Note: These materials are intended as supplements for students in Ant. 301. These pages are in development and will contain errors.

Race

History of the Concept of Race

Political Definitions of Race

Racism

Adaptive Significance of Race

The Human Life Cycle

Growth and Development

Diet and Growth

Altitude Stress

Other Disturbances of Growth

Senescence

Adaptability

Temperature Extremes

Solar Radiation

Altitude

Undernutrition

Homeostasis

Hormones and Energetics

Respiration

Immunology

As Buettner-Janusch (1966) once wrote, as long as we stick to abstract discussions, we have few problems with the race concept. However, when we seek to classify real populations and individuals into racial categories, we usually cannot do it in a way that satisfies anyone but ourselves. The basic problem is that a classification is inherently typological and the categories cultural constructs (return to outline).

History of the Concept of Race

Racial typologies which predate the discovery of genetics were straightforward applications of taxonomy and ideal types to human groups. Linnaeus (1758) proposed this classification:

Linnaeus's Human Taxonomy

MAMMALIA

Order 1. Primates [Fore-teeth cutting; upper 4, parallel; Two pectoral teats.]

1. Homo sapiens [Diurnal, varying by education and situation.]

Wild Man (Ferus) [Four-footed, mute, hairy.]

American (Americanus) [Copper-colored, choleric, erect. Hair black, straight, thick; nostrils wide, face harsh; beard scanty; obstinate, content free. Paints himself with fine red lines. Regulated by customs.]

European (Europaeus) [Fair, sanguine, brawny. Hair yellow, brown, flowing; eyes blue; gentle, acute, inventive. Covered with cloth vestments. Governed by laws.]

Asiatic (Asiaticus) [Sooty, melancholy, rigid. Hair black; eyes dark; severe; haughty; covetous, covered with loose garments. Governed by opinions.]

African (Afer) [Black, phlegmatic, relaxed. Hair black, frizzled; skin silky; nose flat; lips tumid; crafty, indolent, negligent. Anoints himself with grease. Governed by caprice.]

2. Homo monstrous [Varying by climate or art.]

Alpini [Small, active, timid.]

Patagonici [Large, indolent.]

Monorchides [Less fertile.]

Imberbes [Beardless.}

Macrocephali [Head conic.]

Plagiocephali [Head flattened.]

As biologists became more sophisticated in taxonomy, cultural characteristics played a lesser role in classifications. After a series of seminar discussions in 1962 organized by S.L. Washburn and sponsored by the Wenner-Gren Foundation for Anthropological Research, the term "race" was applied to populations rather than individuals. The older "types" were abandoned in favor of breeding populations or demes. There have been relatively few serious scientific attempts to form racial vocabularies or racial taxonomies since this turning point in human biology. The importance of racial vocabulary and study of race was greatly diminished, and there was a call from some quarters (Montagu, for example) to abandon the term race altogether.

After the beginning of the 1960s the scientific concepts of racial variation no longer coincided with the vocabulary or the perceptions of the general public. Ashley Montagu (1964) argued that the traditional anthropological concept of race is artificial, wrong, and leads to confusion. Consequently he proposed that the term be dropped altogether. An alternative concept, the ethnic group, fits the social and political needs of the current period. Montagu provides this definition:

An ethnic group represents one of a number of populations, comprising the single species, Homo sapiens, which individually maintain their differences, physical and cultural, by means of isolating mechanisms such as geographic and social barriers. These differences will vary as the power of the geographic and social barriers acting upon the original genetic differences varies (Montagu, 1964: 317).

Ethnic group is less typological in a biological sense, because it recognizes that immigrants with different biological backgrounds blend into the group. Admixture and gene flow between human populations appears to have been important in the past, and most anthropologists do not think human populations have ever been isolated long enough to be considered subspecies. The Mendelian population is the biological concept that is probably closest to what modern anthropologists mean by the word "race" if they chose to use the term.

Whether the phenomenon is described as a deme, breeding population, Mendelian population, cline, or ethnic group, knowledge about population variation remains essential to medicine and biology. It is important to understand the process of racial differentiation, but it is the ethnic group, not race, that is socially and politically relevant in our modern world (return to outline).

