Explaining Human Biodiversity

Learning Objectives:

• Identify the differing sources of human variation
• Discuss physical characteristics in human populations that may represent adaptations stemming from natural selection
• Distinguish race as a cultural construct with physiological and social consequences evident among populations
• Explain how continuous variation of traits in human populations are understood, measured and analyzed

Sources of Human Variation

Although modern humans represent a single species, we clearly are not all alike. As a species, humans exhibit a great deal of phenotypic variation, which is shaped, in part, by both their genetic makeup and unique life histories, which are products of a tremendous amount of gene exchange and adaptation over time. However, we clearly have some traits in common with other people.

Biological anthropologists who focus their research on human variability study the range and frequency of possible genetic, physical and mental characteristics in humans. Common areas of study include life history (e.g., growth and aging), physical traits (e.g., skin color, body shape, cognitive ability), and biochemical traits (e.g., blood group, immunity, nutrition). Human variation is typically quantified using:

• biostatistics, or the branch of statistics that deals with data relating to living organisms;
• bioinformatics, or the science of collecting and analyzing complex biological data such as genetic codes; and
• demography, which is the statistical study of the size, structure, and distribution of populations, and spatial or temporal changes in them in response to birth, migration, aging, and death.

To understand variation among human populations, we must consider four aspects of variability:

• evolutionary mechanisms affecting genetic diversity within and between populations;
• critical periods in an individual’s growth and development;
• interactions with the physical environment; and
• cultural differences that determine living conditions and construct unique life experiences.

Examining trait variation

A population’s total complement of genes is referred to as a gene pool. These genes may have two or more alternate forms, known as alleles. When a single gene has two or more alleles with frequencies of about 1% or greater, it is polymorphic. This type of discontinuous variation divides the population into two or more distinct forms. Common examples of polymorphic traits in humans include eye color and blood type. The expression of polymorphic traits varies in populations from different parts of the world. Very specific populations found in distinct habitats that can be distinguished regionally on the basis of discrete phenotypes are referred to as being polytypic.

Certain periods of human growth and development are controlled by genetics; however, these critical periods are also highly susceptible to influences from an individual’s physical and cultural surroundings. Physical differences among humans may result from how well the requirements of growth are met, but people around the world have differing beliefs and practices concerning food choice and nutritional needs.

The physical environment influences human variability through selective pressure and evolutionary mechanisms of change, such as natural selection, acting on populations. However, our bodies allow us to to individually maintain performance across a range of environmental conditions (e.g., temperature, humidity and altitude) through short-term adjustments called acclimatization (also known as acclimation). These short-term regulated responses to novel environments or new conditions may take minutes to weeks, depending on the degree to which an individual can change certain traits.

Adaptations are heritable traits that aid the survival and reproduction of an organism in its present environment. Some adaptations may occur during an individual’s lifetime. These are known as functional adaptations. While functional adaptations also take place on an individual level, these changes can over several generations bring about genetic changes in a population. Acclimatization and adaptations maintain a relatively stable equilibrium in physiological processes, which is known as homeostasis. Potentially harmful agents or disruptions that affect organisms are known as stressors. Survivability in some environmental extremes, such as hot climates and high altitudes, may be heavily influenced in the long-term by adaptations, but humans have also made use of material culture to make living possible in these same extremes.

Most cultural practices that guide who we are, such as the way we dress, the food we eat, or the kind of house we live in, may seem superficial, but all of these practices to some extent influence our interactions with the physical environment and possibly the genetic expression of specific traits. The process by which humans use cultural knowledge to modify their environments is known as niche construction. Not all human modifications have positive results for populations. For example, overcrowding, the spread of disease and a lack of quality food resources have all had an evolutionary impact on human populations.

Because of the complex interrelationships among genetic, environmental, and cultural influences, the relative importance of each of these factors can be identified through detailed analysis of specific traits in human populations. As we have seen, the genetic makeup of modern humans has the potential to produce a great deal of variation; however, many physical traits vary independently of one another. Biological anthropologists use an approach known as univariate analysis to examine clines, or measurable gradients in a biological trait of a species across its geographical range. Maps of clinal distribution (Figure 3.1) indicate that clines can show smooth, continuous

Figure 3.1: Clinal distribution of a genetic trait in European populations that is shared with a common ancestor originating from the Western region about 3,000 years ago

gradation in a character, or they may show more abrupt changes in the trait from one geographic region to the next. Plotting the distribution of individual traits in human populations sheds light on the genetic, environmental, and cultural factors that influenced their distribution. Mathematical models are also used to analyze evolutionary processes in a gene pool and track changes to specific physical traits within a population.

