Europeans, as well as members of other human groups in Africa, the Arabian Peninsula and the Indian subcontinent retain the capacity to digest milk lactase in adulthood.
Tibetans have lower blood levels of hemoglobin and a higher density of capillaries. Both traits seem to have a genetic basis.
In West African villages that speak Kwa languages, sickle-cell anemia is much more prevalent than in other Africans.
These features that characterize human populations do not correspond to the color of the skin. Neither the differences in skin color correspond to many other features that also vary according to other patterns and the effect of various selective pressures.
A useful concept?
It has been observed, for example, that North Americans of African origin (usually called “African Americans”) are more likely to suffer from certain diseases. That is why they defend the use of the term “race” to differentiate blacks from whites. An example is that of the greater propensity – of genetic base – of African-Americans to suffer from prostate cancer. Most of them are descended from enslaved people from West African villages where the responsible genetic variant is very common. When the gene in question has, in those same people, European ancestry, the frequency of that variant is much lower. And they all have dark skin.
The biological categories are problematic. In the animal world, different lineages and groups of lineages are differentiated, not without difficulties. We classify animals into phyla, classes, orders, families, genera, species and, in some cases, subspecies. Intermediate categories can also be defined. But we do not have races. Below the species or subspecies, there are populations.
In domestic animals it is usual to talk about races, but that is a very special case, since they have been obtained by artificial selection of certain attributes. It is, therefore, a category not transferable to the rest.
Of course there is genetic diversity in the human species. It has occurred, as in other animals, due to random mutations and the effect of natural selection on the frequency of genetic variants in each population, gene flow caused by migrations and crossings between individuals of different populations, and the genetic drift. But there are no homogeneous sets of variants that allow us to define large human groups that we can call races.
There is, therefore, no basis to invoke its existence. As there is none to justify, on non-existent bases, other differences.