Figure 12.1 Frequencies of the three alleles for the A, B, and O blood groups for selected populations around the world illustrate the polytypic nature of Homo sapiens. The frequency of the alleles differs among populations.
Genes become more common in subsequent generations. Similarly, the polymorphism of the human species has allowed us to thrive in a wide variety of environments.
When polymorphisms of a species are distributed into geographically dispersed populations, biologists describe this species as polytypic (“many types”); that is, genetic variability is unevenly distributed among populations. For example, in the distribution of the polymorphism for blood type (four distinct phenotypic groups: A, B, O, or AB), the human species is polytypic. The frequency of the O allele is highest in American Indians, especially among some populations native to South America; the highest frequencies of the allele for type A blood tend to be found among certain European populations (although the highest frequency is found among the Blackfoot Indians of the northern Plains in North America); the highest frequencies of the B allele are found in some Asian populations (Figure 12.1). Even though single traits may be grouped within specific geographic regions, when a greater number of traits are considered, specific human “types” cannot be identified. Instead each of these traits is independently subject to evolutionary forces.
As mentioned above, today anthropologists study biological diversity in terms of clines, or the continuous gradation over space in the form or frequency of a trait. As mentioned in Chapter 2, the spatial distribution or cline for the sickle-cell allele allowed anthropologists to identify the adaptive function of this gene in a malarial environment. Clinal analysis of a continuous trait such as body shape, which is controlled by a series of genes, allows anthropologists to interpret human global variation in body build as an adaptation to climate.
Generally, people long native to regions with cold climates tend to have greater body bulk (not to be equated with fat) relative to their extremities (arms and legs) than do people native to regions with hot climates, who tend to be relatively long and slender. Interestingly, these differences show up as early as the time of Homo erectus, as described in Chapter 8. A person with larger body bulk and relatively shorter extremities may suffer more from summer heat than someone whose extremities are relatively long and whose body is slender. But they do conserve needed body heat under cold conditions. A bulky body tends to conserve more heat than a less bulky one, because it has less surface area relative to volume. In hot, open country, by contrast, people benefit from a long slender body that can get rid of excess heat quickly. A small slender body can also promote heat loss due to a high surface area-to-volume ratio.
In addition to very long-term effects that climate may have imposed on human variation, climate can also
Polytypic Describing the expression of genetic variants in different frequencies in different populations of a species.
Contribute to human variation through its impact on the process of growth and development (developmental adaptation). For example, some of the physiological mechanisms for withstanding cold or dissipating heat have been shown to vary depending upon the climate that an individual experiences as a child. Individuals who grow up in very cold climates develop circulatory system modifications that allow them to remain comfortable at temperatures that people from warmer climates cannot tolerate. Similarly, hot climate promotes the development of a higher density of sweat glands, creating a more efficient system for sweating to keep the body cool.
Cultural processes complicate studies of body build and climatic adaptation. For example, dietary differences particularly during childhood will cause variation in body shape through their effect on the growth process. Another complicating factor is clothing. Much of the way people adapt to cold is cultural rather than biological. For example, Inuit peoples of Arctic Canada live where it is very cold much of the year. To cope with this, they long ago developed efficient clothing to keep the body warm. Inside their clothing, the Inuit are provided with what amounts to an artificial tropical environment. Such cultural adaptations allow humans to inhabit the entire globe.
Some anthropologists have also suggested that variation in features such as face and eye shape relate to climate. For example, biological anthropologist Carleton Coon and his colleagues once proposed that the “Mongoloid face,” common in populations native to East and Central Asia, as well as Arctic North America, exhibits features adapted to life in very cold environments.225 The epicanthic eye fold (which minimizes the eye’s exposure to the cold), a flat facial profile, and extensive fatty deposits may help to protect the face against frostbite.