Biological anthropologists also use multivariate analysis to examine a number of different traits to better understand the interrelationships that exist among them. An excellent example of this approach is seen in studies conducted on twins to reveal the importance of environmental and genetic influences for traits, phenotypes, and disorders. The classical twin design compares the almost 100% genetic similarity of monozygotic (identical) twins to the average 50% genetic similarity in dizygotic (fraternal) twins. If identical twins are considerably more similar than fraternal twins in a study, this implicates that genes play an important role in these traits. By comparing many hundreds of families with twins, researchers can then understand more about the roles of genetic effects, shared environment, and unique environment in shaping behavior. Modern twin studies have concluded that almost all traits are in part influenced by genetic differences, with some characteristics showing a stronger genetic influence (e.g., height), others an intermediate level (e.g., personality traits) and some more complex, with evidence for different genes affecting different aspects of the trait (e.g., autism).

Human Growth and Development

Growth is the process of increasing in physical size, while development is the progressive acquisition of various physiological abilities. In humans, our genes determine the timing of growth, but environmental and cultural factors influence our development throughout our life history. Human life history can be divided into four major periods of development: embryonic, prenatal, postnatal and adult.

Human embryonic development lasts from implantation of a zygote, or the cell that results from the union of a sperm and ovum, until about 8 weeks from the time of conception. During this time the embryo is developing through a process where cells begin to take on different functions and layers of cells create different structures. Embryos are highly susceptible to mutation and disruption from environmental factors. All of the essential structures have been formed by the time embryonic development comes to an end. The fully developed embryo at this point is now referred to as a fetus.

Prenatal development takes place from about the 10th week of pregnancy through to birth. During this period there is a dramatic change in size that takes place. When prenatal development begins, the fetus is around 4 cm (1.5 in) long and weighs about 3 gm (0.1 oz), and, by the time this period ends at 38 to 40 weeks, it will probably be over 0.5 m (20 in) long and weigh about 3.4 kg (7.5 lb). As the fetus grows, development of bodily functions will continue in preparation for life outside the womb. Nutritional and environmental disruptions can greatly affect how the organ systems formed during embryonic development will continue to grow and develop. For example, the lungs become capable of breathing air, the brain develops more fully, and fat deposits accumulate in order to help maintain body heat after birth. At this time, the fetus can also respond to sound, light and touch.

Postnatal development includes various periods from birth through adolescence.

• The neonatal period lasts from birth until a newborn about is 4 weeks old. Rather than having all of its need being met directly in the womb, it must now learn to adapt and survive in a new environment. How well the newborn adapts in the first month of life is essential to its survival, since it is at its greatest risk for disease or death than at any other time.
• The infant period takes place over the second month following birth through weaning, which on average is around 3 years of age. This is a time of great physical and cognitive development, including the ability to sit up, crawl and start walking, as well as using simple gestures, imitating words, and exploring objects and their surroundings.
• Childhood lasts until about 7 years of age. While overall physical growth slows during childhood, this is a time of great cognitive, emotional and social development. Early in childhood, rapidly growing brains require diets rich in fats and protein, and individuals are learning behaviors that benefit their survival, but they are still heavily dependent on adults for resources and care. Around 6 years of age, brain growth is complete and primary teeth are being replaced with permanent teeth.
• The preadolescent period is a time of transition around 7 years and commonly ends with the beginning of puberty, or the process of physical changes resulting in the body being capable of sexual reproduction. This is initiated by the release of sex hormones, which occurs around 10-11 years in females and 11-12 years in males. During puberty, reproductive organs mature and secondary sex characteristics, such as female breast development and male facial hair, appear. The major landmark of puberty for females is the onset of menstruation, and for males, it is the first ejaculation, with both of these events occurring on average between the ages of 12-13.
• Adolescence is a transitional period of physical and psychological development that generally occurs over the 5 to 10 years following puberty. This transition is marked by a growth spurt, or a rapid increase in an individual’s height and weight resulting from the release of hormones. Males experience their growth spurt about two years later, on average, than females, and during the peak velocity of growth, adolescents grow at a rate nearly identical to that of an infant. The accelerated growth in different body parts happens at different times, but for all adolescents it has a fairly regular sequence. The first places to grow are the extremities (e.g., head, hands and feet) followed by the arms and legs, then the torso and shoulders. This non-uniform pattern of growth is one reason why an adolescent body may seem out of proportion. Growth centers, or epiphyses, in long bones of the arms and legs begin to fuse to the central shaft, or diaphyses, of the bones through mineralization of growth plates, or metaphyses, that previously allowed for the lengthening of bone to occur during growth (Figure 3.2). Adolescence ends when these centers are fully fused, and thereby terminating physical growth in height.

In adult development, basic structure of the body is present, but it continues through physiological changes involving the replacement of cells and tissues. Aging is the process of growing older, while senescence is the gradual deterioration of physiological function with age. As we senesce, our bodies have a harder time responding to stressors, which in turn increases homeostatic imbalance, and increases risk of aging-associated diseases such as osteoporosis, cancer and heart disease. Senescence also decreases our capability to reproduce. Menopause, or the point in time when menstrual cycles permanently cease due to loss of ovarian function, marks the end of the reproductive phase in females, while males experience a notable decrease in sperm viability and motility.