Political Definitions of Race

The Office of Management and Budget in the United States government has established a classification that is used in reporting federal statistics (Hoyme and Iscan, 1989). These categories are also likely to be used in law enforcement records and reports, but they represent bureaucratic categories, not scientific or anthropological classifications:

1. American Indian or Alaskan native

2. Asian or Pacific Islander

3. Black

4. Hispanic

5. White

Racism

All cultures make the distinction between "us" and "other." Persecution of "others" has long been a human behavior pattern. However the unique form of racism seen in the western world developed as part of a struggle between classes (nobles versus serfs; royalty versus commoners; Northern Europeans versus other Europeans; all Europeans versus non-Europeans). In nineteenth century United States, Europeans were seen as superior to Black slaves, and a relation developed that was different from servility. The slaves were converted into a pariah group, outsiders who were banned from society and the major institutions of social structure.

People in different cultures evaluate physical differences according to what they have been taught ("folk assessment"). Sometimes these "folk assessments" have little to do with biology. For example, in the late 1950s most restaurants and all movie theaters in the area of The University of Texas at Austin were forbidden to black Americans. They could cook or clean, but never were permitted to use the facilities. However, the ban did not apply to black foreign students. "We reserve the right to refuse service to anyone" did not really mean anyone, it was directed at those who were both American and black. The institution "race" in this instance was culturally defined and independent of biology.

In every culture there are individuals who want either to change or to maintain these entrance requirements for racial institutions. These persons are appropriately labeled "racists." Entrance requirements of many social organizations and culturally defined categories are based on "folk assessments." This is racism. From the perspective of culture, "race" is an institution with entrance requirements and with culturally defined expectations for behavior and culture-loaded values.

These institutions often result in social inequality. Washburn (1963) makes the point that a single lynching can stir a whole community to action, yet discrimination through denying education, medical care, and economic progress kills on a mass scale. This table compares life expectancy in years in the United States for white males and black males (figures prior to 1960 predate the American civil rights movements).

Notice that the difference in life expectancy at 1900 (16 years) is half the life expectancy of a black American male. Even in 1960, the six-year difference reflects enormous costs. For the 19 million black Americans in 1958, this six year difference computes to over 100 million years of human life lost to blacks. Consider the cost of differences in infant mortality. In 1958, the death rate for black Americans was 51 per thousand births, about twice that of white Americans. Indeed, these estimates are smaller than the terrible reality. The personal, social, and economic consequences are incalculable and considerations of mortality rates ignore the losses in quality of life, opportunities, unrealized dreams, and contributions to society.

There are no inferior ethnic groups. All humans belong to the same biological species, have generally similar abilities and attributes, and are capable of interbreeding if given an opportunity. The question of IQ scores and intelligence is typical of the confusion about inferiority. It is circular logic to use IQ scores to measure intelligence and define intelligence as what IQ scores measure. Ignoring the numerous and substantial problems of measuring intelligence, it is clear that IQ scores are strongly affected by health and education. Social programs of the 1960s attempted to reduce environmental differences between different racial and socioeconomic groups in the United States by providing more adequate diets, health services, and educational opportunities to all children. As long as the environmental variables of poverty, disease, malnutrition, and illiteracy prevented some children from achieving their genetic potentials, hypothesized minor differences in that potential were moot. There were opposing views and opposing political opinions. Jensen (1969) argued that a 20 percent difference in IQ exists between black and white children, and his papers were used to attack reforms of the 1960s. Why waste funds feeding or educating a child that cannot learn? Racial prejudice is sometimes evident in science under the guise of "biological determinism," which has at its core the fundamental dogma that human nature is a product of heredity. Clearly our human aptitudes have an inherited neurological and biochemical basis. But population variance due to genetic differences is small compared to large environmental variance in conditions that we can readily alter - health, nutrition, and education. Moreover, heredity produces individual genotypes, not group averages. Jensen failed to recognize that heritability in a study sample includes the contribution of environment. There are no large differences between scores of black Americans and white Americans when environmental differences are considered. Indeed, this reality has been known since educated blacks scored better than untutored whites in the standardized tests administered to American soldiers during World War I. Black soldiers from Ohio out scored white recruits from 11 Southern states (see Table 5-3). These scores correlate with the yearly education expenditures of the states from which the recruits were drawn, as well as the per capita income in that state (Montagu, 1963: 64).