Although experimental studies have failed to sustain the frostbite hypothesis, it is true that a flat facial profile generally goes with a round head. A significant percentage of body heat may be lost from the head. A round head, having less surface area relative to volume, loses less heat than a longer, more elliptical head. As one would predict from this, populations with more elliptical-shaped heads are generally found in hotter climates; those with roundershaped heads are more common in cold climates. However, these same features also could be present in populations due to genetic drift.
The epicanthic eye fold is common among people of East Asia. While some anthropologists have suggested that this feature might be an adaptation to cold, genetic drift could also be responsible for the frequency of this trait.
Culture and Biological Diversity
Although cultural adaptation has reduced the importance of biological adaptation and physical variation, at the same time cultural forces impose their own selective pressures. For example, take the reproductive fitness of individuals with diabetes—a disease with a known genetic predisposition. In North America and Europe today, where medication is relatively available, people with diabetes are as biologically fit as anyone else. However, if people with diabetes do not have access to the necessary medication, as is true in many parts of the world, their biological fitness is lost and they die out. Because financial status affects access to medical resources, one’s biological fitness may be determined by this cultural factor.
Culture can also contribute directly to the development of disease. For example, one type of diabetes is very common among overweight individuals who get little exercise—a combination that describes 61 percent of people in the United States today. As peoples from traditional
Loss of traditional cultural practices brought about by forced reservation life has resulted in high rates of diabetes among American Indians. The Pima Indians of Arizona have the highest rates of diabetes in the world today. Diabetes was not a problem for the Pima before the plentiful high-carbohydrate diet and low activity patterns typical of U. S. culture replaced their traditional lifeways. Despite the sociopolitical roots of this disease in their community, the Pima have participated in government-funded research aimed at both understanding the genetic origins of diabetes and finding effective treatment for it. Here a Pima woman prepares to give herself an insulin injection.
Consume milk or milk products. Only 10 to 30 percent of Americans of African descent and 0 to 30 percent of adult Asians are lactose tolerant.226 By contrast, lactase retention and lactose tolerance are normal for over 80 percent of adults of northern European descent. Eastern Europeans, Arabs, and some East Africans are closer to northern Europeans in lactase retention than they are to Asians and other Africans.
Generally speaking, a high retention of lactase is found in populations with a long tradition of dairying. For them, fresh milk is an important dietary item. In such populations, selection in the past favored those individuals with the allele that confers the ability to assimilate lactose, selecting out those without this allele.
Because milk is associated with health in North American and European countries, powdered milk has long been a staple of economic aid to other countries. In fact, such practices work against the members of populations in which lactase is not commonly retained into adulthood. Those individuals who are not lactose tolerant are unable to utilize the many nutrients in milk. Frequently they also suffer diarrhea, abdominal cramping, and even bone degeneration, with serious results. In fact, the shipping of powdered milk to victims of South American earthquakes in the 1960s caused many deaths.
Among Europeans, lactose tolerance is linked with the evolution of a non-thrifty genotype as opposed to the thrifty genotype that characterized humans until about 6,000 years ago.227 The thrifty genotype permits efficient storage of fat to draw on in times of food shortage. In times of scarcity, individuals with the thrifty genotype conserve glucose (a simple sugar) for use in brain and red blood cells (as opposed to other tissues such as muscle), as well as nitrogen (vital for growth and health).
Regular access to glucose through the lactose in milk led to selection for the non-thrifty genotype as protection against adult-onset diabetes, or at least its onset relatively late in life (at a nonreproductive age). Populations that are lactose intolerant retain the thrifty genotype. As a
Cultures throughout the world adopt a Western high-sugar diet and low activity pattern, their incidence rates will rise for diabetes and obesity as well.