Figure 3.2: Xray of lower leg of 12 year-old child showing growth plates prior to fusion and termination of physical growth

Adaptive Aspects

Human skin color

Although human skin color may appear dramatically different in different individuals, it is a physical characteristic that exhibits continuous variation; that is, the range of small differences from one extreme to another that we see among individuals. Understanding of the genetic mechanisms underlying human skin color variation is still incomplete, however there are a number of genes that affect human skin color in specific populations, and this happens independently of other physical features such as eye and hair color. Different populations have different allele frequencies of these genes, and it is the combination of these allele variations that bring about the complex, continuous variation in skin coloration we can observe today in modern humans.

A number of substances affect skin color in humans, although the single most important substance is the pigment melanin, which determines the lightness or darkness of tan, brown and black skin color. Melanin is produced within the skin in cells called melanocytes. All modern humans have about the same number of melanocytes, but genetics control their arrangement and the amount of melanin they produce. People with light skin have less melanin and their skin color is determined mainly by the bluish-white connective tissue below the dermal layer in skin and by hemoglobin, a protein that gives red blood cells their color, circulating in the veins of the dermal layer. The environment also directly influences skin color. Exposure to the ultraviolet radiation (UVR) in sunlight stimulates the production of melanin, resulting in a tanning response.

Adaptive aspects of light and dark skin color may have played a role in the presence of light and dark skin color in different human populations. There is a direct correlation between the geographic distribution of ultraviolet radiation (UVR) and the distribution of indigenous skin pigmentation around the world (Figure 3.3). Areas that receive higher amounts of UVR, generally located closer to the equator, tend to have darker-skinned populations. Areas that are far from the tropics and closer to the poles have lower intensity of UVR, which is reflected in lighter-skinned populations.

Research suggests that skin color adapts to intense sunlight irradiation to provide partial protection against the UVR that produces sunburn and skin cancer damage and thus mutations in the DNA of the skin cells. Extended UVR exposure also results in a significant decrease in folate, a B-vitamin that is extremely important during periods of rapid growth such as pregnancy, infancy, and adolescence. Low folate levels during early pregnancy have been correlated with embryonic defects such as spina bifida, when the spine and spinal cord don’t form properly, and anencephaly, or the absence of a major portion of the brain, skull, and scalp. Those with darker color skin in high UVR regions would be better protected against these damaging effects than those with lighter skin.

Figure 3.3: Global map of indigenous skin pigmentation prior to major population movements of the past 500 years

However, darker skin is not advantageous in all environmental settings. In regions with less sunlight, other selective pressures resulted in an adaptive advantage for those with lighter color skin. When human skin is exposed to UVR in sunlight, synthesis of Vitamin D is stimulated in the skin. Vitamin D is necessary for the maintenance of teeth and bones, supporting the health of the immune system, and regulating insulin and lung function. It also influences the expression of genes involved in cancer development.

Over time, natural selection led to populations in equatorial regions with ample exposure to sunlight evolving darker skin pigmentation to avoid the deleterious effects of UVR, while populations in who lived in northern temperate and cold climates would have increased chances of survival and, ultimately, reproductive success if they had lighter skin that absorbed higher levels of UVR and synthesize more Vitamin D.

Thermoregulation

As in other mammals, body temperature in humans is an important aspect of homeostasis. Thermoregulation is s a process that allows your body to maintain its core internal temperature (Figure 3.4). Body heat is generated mostly in the deep organs, especially the liver, brain, and heart, and in contraction of skeletal muscles. Body temperatures that are too high or too low cause serious stress, placing the individual in great danger of injury or even death. For humans, adaptation to varying climatic conditions includes both physiological mechanisms resulting from evolution and behavioral mechanisms resulting from conscious cultural adaptations.

Figure 3.4: Distribution of body heat in hot and cold surroundings; in hot environments vasodilation carries heat close to the body surface, while in cold climates vasoconstriction retains heat deep in the body core

In hot conditions, glands in the skin excrete sweat, a fluid containing mostly water with some dissolved minerals, through pores onto the skin’s surface. This causes heat loss via evaporative cooling; however, a lot of essential water is lost. Tiny muscles under the surface of the skin relax so that their attached hair follicles remain flat. These flat hairs increase the flow of air next to the skin increasing heat loss by convection. The smooth muscle walls of small blood vessels relax allowing vasodilation, or the increased blood flow and redirecting of blood into the superficial capillaries in the skin, which increases heat loss by convection and conduction.

Hyperthermia is a condition where an individual’s body temperature is elevated beyond normal due to failed thermoregulation. The person’s body produces or absorbs more heat than it dissipates. The normal human body temperature is 36.5 °C (97.7 °F), but it can be as high as 37.7 °C (99.9 °F) in the late afternoon. Hyperthermia requires an elevation from the temperature that would otherwise be expected. Such elevations range from mild to extreme; body temperatures above 40 °C (104 °F) can be life threatening. An early stage of hyperthermia can be heat exhaustion, whose symptoms can include heavy sweating, rapid breathing and a fast, weak pulse. If the condition progresses to heat stroke, then vasodilation and an inability to cool the body through perspiration may cause the skin to feel hot and dry.