Army Comprehensive Alpha Tests: White Recruits from 11 Southern States Compared with Negro Recruits
from 4 Northern States

*Computed from the data in R.M. Yerkes, Psychological Examining in the United States Army, Memoirs of the National Academy of Sciences, XV, 1921, pp. 690-691, Tables 205-206.
(copied from 1963. Mankind Monographs. VI The Anatomy of a Controversy, Part II. Mankind Quarterly: Edinburgh. p. 63

Racism did not vanish with the civil rights movement and no story or statement adequately communicates the travesty and pain associated with discrimination. A society can ill afford the lost potential and the division that is the legacy of racism (return to outline).

Adaptive Significance of Race

Racial labels are social constructs, but is there an underlying reality? Yes and no. The underlying reality among primates is the Mendelian population, a population phenomenon. If gene flow across a species range were such that each individual had a nearly equal chance of mating with a genotype representative of each member of the opposite sex, there would be no subpopulations. Genotype variations between Mendelian populations reflect the ongoing processes of evolution in contemporary demes. Continuing gene flow between demes partially counters the effect of differentiation.

There is no underlying reality in the sense that all human populations share most of the same genotype. The within group genetic variation is large compared to the between group variance. How is this possible? It may reflect gene flow between demes and a recent common ancestry. If evolutionary processes produce rapid change in human populations, then all humanity shares a recent common ancestry and the local differences are due to evolutionary processes (particularly selection) on a small number of genetic characters (return to outline).

Growth and Development

Humans experience two kinds of changes as they develop from embryo to adult: physical growth, a series of changes in size and form; and maturation, a sequence of developmental changes. The most accurate research design to study growth is the longitudinal record, where each individual is measured at successive ages. Since children require decades to grow from neonates to adults, longitudinal information is difficult to acquire. Although it produces information of much less quality, the cross-sectional method documents a sample of individuals from different age groups. This latter approach is easier to do and can usually incorporate a larger number of children. Some investigators combine the two methods, documenting children of different ages for several years, in a mixed longitudinal study.

Age Categories Based on Skeletal Remains

Fetal (conception to birth): skeletally characterized by initial ossification of primary centers of ossification and terminated by birth. Since termination of this category is variable (although normally nine months), it is difficult to distinguish individuals who are in the late fetal or early infancy state of development on the basis of skeletal evidence. In practice, osteologists are usually able to distinguish fetal remains only in the earliest stages of development, and these on the basis of the poorly ossified cortical bone and the extremely diminutive size of the bones.

Infancy (0-2 years): characterized by diminuitive size and incomplete formation of bone, and defined by eruption of the complete deciduous dentition. Several features of the skull are characteristic of this period: The mandible is in two separate pieces that fuse at the mental symphysis at the end of the first year; the sphenoid is in three separate pieces at birth, and these fuse near the end of the first year; the frontal is in two pieces that fuse during the second year; the occipital is in four pieces at birth. Postcranially the vertebrae are in three pieces, a centrum and each half of the neural arch.

Early childhood (3 - 5 years): defined by the period of functional occlusion of the deciduous dentition prior to eruption of the first permanent molar. The occipital provides the best indication of age during this period: the two lateral and squamous portions fuse about the fourth year and the basilar portion fuses to these by the sixth year. Postcranially the period is indicated by the appearance around year five of all of the carpal bones in females, and all but the scaphoid and trapezoid in males. The halves of the vertebral neural arches begin to fuse to one another about year two, and to the centrum between three and seven.

Late childhood (6 -12 years): defined by eruption of the first and second permanent molars. During this period all of the deciduous teeth are replaced. Postcranially the ilium, ischium, and pubis fuse toward the end of late childhood.

Adolescence (13 - 24 years): the period of final skeletal maturation. The final stages of growth occur and end with complete fusion of most of the epiphyses. During early adulthood the third molar erupts, if it erupts at all.