Another example of culture acting as an agent of biological selection is lactose tolerance: the ability to digest lactose, the primary constituent of fresh milk. To digest milk, the body has to make a particular enzyme—lactase. Most mammals as well as most human populations— especially Asian, Native Australian, Native American, and many (but not all) African populations—do not continue to produce lactase into adulthood. Adults with lactose intolerance suffer from gas pains and diarrhea when they
Biocultural Connection
Beans, Enzymes, and Adaptation to Malaria
Some human adaptations to the deadly malarial parasite are biological while others are strictly tied to cultural practices such as local cuisine. The phenotype of the sickle-cell allele, for example, manifests specifically in red blood cells. Biological and dietary adaptations to malaria converge with the interaction between one form of the glucose-6-phosphate-dehydrogenase (G-6-PD) enzyme and fava bean consumption.
The fava bean is a broad flat bean (Vivia faba) that is a dietary staple
Fava beans, a dietary staple in the countries around the Mediterranean Sea, also provide some protection against malaria. However, in individuals with G-6-PD deficiency, the protective aspects of fava beans turn deadly. This dual role has led to a rich folklore surrounding fava beans.
In malaria-endemic areas along the Mediterranean coast. G-6-PD is an enzyme that serves to reduce one sugar, glucose-6-phosphate, to another sugar—in the process releasing an energy-rich molecule. The malaria parasite lives in red blood cells off of energy produced via G-6-PD. Individuals with a mutation in the G-6-PD gene, so-called G-6-PD-deficiency, produce energy by an alternate pathway not involving this enzyme that the parasite cannot use. Furthermore, G-6-PD-deficient red blood cells seem to turn over more quickly, thus allowing less time for the parasite to grow and multiply. While a different form of G-6-PD deficiency is also found in some sub-Saharan African populations, the form found in Mediterranean populations is at odds with an adaptation embedded in the cuisine of the region.
Enzymes naturally occurring in fava beans also contain substances that interfere with the development of the malarial parasite. In cultures around the Mediterranean Sea, where malaria is common, fava beans are incorporated into the diet through foods eaten at the height of the malaria season. However, if an individual with G-6-PD deficiency eats fava beans, the result is that the substances toxic to the parasite become toxic to humans. With G-6-PD deficiency, fava bean consumption leads to hemolytic crisis (Latin for "breaking of red blood cells”) and a series of chemical reactions that release free radicals and hydrogen peroxide into the blood stream. This condition is known as favism.
The toxic effect of fava bean consumption in G-6-PD individuals has prompted a rich folklore around this simple food, including the ancient
Greek belief that fava beans contain the souls of the dead. The link between favism and G-6-PD deficiency has led parents of children with this condition to limit consumption of this favorite dietary staple.
Unfortunately, this has sometimes become a generalized elimination of many excellent sources of protein such as peanuts, lentils, chickpeas, soy beans, and nuts. Another biocultural connection is again at the root of this unnecessary deprivation. The Arabic name for fava beans is foul (pronounced "fool”), while the soy beans are called foul-al-Soya, and peanuts are foul-al-Soudani; in other words, the plants are linked linguistically even though they are unrelated biologically.®
An environmental stressor as potent as malaria has led to a number of human adaptations. In the case of fava beans and G-6-PD deficiency, these adaptations can work at cross purposes. Cultural knowledge of the biochemistry of these interactions will allow humans to adapt, regardless of their genotype.
BIOCULTURAL QUESTION
How does what you have learned from this chapter about the falsehood of the biological category of race relate to the way the varied adaptations to malaria described here work against one another? 228 229
Consequence, when they are introduced to Western-style diets (characterized by abundance, particularly of foods high in sugar content), the incidence of obesity and diabetes skyrockets. This chapter’s Biocultural Connection describes another example of genetic and cultural adaptations working at cross purposes.
Melanin The chemical responsible for dark skin pigmentation that helps protect against damage from ultraviolet radiation.