In cold conditions, heat is lost mainly through the extremities (e.g., hands, feet, head). Those same tiny muscles under the surface of the skin now contract so that their attached hair follicles stand erect, which acts as an insulating layer, trapping heat. Small blood vessels carrying blood to superficial capillaries under the surface of the skin contract allowing vasoconstriction, or the rerouting blood away from the skin and towards the warmer core of the body. This prevents blood from losing heat to the surroundings and also prevents the core temperature dropping further. Muscles can also receive messages from the brain to start shivering. Shivering is more effective than exercise at producing heat because the individual remains still. This means that less heat is lost to the environment through convection.

Hypothermia is reduced body temperature that happens when a body dissipates more heat than it absorbs. In humans, it is defined as a body core temperature below 35.0 °C (95.0 °F). Frostbite occurs only when water within the cells begins to freeze, which destroys cell structure and causes tissue damage.

Human body form

The human body includes various sizes and forms. Some of us are taller and thinner, while others are shorter and thicker in height and volume respectively. Over multiple generations, these distinct differences are the result of adaptations to environmental conditions and the body’s ability to maintain a relatively stable equilibrium in physiological processes.

Bergmann’s rule generally states that larger body sizes are observed in populations that live farther from the equator or in high-altitude regions. The principle is named after a 19th century German biologist, Karl Bergmann, who published his observations on the subject. This tendency was initially attributed to the fact that there is a decrease in the surface area to volume ratio when species increase in size, allowing them to retain body heat more efficiently in cold climates. However, many exceptions to the rule are known. The general explanation for the phenomenon is attributed to the importance of the surface area to volume ratio, with warm-blooded animals in colder climates having an advantage in the reduction of the surface area to volume ratio, in order to reduce heat loss. For example, polar bears have large, compact bodies, relative to bears in warmer climates, thus offering a smaller relative surface area for losing heat.

Bergmann’s rule has also been applied to populations of humans. Pygmy peoples, or ethnic groups whose average height is unusually short, are found only in the tropics, and other tropical ethnic groups tend to be shorter than those in temperate regions. Also, in Europe, Southern European populations, such as Italian or Greek ethnic groups, tend to be shorter on average than Northern European populations, such as Swedish or Norwegian ethnic groups. The same goes for Asian populations, as northern Asian ethnic groups are on average larger than their Southeast Asian counterparts. The Inuit of Alaska, Greenland and northern Canada have a greater body volume due to accumulation of fat to retain body heat, which is a functional adaptation to severe cold.

A corollary of Bergmann’s rule is Allen’s rule, which indicates a trend in terms of appendages of warm-blooded animals according to latitude. This principle states that individuals in populations of the same species located in warm climates near the equator tend to have longer limbs than individuals in populations located in colder climates further from the equator. For example, the Inuit people, who live and hunt in northern climates, tend to have a more stocky body with shorter arms and legs than the Maasai people of Kenya and Tanzania, who have a taller, slender body shape, with long limbs (Figure 3.5). The principle is attributed to the same factor of heat conservation seen in Bergmann’s rule. That is, longer appendages offer more surface area, and thus greater opportunity to dissipate heat, while shorter appendages offer less surface area and are more effective in maintaining body heat.

Because the human skull and facial features vary tremendously in shape, numerous theories explaining this variation have been advanced over the centuries. As with body form, the shape of the skull and face may represent adaptations to the physical environment. People living in colder climates tend to have more rounded heads, which conserve heat better, while people in warmer climates tend to have narrow skulls. Higher, narrower nasal openings have more surfaces that moisten inhaled air. People living in drier climates tend to have more surface area in their nasal openings, regardless of whether the environment is hot or cold. Like the generalizations related to body form, there are many exceptions to these patterns of possible adaptation.

Figure 3.5: Maasai warrior from Kenya (top) and an Inuit man from Nunavut, Canada (bottom) illustrate variation in human body form according to Bergmann’s and Allen’s rules

Biochemical variation

One of the most well-known biochemical variants in humans is the ABO blood type system. ABO blood type is a polymorphic trait that is controlled by a single gene with three types of alleles: A, B, and O. A and B are both dominant, and O is recessive. They are expressed in four phenotypes: Type A (genotypes AA and AO ); Type B (genotypes BB and BO ); Type AB (genotype AB ); and Type O (genotype OO ).

Antigens are substances that stimulate an immune response through the production of antibodies, which are proteins that counteract a specific antigen when it enters the body. The presence of different antigens and antibodies associated with each blood type is the reason that doctors need to know a person’s blood type before giving a blood transfusion. Type A blood has anti-B antibodies, and vice versa (Figure 3.6). Type O incorporates antibodies that fight against proteins in both Type A and Type B.