Young adulthood (25 - 49 years): period of full maturation in life prior to onset of gross skeletal degeneration. Although initiation of this period is marked by the completion of epiphyseal fusion, termination of the period is more variable and difficult to assess. Meindl and Lovejoy (1985) note that active fusion of the cranial sutures occurred during the mid-forties to the mid-fifties in a North American sample. Todd (1920-21), McKern and Stewart (1957), and Gilbert and McKern (1973) noted that the ventral rampart developed on the pubic symphysis during the midportion of young adulthood, and that the pubic symphysis had not undergone significant deterioration.

In his Natural History, Buffon published measurements recorded by Count Philibert Gueneau de Montbeillard on the stature of his son during the years 1759-1777. This oldest and most famous longitudinal record documents his growth from birth to age 18 years at 6-month intervals. If we plot growth curves for modern children we would note that the average girl is shorter than the average boy until adolescence. From birth until about 11 years of age, girls' rates of stature growth steadily decrease. As girls approach adolescence, they experience accelerated growth usually referred to by scientists as the adolescent growth spurt. The short time the stature of girls exceeds boys is a consequence of girls beginning their adolescent spurt 2 years earlier than boys.

Not all tissues exhibit the same patterns of growth as stature. The brain (and skull) develop more rapidly to their final size and show little if any adolescent spurt. Preadolescent lymphoid tissue grows far beyond its adult amount and then declines starting about the beginning of adolescence. Reproductive organs have a pronounced adolescent spurt. Each of the body's systems and tissues has a characteristic developmental pattern, generally an s-shaped curve with the slowest changes occurring during the middle years of childhood. Americans are usually "growth" conscious about their children and standards of normal growth or stature and weight have been established.

Physical growth is a set of processes that are strongly regulated by genetic factors. Generally children tend to reach a height close to the average height of their parents. As in all complex genetic characters, there is variation, and predictions that work for populations often prove disappointing when applied to specific individuals. Processes of growth are controlled by hormones. Thyroxine, produced by the thyroid gland, is important in regulation of metabolic rate and brain growth. Growth hormone, secreted by the pituitary gland, increases protein synthesis and stimulates cell multiplication. Androgens and estrogens stimulate growth and maturation of bone, muscle growth, and the development of secondary sexual characteristics. The growth processes require adequate nutrition and an absence of disease or environmental stress that would disrupt them. Since both heredity and environment contribute to growth, there can be substantial deviations from the expected growth curve of an individual. If a circumstance temporarily inhibits normal growth progress, elimination of the disturbance or deficiency sometimes is followed by an acceleration of growth, called catch-up growth, or an extension of the growth period and delay of maturation that allows the individual to achieve their size potential. If the deviations are too severe or if conditions are sustained too long, catch-up growth may not be able to compensate. The prenatal period and the first year of life are known as critical periods of growth, since disturbances during these times are usually not removed by catch-up growth.

The graphs below are from the first longitudinal study. It was made by a French scientist, de Montbeillard, who measured his son's growth from birth to 18 years (1759-1777). The graphs are modified from Tanner, Growth at Adolescence (1962).

Height in cm ploted against age in years

Height gain in cm ploted against age in years

Measurements of maturation are different from linear records of physical growth. For example, most of the bones of mammals are first formed as cartilage. The replacement of cartilage or membrane by a mineralized matrix is called ossification. Bones exhibit growth as osteoblast activity adds to their mineralized matrix, increasing their size or changing their shape. In the second or third intrauterine month, blood vessels begin to invade the cartilage precursors of bone and osteoblasts and mineral depositing cells begin to develop in local areas as ossification centers. In each bone, specific spots, called centers of ossification begin to form a bony matrix. The location and timing of the appearance of centers of ossification is consistent in healthy children. These centers in the long bones form the body or shaft of the bone, technically called a diaphysis. Secondary centers of ossification, called epiphyses, are separated from the diaphysis by a cartilage plate called the epiphyseal plate. Much of the longitudinal growth of a bone occurs along this plate in a region called the metaphysis. Skeletal maturation refers to the appearance of ossification centers and the loss of cartilage epiphyseal plates as ossification unites the epiphyses to the diaphysis. The flat bones of the skull join each other in irregular junctions called sutures. With increasing age, sutures may ossify to form continuous plates of bone across the juncture. Teeth also exhibit a specific sequence of formation and eruption. All of these processes are consistent in the age at which they occur in the individual and can be used as guidelines to measure the degree of maturation.

