World Hot Spots
Race, Genetics and HistorySeptember 04, 2000
Compiled By Amon Hotep
The racial differences that impressed early Europeans and that continue to bother many people today include skin color, eye shape, hair type, body and facial form—in short, the traits that often allow us to determine a person's origin in a single glance. Ignoring admixture, it is fairly easy to recognize a European, an African, and an Asian, to mention those standard types with which we are most familiar. Many of these characteristics—almost homogeneous on a particular continent—give us the impression that "pure" races exist, and that the differences between them are pronounced. These traits are at least partly genetically determined. Skin color and body size are less subject to genetic influence since they are also affected by exposure to the sun and diet, but there is always a hereditary component that can be quite important.
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These characteristics influence us a lot, because we recognize them easily. What causes them? It is almost certain they evolved in the most recent period of human evolution, when "modern" humans, or early humans practically undistinguishable from ourselves, first appeared in Africa, grew in numbers, and began to expand to the other continents. Evidence and details will be discussed later. What interests us here is that this diaspora of Africans to the rest of the world exposed them to a great variety of environments: from hot and humid or hot and dry environments (to which they were already accustomed) to temperate and cold ones, including the coldest ones of the world, as in Siberia. We can go through some of the steps that this entailed.
1. Exposure to a new environment inevitably causes an adaptation to it. In the 50,000-100,000 years since the African diaspora, there has been an opportunity for substantial adaptation, both cultural and biological. We can see traces of the latter in skin color and in size and shape of the nose, eyes, head, and body. One can say that each ethnic group has been genetically engineered under the influence of the environments where it settled. Black skin color protects those who live near the equator from burning under the sun's ultraviolet radiation, which can also lead to deadly skin cancers. The dairy-poor diet of European farmers, based almost entirely on cereals that lack ready-made vitamin D, might have left them vulnerable to rickets (our milk still has to be enriched with this vitamin). But they were able to survive at the higher latitudes to which they migrated from the Middle East because the essential vitamin can be produced, with the aid of sunlight, from precursor molecules found in cereals. For this Europeans have developed the whiteness of their skin, which the sun's ultraviolet radiation can penetrate to transform these precursors into vitamin D. It is not without reason that Europeans have, on average, whiter skin the further north they are born.
The size and shape of the body are adapted to temperature and humidity. In hot and humid climates, like tropical forests, it is advantageous to be short since there is greater surface area for the evaporation of sweat compared to the body's volume. A smaller body also uses less energy and produces less heat. Frizzy hair allows sweat to remain on the scalp longer and results in greater cooling. With these adaptations, the risk of overheating in tropical climates is diminished. Populations living in tropical forests generally are short, Pygmies being the extreme example. The face and body of the Mongols, on the other hand, result from adaptations to the bitter cold of Siberia. The body, and particularly the head, tends to be round, increasing body volume. The evaporative surface area of the skin is thus reduced relative to body volume, and less heat is lost. The nose is small and less likely to freeze, and the nostrils are narrow, warming the air before it reaches the lungs. Eyes are protected from the cold Siberian air by fatty folds of skin. These eyes are often considered beautiful, and Charles Darwin wondered if racial differences might not result from the particular tastes of individuals. He called the idea that mates were chosen for their attractive quality "sexual selection." It is very likely that some characteristics undergo sexual selection—eye color and shape, for example. The shape of Asian eyes is not appreciated in Asia today. If it is admired elsewhere, why is it not found in other parts of the world? Of course it is also characteristic of the Bushmen of southern Africa, and other Africans have slanted eyes. It probably diffused by sexual selection from northeastern Asia to Southeast Asia, where it is not at all cold. It is also possible that the trait may have originated more than once in the course of human evolution. If it first appears that climatic factors were most important in the creation of racial differences, we should not neglect sexual selection as a possible side explanation. Unfortunately, the genetic bases for these adaptations are not well known; each of these traits is very complex. Considerable local variation in tastes further complicates the matter.
2. There is little climatic variation in the area where a particular population lives, but there are significant variations between the climates of the Earth. Therefore, adaptive reactions to climate must generate groups that are genetically homogeneous in an area that is climatically homogeneous, and groups that are very different in areas with different climates.
We could ask if sufficient time has passed since the settling of the continents to produce these biological adaptations. The selection intensity has been very strong, so the answer is probably yes. We could note in this regard that the Ashkenazi Jews who have lived in central and eastern Europe for at least 2,000 years have much lighter skin than the Sephardi Jews who have lived on the Mediterranean perimeter for at least the same length of time. This could be an example of natural selection, but it might also result from genetic exchange with neighboring populations. Some available genetic information favors the second interpretation, but better genetic data are desirable before we can exclude the influence of natural selection.
3. Adaptations to climate primarily affect surface characteristics. The interface between the interior and exterior plays the biggest part in the exchange of heat from the interior to the exterior and vice versa. A simple metaphor can help explain this statement: if you want to decrease the cost of heating your house in the winter, or cooling it in the summer, you must increase the house's insulation so that the thermal flow between the inside and outside is minimal. Thus, body surface has been largely modified to adapt different people to different environments.