Type O has the highest frequency of expression, while Type AB has the lowest. Types A and B occur frequently, but in smaller percentages than Type O. However, the three blood-group alleles are found throughout the world in varying prevalence from population to population (Figure 3.7). Type B prevalence is greatest in Asia and declines as one moves west. This pattern is attributable to population movements by Asian

Figure 3.6: Characteristics of the blood types in the ABO blood group

nomads who spread to eastern and central Europe in several waves over the last 2,000 years. Dissemination of Type B was less with smaller groups who migrated farthest, and concentration of Type B remained greatest in regions of origin. A second cline of the ABO blood group in Europe and Asia exists with Type A, which is greatest in Europe and decreases as one moves east, a pattern opposite to that of Type B.

When a mutant variation in alleles is harmful, your first inclination might be to describe it as a maladaptation, or an adjustment in an organism that undermines the ability to cope with environmental challenges. But when a harmful trait proves helpful in a population’s survival, the frequency of the alleles involved in its expression will be increased by natural selection. One example of this process is seen in sickle cell disease (SCD), a group of blood disorders that occur when a person inherits two recessive alleles of the hemoglobin gene, one from each parent. The most common type is known as sickle cell anemia ( SCA ) (Figure 3.8). It results in an abnormality in the oxygen-carrying protein hemoglobin found in red blood cells. Abnormal hemoglobin molecules collapse into a rigid, sickle-like shape, inhibiting the distribution of oxygen to tissues.

Figure 3.7: Clinal distributions of ABO blood types among indigenous populations

Figure 3.8: Comparison of normal (top) and sickle-shaped (bottom) red blood cells

Figure 3.9: Sickle cell anemia is an autosomal recessive inherited condition; those with heterozygous alleles are not affected and are “carriers” of the trait

People who are homozygous ( HbS HbS ) for sickle-cell anemia usually die at a fairly young age, and only individuals who are heterozygous ( HbS HbA ) for the trait are “carriers,” passing the trait to the next generation. Two heterozygous “carrier” parents have a 25% chance of having a child with the trait for the disease (Figure 3.9).

Figure 3.10: Geographic distribution of sickle-cell allele and malaria in Africa

Sickle cell disease occurs more commonly among people whose ancestors lived in tropical and sub-tropical sub-Saharan regions of Africa where malaria is or was common (Figure 3.10). Where malaria is common, carrying a single sickle cell allele is a heterozygous advantage – humans with one of the two alleles of sickle cell disease ( HbS ) show less severe symptoms when infected with malaria. Consequently, the survival of those with heterozygous genotypes for the trait balances the deaths of those who are homozygous recessive for the trait. This is what geneticists refer to as a balanced polymorphism, when homozygous and heterozygous genes exist in a state of relative stability, or equilibrium, within a population.

An example of the role of cultural practices shaping natural selection on genes that affect human health can be seen in our ability to digest certain foods. Lactase persistence (also known as lactose tolerance) is the continued activity of the digestive enzyme lactase in adulthood. Since the only function of lactase is the digestion of lactose, the sugar found in milk, in most mammal species, the activity of the enzyme is dramatically reduced after weaning, which in humans is around the age of 3 years. In some human populations, though, lactase persistence has recently evolved as an adaptation to the consumption of nonhuman milk and dairy products beyond infancy.

There are a number of polymorphisms within the human lactase gene, and “persistence genes” appear to have arisen multiple times independently in human populations. Lactase persistence is inherited as a dominant trait, while lactase deficiency is the result of being homozygous for the recessive lactase allele that is poorly expressed after early childhood.

The majority of people around the world are affected by varying degrees of lactase deficiency (also referred to as lactose intolerance) as adults (Figure 3.11). However, not all genetically lactase deficient individuals are noticeably lactose intolerant, and not all lactose intolerant individuals have the lactase deficiency genotype. Without lactase, milk ferments in the intestine, causing diarrhea, bloating and cramps.

Figure 3.11: Percentage of people worldwide with lactase deficiency

High altitude living

Humans have settled in an amazing range of environmental landscapes, and the physical environment plays a role in causing differences in human populations. These populations have sometimes adapted to similar environmental differences in different ways.

Acclimatization is the process of adjusting to a change in environment (e.g., altitude, temperature, humidity) that allows an individual to maintain physical capabilities across a range of environmental conditions. Individuals that visit or move to high altitudes, for example, may initially have difficulty breathing, but after living in this environment for some time their bodies will acclimatize and they will be able to breathe easier. This physiological adjustment is temporary: When these individuals return to lower altitudes, their bodies and breathing will revert to their earlier states. Acclimatization can be differentiated from functional adaptations, where individuals born or raised in certain environmental conditions may develop non­reversible physical characteristics.

Because of the lower barometric pressure at high altitude, people take in less oxygen, making the air feel “thinner.” So, at high altitudes, most humans experience dizziness and breathing difficulties, which are symptoms of hypoxia, or oxygen deficiency. High-altitude adaptation in humans is an example of functional adaptation in certain human populations, including those of Tibet in Asia, the Andes of the Americas, and Ethiopia in Africa, who have acquired the ability to survive at extremely high altitudes. Indigenous inhabitants of these regions have undergone extensive physiological and genetic changes, particularly in the regulatory systems of oxygen respiration and blood circulation, when compared to the general lowland population.