Puberty represents another type of maturation. The adolescent growth spurt occurs between the ages of 12.5 years and 15.5 years in boys and about 2 years earlier in girls on average. The age at which this occurs varies, but the sequence of changes is consistent. Although all skeletal and muscular dimensions may be slightly altered during the growth spurt, the major effect is in trunk growth and muscle development, including heart muscles. In boys there is accelerated growth of the testes, scrotum, and pubic hair. Axillary hair, facial hair, and the larynx also begin to grow with larynx enlargement accelerating near the time the penis approaches maximum development. Sperm, present from the start of testis enlargement, usually first appear in morning urine shortly after the height spurt reaches peak velocity.

In girls the first signs of puberty are the development of breast-buds and pubic hair. The uterus and vagina develop as the breasts mature. Menarche (the first menstrual period) is usually observed after the height growth spurt reaches maximum velocity. The first year or two after menarche represent a period of continued growth and relative infertility - about 75% of the cycles during this time are anovulatory. Better fed girls may exhibit a higher proportion of ovulatory cycles right after menarche. There is a general correlation between the different components of maturation. For example, girls who exhibit early skeletal maturation are also early with menarche (return to outline).

Diet and Growth

The first dietary crisis in a developing individual is prenatal. Maternal health and nutrition are important contributors to a healthy birth weight since the mother is limited in her ability to divert nutrients from her body to assist the fetus. After the infant is born, the infant receives nutrition and protective antibodies from the mother's breast milk, thus maternal health and diet remain important to the neonate. Crisis occurs when the mother's milk does not entirely meet the nutritional requirements of the child -- a crisis which is heightened by weaning, particularly in the absence of suitable weaning foods. This is a period of elevated risk of acquiring nutritional and infectious disease. Countries with a severe weaning crisis have high infant mortality rates. If the infant mortality rate is above 100 per thousand in the first year, almost 40% of the children die before the age of 5 years.

Undernourishment generally means a shortage of calories. The under nourished child has higher health risks and does not reach the size that would be expected from their genetic background. The malnourished child grows and matures more slowly than normal and attains a lesser stature. One immediate consequence of chronic undernutrition of the mother is retarded prenatal growth and low birth weight of her children. Low birth weight children are high risk for neonatal infection (due to impaired immune systems) and have other physiological problems including temperature instability, poor respiration, and metabolic disorders.

Malnutrition appears to have an unfortunate effect on brain growth during prenatal and neonatal development. Because of the timing of brain growth, severe malnutrition between the last trimester and the end of the first postnatal year produces permanent disruptions of development. Mental abilities are not solely determined by brain size however. If the malnutrition is not too severe, early acquisition of an adequate diet and an enriched environment may allow a mildly affected child to achieve high order dendritic branching and cortical thickness that is comparable to a normal individual.

Protein-calorie malnutrition (PCM) refers to a diet that is low in protein and generally inadequate. In a neonate, it quickly leads to marasmus, a condition of extreme protein-calorie malnutrition. A child with marasmus has exhausted its protein and calorie stores. It has little or no subcutaneous fat and exhibits extreme muscular wasting as the body literally begins to cannibalize itself for nutrients to maintain metabolism. Such children have reduced brain weight, brain damage, loss of muscle tone, and few interactions except hunger related activities. Growth is drastically curtailed and the child has the skeletal appearance of starvation.