4. We can see only the body's surface, as affected by climate, which distinguishes one relatively homogeneous population from another. We are therefore misled into thinking that races are "pure" (meaning homogeneous) and very different, one from the other. It is difficult to find another reason to explain the enthusiasm of nineteenth-century philosophers and political scientists like Gobineau and his followers for maintaining "racial purity." These men were convinced that the success of whites was due to their racial supremacy. Because only visible traits could be studied then, it was not absurd to imagine that pure races existed. But today we know that they do not, and that they are practically impossible to create. To achieve even partial "purity" (that is, a genetic homogeneity that is never achieved spontaneously in populations of higher animals) would require at least twenty generations of "inbreeding" (e.g., by brother-sister or parent-children matings repeated many times). Such inbreeding would have severe consequences for the health and fertility of the children, and we can be sure that such an extreme inbreeding process has never been attempted in our history, with a few minor and partial exceptions.
In more recent times, the careful genetic study of hidden variation, unrelated to climate, has confirmed that homogeneous races do not exist. It is not only true that racial purity does not exist in nature: it is entirely unachievable, and would not be desirable. It is true, however, that "cloning," which is now a reality in animals not very remote from us, can generate "pure" races. Identical twins are examples of living human clones. But creating human races artificially by cloning would have potentially very dangerous consequences, both biologically and socially.
We shall also see that the variation between races, defined by their continent of origin or other criteria, is statistically small despite the characteristics that influence our perception that races are different and pure. That perception is truly superficial—being limited to the body surface, which is determined by climate. Most likely only a small bunch of genes are responsible, and little significance is attached to them, especially since we are progressively developing a totally artificial climate.
Is it possible to reconstruct human evolution by studying the types of living populations only? We can simplify the process of doing so by concentrating most of our studies to indigenous people, when it is possible to recognize them and differentiate them from recent immigrants to a region. But we learn much about human origins and evolution from a single gene like ABO.
We will introduce here the word "gene." Everybody has heard it, but few know its precise meaning. The old definition, "unit of inheritance," is still difficult to understand in fact, it was used when we did not know what a gene was in chemical terms. Today we can give a much more concrete definition: a gene is a segment of DNA that has a specified, recognizable biological function (in practice, most frequently that of generating a particular protein). It is, therefore, part of a chromosome, a rod found in the nucleus of a cell that contains an extremely long DNA thread, coiled and organized in a complicated way. A cell usually has many chromosomes, and their distribution to daughter cells is made in such a way that a daughter cell receives a complete copy of the chromosomes of the mother cell. When studying evolution, however, we may, and often must, ignore what a gene is doing, because we don't know. But a gene remains useful for evolutionary studies (and others) if it is present in more than one form, and the more forms of a gene (allele) that exist, the better the gene suits our purposes. With only three alleles, ABO can hardly be very informative. In Africa, the place of origin, one finds all alleles. But this is also true of Asia and Europe. In Asia, however, the B allele is more frequent than in the other continents; group A is somewhat more common in Europe; and Native Americans are almost entirely blood group O. What conclusions can we draw? That A and B genes were probably lost in the majority of Native Americans, but why? Many have speculated about the reason, but it is impossible to provide an entirely satisfactory answer.
The first hypothesis connecting the historical origin of a people and a gene that was subsequently confirmed by independent evidence was made on the basis of the RH gene in the early forties. The simplest genetic analysis recognizes two forms: RH + and RH-. Globally, RH+ is predominant, but RH- reaches appreciable frequencies in Europe with the Basques having the highest frequency. This suggests that the RH- form arose by mutation from the RH+ allele in western Europe and then spread, for unspecified reasons, toward Asia and Africa, never greatly diminishing the frequency of the RH+ gene. The highest frequencies of the negative type are generally found in the west and northeast of Europe. Frequencies steadily decline toward the Balkans, as if Europe was once entirely RH-(or at least predominantly so) before a group of RH+ people entered via the Balkans and diffused to the west and north, mixing with indigenous Europeans. This hypothesis would have remained uncertain if it had not been substantiated by the simultaneous study of many other genes. Archeology also lent support to the argument, as we shall see later.
Reconstructing the history of evolution has proved a daunting task. The accumulation of data on many genes in thousands of people from different populations has produced a dizzying amount of information that describes the frequency of the different forms of more than 100 genes—a body of knowledge that is very useful for testing evolutionary hypotheses. Experience has shown that we can never rely on a single gene for reconstructing human evolution. It might appear that a single system of genes like HLA, which today has hundreds of alleles, would be sufficient. The HLA genes play an important role in fighting infections and recently have become important in matching donors and recipients for tissue and organ transplants. They possess a great diversity of forms, as is necessary for a potential defense against the spread of tumors among unrelated individuals, but they are also subject to extreme natural selection related to their role in fighting infection. If the conclusions we reach about evolution through observations made using HLA are different from those obtained using other genes, we need to explain the reasons, because they may lead to different historical interpretations. It is very useful, and I think essential, to examine all existing information. The broadest synthesis has the greatest chance of answering the questions we ask, and the least chance of being contradicted by later findings.
Therefore, it is also worth gathering information from any discipline that can provide even a partial answer to our problems. Within genetics itself, we want to collect as much information about as many genes as possible, which would allow us to use the "law of large numbers" in the calculation of probabilities: random events are important in evolution, but despite their capriciousness, their behavior can be accounted for through a large number of observations. Jacques Bernoulli, in his Ars conjectandi of 1713, wrote, "Even the stupidest of men, by some instinct of nature, is convinced on his own that with more observations his risk of failure is diminished."
Many studies have been invalidated because of an inadequate number of observations. When we study polymorphisms directly on DNA, there is no dearth of evidence: we can study millions. We may not need to study them all, because at a certain point additional data fail to provide new results or lead to different conclusions.
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