Where the Tibetan highlanders live, the density of oxygen molecules in the air is only about 60% of that at sea level. The Tibetans, who have been living in this region for 3,000 years, do not exhibit the elevated hemoglobin concentrations to cope with oxygen deficiency as observed in other populations who have moved temporarily or permanently at high altitudes. Instead, the Tibetans inhale more air with each breath and breathe more rapidly than sea-level populations. Tibetans have better oxygenation at birth but a lower concentration of hemoglobin, enlarged lung volumes throughout life, and a higher capacity for exercise.

In contrast to Tibetans, Andean highlanders, who have been living at high-altitudes for no more than 11,000 years, show different pattern of hemoglobin adaptation. When Andean highlanders spend some weeks in the lowland, their hemoglobin drops to the average of other people. This shows only temporary and reversible acclimatization taking place. However, in contrast to lowland people, Andean highlanders do have increased oxygen levels in their hemoglobin, conferring the ability to carry more oxygen in each red blood cell. This leads to a more effective transport of oxygen in their body, while their breathing is essentially at the same rate, enabling them to overcome hypoxia and normally reproduce without risk of death for the mother or baby.

Highland Ethiopians exhibit elevated hemoglobin levels, like Andeans and lowlander peoples at high altitudes, but do not exhibit the Andeans’ increase in oxygen levels in hemoglobin. But, similar to the Andean highlanders and Tibetans, the Ethiopian highlanders are immune to the extreme dangers posed by high-altitude environment, and their pattern of adaptation is definitely unique from that of other highland peoples.

Concept of Race

Groups of humans have always identified themselves as distinct from neighboring groups. The idea of race as we understand it today came about during the historical process of exploration and conquest which brought Europeans into contact with groups from different continents, and of the ideology of classification and typology found in the natural sciences. By the 19th century, the term race was often used in a general biological taxonomic sense, to denote phenotypically differentiated human populations.

Humans in both the past and present have used various racial classifications to categorize people and developed stereotypes about the behavior and mental abilities of different “racial categories.” These categories are dependent on the criteria selected, such as skin color. hair color and texture, eye color, blood groups, body build, and facial structure. One of the earliest scientific efforts to organize human variation into racial categories was undertaken by Swedish taxonomist Carl von Linné in the early 18th century. He divided Homo sapiens into four races based on skin color: Homo europaeus (white Europeans), Homo americanus (red North American Indians), Homo asiaticus (yellow Asians), and Homo afer (black Africans). His classification of humans was influenced by ancient and medieval theories, as well as European perceptions of their superiority. For example, he described American Indians with reddish skin as needing regulation and Africans with black skin as being negligent and governed by impulse. In contrast, Europeans with white skin were described as gentle, acute, inventive, and governed by laws.

Following Linnaean taxonomy in the later 18th century, the German scientist Johann Blumenbach devised a racial classification system that is still sometimes used in popular, unscientific discussions of race. He divided humans into five distinct groups-Caucasian, Mongolian, Malay, Ethiopian, and Native American-corresponding to the colors white, yellow, brown, black, and red, respectively. Blumenbach based his racial typology primarily on skin color as well as geography, but he considered other traits as well, including facial features, chin form, and hair color. While Blumenbach emphasized the unity of all humanity, his typologies were modified during the nineteenth and early twentieth centuries as the three races of mankind, the Caucasoid, Mongoloid, and Negroid as a means of justifying slavery, colonialism, and racism throughout the world. In addition, a number of physical anthropologists in the United States and Europe, such as Samuel Morton, Ernst Haeckel, Rudolph Virchow, and others, began to assert that the Caucasian race had larger brains and higher intellectual capacities than non-Caucasians.

Because early researchers such as Carl von Linné and Johann Blumenbach created their typologies before Charles Darwin and Gregor Mendel had published their findings, they did not incorporate the modern principles of natural selection, heredity, and population genetics. They also encountered another fundamental problem in distinguishing races. By correlating physical characteristics with cultural differences, these classification systems assumed erroneously that populations which shared certain physical traits, especially skin color, also shared other physical characteristics and behaviors. However, instead of falling into discrete divisions, many of these added physical characteristics and behaviors also exhibited continuous variation. The great disagreement among scientists over the number and traits of different races is a good indication of the problematic nature of this the concept. These early beliefs gave rise to many current popular misconceptions and generalizations concerning the values, traditions, and behaviors of different peoples.

In the early 20th century, many anthropologists taught that race was an entirely biologically phenomenon. There was a belief that all members of each race possess physical and mental characteristics or abilities specific to that race, especially so if distinguishing it as inferior or superior to another race or races, a position commonly called racial essentialism. This, coupled with the misuse of science to justify the claim that some groups are racially superior to others, formed the basis of what is now called scientific racism. Studies of culture and population genetics of the 1950s and 1960s eroded the scientific standing of racial essentialism and weakened any arguments based on scientific racism, leading many anthropologists of the time to revise their conclusions about sources of phenotypic variation.