Kwashiorkor is a disorder seen when a child has an adequate intake of calories and a deficiency of dietary proteins, the source of nitrogen and amino acids. This condition usually occurs where children are given carbohydrate rich but protein poor diets. The kwashiorkor child has stunted growth but does not have the appearance of starvation. Abnormal amounts of water are retained in the body, giving the child a round face, a bulging stomach, and a body weight that hides the underlying malnutrition. The skin and hair are partly depigmented. If protein deficiency alternates with periods of higher protein intake, the hair can actually have depigmented bands that are reliable indicators of kwashiorkor. The hair may fall out. The child is apathetic and lethargic except for a monotonous crying. Although it is irritable, the kwashiorkor child seems to have little appetite.

There is a trend for children of all ages to be larger as health and nutritional conditions improve. This change in average size is called the secular trend. Thus children in upper economic classes are larger than impoverished children, and children in modern industrialized nations are taller than their ancestors from the previous century. The children of immigrants are often taller than their parent population as a consequence of better nutrition. During times of war, one can sometimes see clear expressions of decreased nutrition in the form of decrements in growth or negative secular trend. There is also a secular trend in maturation. Children from better nutritional backgrounds mature more rapidly. Delayed maturation in poor nutritional conditions may be a way of extending the growth period to allow catch-up growth. The most convenient landmark for secular trends in maturation is the age at menarche. The decrease in the age of menarche is due to progressive dietary improvement from prolonged poor environmental conditions.

There is a synergistic interaction between infection and malnutrition. A malnourished child cannot martial the nutrients needed for an active immune response. Immunodeficiency makes the malnourished individual more vulnerable to infectious disease, and once an infection is established it is likely to be more severe than in a well nourished person. The disease state may intensifies malnutrition, putting the individual at higher risk for other diseases, with progressively less ability to cope (return to outline).

Altitude Stress

There is a significant reduction in birth weight with higher altitudes as a consequence of hypoxic stress. Maternal oxygen transfer is enhanced by changes such as larger and more complex placentas, increased placental cord hemoglobin, and increased capillary area. There is some acclimatization since populations who are indigenous to high altitudes have relatively larger birth weights than recent immigrants. Altitude effects are important even in well nourished societies. For example, Leadville, Colorado (3050 meters) has two to three times the proportion of infants below 2500 g as an average sea level population. A remarkable feature of children who grow in a high-altitude environment is an increase in the size of the thorax. Adults in high-altitude populations have a larger maximum chest circumference and a concomitant increment in lung volume. There is also an increment in size and muscle mass of the heart, especially the right ventricle (return to outline).

Other Disturbances of Growth

Some of the better understood abnormal growth patterns are the consequence of disturbances of hormonal production. For example, if there is too much production of growth hormone (somatotrophin or hGH) beginning at birth, the individual experiences accelerated growth and abnormal height, sometimes exceeding eight feet in stature. Such gigantism results in shortened life span and health problems. A growth rate equal to or greater than 12" in a year signals the need for endocrinological evaluation. An excess of growth hormone secretion is most frequently due to pituitary gland tumors. If the abnormal excess production of somatotrophin occurs after puberty, the epiphysis of the long bones have fused to their shafts, and the increment in stature is not possible. Instead the bones soften and thicken, producing acromegaly. The skin and subcutaneous tissue becomes thicker, and late growth continues in the hands, feet, and face. If the pituitary does not produce enough growth hormone, the child does not follow a normal growth curve, but its body proportions are fairly normal. Abnormal thyroid activity also disturbs growth, and the most dramatic consequences result from too little thyroid activity (hypothyroidism). In a child this can result in cretinism, a dwarf that retains the body proportions of a neonate and which is mentally handicapped.. All of these hormonal imbalances are medically treated today (return to outline).

Senescence

Although the life span has not increased since Roman times in the west, the increased life expectancy (median age at death) and decreased birth rate experienced in industrialized nations produces populations that have large numbers of older members. After birth, mitosis and cell replacement generally does not occur in the central nervous system. Although cells grow and change, there is a continual decrease in nerve cells during life. After puberty, especially after the adolescent growth spurt, mitosis has a progressively more difficult time maintaining the numbers of cells that comprise body tissues. With older ages, mitosis declines and body tissues do not maintain their cell reproducibility. Senescence is in part a generalized atrophy resulting from progressive loss and degradation of tissue functional capacity. The loss in cells results in a shrinkage of the brain in older age groups. The aging healthy brain however retains its ability to increase dendritic branching and increase cortical thickness, a characteristic that is assumed to correlate with learning.