Skin Color: A Case Study in Adaptation
Generally, the notion of race is most commonly equated with skin color. Skin color is subject to great variation and is attributed to several key factors: the transparency or thickness of the skin; a copper-colored pigment called carotene; reflected color from the blood vessels (responsible for the rosy color of lightly pigmented people); and, most significantly, the amount of melanin (from melas, a Greek word meaning “black”)—a dark pigment in the skin’s outer layer. People with dark skin have more melanin-producing
Figure 12.2 This map illustrates the distribution of dark and light human skin pigmentation before 1492. Medium-light skin color in Southeast Asia reflects the spread into that region of people from southern China, whereas the medium darkness of people native to southern Australia is a consequence of their tropical Southeast Asian ancestry. Lack of dark skin pigmentation among tropical populations of Native Americans reflects their more recent ancestry in northeastern Asia a mere 20,000 or so years ago.
Cells than those with light skin, but everyone (except those with a condition called albinism) has a measure of melanin. Exposure to sunlight increases melanin production, causing skin color to deepen.
Melanin is known to protect skin against damaging ultraviolet solar radiation;17 consequently, dark-skinned people are less susceptible to skin cancer and sunburn than are those with less melanin. They also seem to be less susceptible to destruction of certain vitamins under intense exposure to sunlight. Because the highest concentrations of dark-skinned people tend to be found in the tropical regions of the world, it appears that natural selection has favored heavily pigmented skin as a protection against exposure where ultraviolet radiation is most constant.
The inheritance of skin color involves several genes (rather than variants of a single gene), each with several alleles, thus creating a continuous range of phenotypic expression for this trait. In addition, the geographic distribution or cline of skin color, with few exceptions, tends to be continuous (Figures 12.2 and 12.3). The exceptions have to do with the historic movement of certain populations from their original homelands to other regions, or the practice of selective mating, or both.
Because skin cancer generally does not develop until later in life, it is less likely to have interfered with the reproductive success of lightly pigmented individuals in the tropics and so is probably not the agent of selection. On the other hand, severe sunburn, which is especially dangerous to infants, causes the body to overheat and interferes with its ability to sweat and rid itself of excess heat. Furthermore, it makes one susceptible to other kinds of infection. In addition to all this, decomposition of folate, an essential vitamin sensitive to heavy doses of ultraviolet radiation, can cause anemia, spontaneous abortion, and infertility.230
In northern latitudes, light skin has an adaptive advantage related to the skin’s important biological function as the manufacturer of vitamin D through a chemical reaction dependent upon sunlight. Vitamin D is vital for maintaining the balance of calcium in the body. In northern climates with little sunshine, light skin allows enough sunlight to penetrate the skin so as to stimulate the
17 Neer, R. M. (1975). The evolutionary significance of vitamin D, skin pigment, and ultraviolet light. American Journal of Physical Anthropology 43, 409-416.
Figure 12.3 The east-west gradient in the frequency of type B blood in Europe contrasts with the north-south gradient in skin color shown in Figure 12.2. Just as the clines for skin color and blood type must be considered independently, so too must be whatever genes are involved in the complex of abilities known as “intelligence.”
Production of vitamin D, which is essential for healthy bones and for balance within the central nervous system. Without sufficient vitamin D, bone growth in children is impaired, resulting in misshapen, fragile bones, a condition known as rickets (Figure 12.4). There is an adult form of bone disease resulting from vitamin D deficiency as well. Dark pigmentation can interfere with vitamin D synthesis and with calcium balance when there is limited natural light.
Figure 12.4 Bone diseases such as osteomalacia and rickets caused by vitamin D deficiency can deform the birth canal of the pelvis to the degree that it can interfere with successful childbirth. Because sunshine is vital to the body's production of vitamin D, this disease was very common in the past among the poor in northern industrial cities because they had limited exposure to sunlight. Dietary supplements have reduced the impact of bone diseases, such as rickets, although they continue to be a problem in cultures that require women and girls to dress so that they are completely veiled from the sun.
The severe consequences of vitamin D deficiency can be avoided through cultural practices. Until recently, children in northern Europe and northern North America were regularly fed a spoonful of cod liver oil during the dark winter months. Today, pasteurized milk is often fortified with vitamin D.
World History