An overwhelming majority of anthropologists and biologists now view the concept of race as an invalid genetic or biological designation. Among the first to challenge the concept of race on empirical grounds were the anthropologists Franz Boas, who provided evidence of phenotypic plasticity due to environmental factors, and Ashley Montagu, who relied on evidence from genetics. E.O. Wilson, a prominent biologist, challenged the concept from the perspective of general animal systematics and further rejected the claim that races were equivalent to taxonomic subspecies.

Although the diversity of human populations is undeniable, delineating specific races has little practical or scientific value in studying human variation. More importantly, racial classifications do not explain the reason for observed variation. If races constituted fundamental divisions within the human species, such differences would be readily measurable; in fact, they are not. Human genetic variation is predominantly within races, continuous, and complex in structure, which is inconsistent with the concept of genetic human races. According to Jonathan Marks:

“By the 1970s, it had become clear that (1) most human differences were cultural; (2) what was not cultural was principally polymorphic – that is to say, found in diverse groups of people at different frequencies; (3) what was not cultural or polymorphic was principally clinal – that is to say, gradually variable over geography; and (4) what was left – the component of human diversity that was not cultural, polymorphic, or clinal -was very small.

A consensus consequently developed among anthropologists and geneticists that race as the previous generation had known it – as largely discrete, geographically distinct, gene pools – did not exist“ (Marks 2008:28).

Modern scholarship views racial categories as socially constructed; in other words, they are not intrinsic to human beings but rather an identity created, often by socially dominant groups, to establish meaning in a social context. This often involves the subjugation of groups defined as inferior to a dominant group. Therefore, race is a constructed grouping of humans based on shared physical or social qualities into categories generally viewed as distinct by society. This view completely rejects the notion that race is biologically defined. In turn, racism is any prejudice, discrimination, or antagonism directed against someone of a different race based on the belief that one’s own race is superior.

An example of race being created and realized through culture and socialization is found in health-related factors. There are well-defined inequalities between racially defined groups for a range of biological outcomes, such as cardiovascular disease, diabetes, stroke, certain cancers, low birth weight, and preterm delivery. Initially, biomedical researchers saw these patterns as evidence of fundamental genetic differences between alleged races. However, an ever-growing body of evidence has established that social inequalities are in fact the origin and persistence of racial health disparities. This, once again, reinforces the concept that race is a cultural construct that is inconsistent with patterns of global human genetic diversity.

Heredity and intelligence

Intelligence is the ability to acquire and apply knowledge and skills. It gives humans the cognitive abilities to learn, form concepts, understand, and reason, including the capacities to recognize patterns, comprehend ideas, plan, solve problems, and use language to communicate. Intelligence enables humans to experience and think.

Most scientists agree that intelligence varies among individuals, but it is difficult to measure intelligence objectively because intelligence is unique to the culture that one is studying. Nevertheless, a number of tests have been developed in an attempt to quantifiably measure intelligence, the most prominent among them being the intelligence quotient (lQ) test, invented by French psychologist Alfred Binet in 1905. The results of such tests provide a number representing a person’s reasoning ability as compared to the statistical norm or average for their age.

At the outset of World War I, U.S. Army officials were faced with the monumental task of screening an enormous number of army recruits. Psychological IQ tests were administered to over two million soldiers in an effort to classify recruits and to determine which individuals were “officer material” (Figure 3.12). The results of these tests raised questions not only about the mental abilities and backgrounds of the men but also about possible biases in the tests themselves.

Figure 3.12: American soldiers taking a psychological intelligence text in 1917

The concept of intelligence and the degree to which intelligence is measurable is a matter of debate. It remains unclear whether group differences in intelligence test scores are caused by heritable factors or by other correlated demographic variables such as socioeconomic status, education level, and motivation. However, based on the cultural construction of the concept of race as discussed above, many anthropologists argue that cultural factors can lead to certain groups succeeding because culture designs the tests specifically for these groups. This then becomes the root cause of the divisions being realized and naturalized as racial fact.

In a controversial book called The Bell Curve: Intelligence and Class Structure in American Life (1994), the authors argue that race is related to intelligence. Using a bell curve statistical distribution, they place the IQ of people with European ancestry at 100. People of East Asian ancestry exceed that standard slightly, averaging 103, while people of African descent fall below that standard, with an average IQ of 90. These results strongly imply that IQ scores are related to genetic differences among races. In opposition to The Bell Curve, the book Intelligence and How to Get It: Why Schools and Cultures Count (2009) presents results from numerous statistical, historical, and experimental studies, and challenges the argument that IQ is entirely or almost entirely heritable, arguing that nonhereditary factors, such as socioeconomic class, are more impactful than genetic factors in determining intelligence.

Currently, research into differences in IQ based on race or ethnicity can be placed into four general positions:

• scores reflect real differences in average group intelligence, which is caused by a combination of environmental factors and heritable differences in brain function;
• differences in average cognitive ability between races are caused entirely by social and/or environmental factors;
• differences in average cognitive ability between races do not exist, and that the differences in average test scores are the result of inappropriate use of the tests themselves; and
• either or both of the concepts of race and intelligence are poorly constructed and therefore any comparisons between races are meaningless.

While there is some consensus about how to define intelligence, it is not universally accepted that it is something that can be unequivocally measured by a single figure. IQ tests rank people according to standardized performance of memory tasks, problem solving and abstract reasoning, but throughout the world people draw on various forms of intelligence when performing tasks associated with thinking, understanding and perceiving the world around them. To evaluate someone’s intelligence, we need to know what characteristics and abilities carry impact in that person’s environment.

Review of Learning Objectives

Human beings exhibit tremendous variation. Many of the observable differences are the result of cultural variations; However, humans also exhibit tremendous variation in physical traits, ranging from skin color to body build and a variety of biochemical characteristics. These traits are the products of the interaction of evolutionary processes affecting genetic diversity within and between populations, the physical environment, and cultural variables.

Modern studies of human variation focus on explaining why such variation occurs. Because many physical traits vary independently of one another, some researchers have found it useful to examine single, or univariate, traits. Plotting the distribution of individual traits in human populations sheds light on the genetic, environmental, and cultural factors that influenced their distribution. In contrast, multivariate analysis examines the interrelationships among a number of different traits.

Studies suggest that many genetically controlled traits may be the result of natural selection. For example, having darker skin may have been an advantage in tropical regions where it provides protection from ultraviolet radiation in sunlight, which has been shown to have a number of detrimental effects, including sunburn and skin cancer. Most importantly, it decreases folate levels, a factor that causes higher numbers of birth defects, and so directly affects reproductive success. The influence of natural selection and the environment can also be seen in human body and limb forms, which in part may relate to thermoregulation. This characteristic can be illustrated by Bergmann’s rule and Allen’s rule, which state that animals with a larger body form and shorter limbs are found in colder regions and populations with a smaller body form and longer limbs are found in warmer regions. The smaller body forms dissipate excess heat efficiently and function better at higher temperatures, while larger body forms dissipate heat more slowly and so are better adapted to cold climates. The same principle applies to humans: People living in cold climates tend to have stocky torsos with shorter limbs and heavier average body weights, whereas people in warmer regions have tall, slender frames with longer limbs. Adaptive features can also be seen in sweat glands, body hair, cranial and facial features, biochemical characteristics such as blood groups and lactose tolerance.

Physical characteristics, such as skin pigmentation, nose shape, and hair texture, have prompted people throughout history to classify humans into different “races.” Although human populations clearly encompass a great deal of diversity, physical characteristics cannot be used to divide humans into readily discernible groups or races. Instead of falling into discrete divisions, many characteristics exhibit continuous variation. Many attempts at racial classifications have been made, but these have failed because they proved too rigid to account for either the tremendous variation found within individual races or the shared similarities between these supposedly different groups. The word race is also of limited use because the word is used differently in different contexts. It may be used as justification for the discrimination or the marginalization of certain groups. Racial classifications can also be used as self-defined categories that are culturally constructed. For these reasons, modern anthropologists avoid using racial classifications, but rather focus on the distribution and study of specific traits and the explanation of the processes that may have produced them.

References cited

Herrnstein, R. J., & Murray, C. (1994). The bell curve: Intelligence and class structure in American life. Free Press.

Marks, Jonathan (2008). Race: Past, present and future. Chapter 1. In Koenig, Barbara; Soo-Jin Lee, Sandra; Richardson, Sarah S. (eds.). Revisiting Race in a Genomic Age. Rutgers University Press.

Nisbett, R. E. (2009). Intelligence and how to get it: Why schools and cultures count. WW Norton & Company.

Concept review

• List four aspects of variability that must be considered in understanding variation among human populations.
• Describe the three main approaches used in quantifying human variation.
• Define clinal variation and provide and example among humans.
• List and describe the main stages of human life history.
• Provide at least two benefits to our life history pattern.
• Explain why sickle-cell anemia is a good example of a balanced polymorphism.
• Describe the human body’s thermoregulatory response to heat and cold.
• Describe how darker skin color can be beneficial to reproduction in equatorial regions of the world.
• Describe the differences in high-altitude adaptation in Tibetan, Andean and Ethiopian highland populations.
• Define scientific racism and explain it’s historical development.
• Describe the four general positions on differences in IQ based on race or ethnicity.

Applying concepts

• How does culture interact with and impact the human life cycle? For each life stage, provide an example of interaction or impact on our current or future life history patterns.
• Choose one environmental stress (heat, cold, UV radiation, altitude) and describe how humans can adapt culturally, functionally, and genetically through the process of natural selection.
• Modern humans adapt to the environments culturally and biologically. Think of two examples of human cultural adaptations to the environment and consider the ways these two adaptations may affect the biology of future generations.
• Explain why the statement “race is a valid, biological concept” is incorrect.