By middle age, the reproductive system begins to atrophy. The interstitial cells of the ovary and the testicle are less active. In females the cycle of uterine and ovarian function that are associated with fertility ceases at about the age of 50 years, an event designated by the term menopause. After the age of 60 years, tissue decline is usually evident in smaller size or diminished function. An exception is when muscle wasting is masked by fat deposition as excess calories are no longer burned by high rates of physical activity. Another exception is the prostate gland. In some men it enlarges after the age of 60 years until it blocks the passage of urine. With advanced age, the matrix of tissue is likely to exhibit a decrease in water, damage to components, and deposition of calcium salts. Connective tissues lose elasticity. With impairment of growth and repair, many body tissues exhibit degeneration (return to outline).

An adaptation is a heritable feature or suite of features that improve an individual's chances of survival and reproduction in a particular environment. Humans adjust to their environment through an interaction of cultural, genetic, developmental, and physiological characters. Some of these adjustments alter the body or its properties to maintain homeostasis and protect the body from conditions that are beyond its tolerance. Although an ability or talent for culture is a human adaptation, technological improvements devised to solve human problems are not adaptations in an evolutionary sense even though they alter survival. But cultural traits are adaptations in the usage of common language, and one must be careful to avoid confusion. Plasticity refers to the extent that phenotypes can be modified by the environment (return to outline).

Temperature Extremes

Humans, like other primates, are tropical animals. Body temperature in a living human is usually maintained between 36.5 C and 37.5 C. It is generally fatal for body temperature to remain as low as 24 C or as high as 45 C for more than a few moments. Temperature homeostasis is achieved through regulating heat generated by metabolic processes and by controlling heat loss to the environment. Heat gain or loss is due to the combination of conduction, convection, metabolism, and radiation. Extreme heat loss can occur if convection is increased by exposure to elevated wind velocities at sub-body temperatures or if conduction is maximized by immersion in a cold fluid. The minimum metabolic activity (measured at rest after an eight hour fast in a thermally neutral environment) is called the basic metabolic rate (BMR). When the temperature falls, the hypothalamus stimulates blood vessels to constrict, thereby reducing the flow of warmed blood to the surface of the body, and thus heat loss to the environment. A continued drop in temperature may be offset by a release of epinephrine from the adrenal gland, increasing metabolic activity and generating more heat. The body at rest may begin to shiver, generating heat in metabolic processes of muscle activity. To a limited extent, the outer layers of the body may be allowed to cool, using outer tissues as insulation to protect the temperature of the body core. This attribute, known as insulative hypothermia, is assisted by counter-current heat exchange between arteries and veins. The cooler venous blood is warmed by the adjacent artery, returning much of the heat to the body's core, a mechanism that can become maladaptive in subfreezing environments.

The insulating properties of the body are greatly affected by morphological adaptations. For example, body fat in most of us is stored subcutaneously where it serves as an insulating blanket. An increased layer of fat that greatly enhances insulative hypothermia is also an important feature of human infants whose small body size makes their heat loss problems more difficult to manage. As body size increases, volume (or weight) increases slightly faster than surface area. Thus larger people have a larger volume-to-surface-area ratio and retain more heat. Another physical property that enhances preservation of core temperature is presence of relatively short appendages, which reduce the volume-to-surface-area ratio even further. These physical relationships have long been recognized in mammalogy as Bergmann's rule and Allen's rule. Such relationships between body composition and climate can are apparent in humans, and many other mammal species.

When body temperature rises above normal, the hypothalamus may cause the blood vessels to dilate, bringing more blood volume to the body surface and promoting heat loss through the skin, which is cooled by evaporation of perspired water, conduction, convection, and radiation. In regions that are both hot and humid, perspiration is not as effective as it would be under dry heat conditions. Thus native inhabitants of tropical rain forests tend to have small bodies with long extremities and relatively little subcutaneous fat. Fat however is an important medium for storage of excess carbohydrates, and fat stores can be important to surviving periods of nutritional crises such as pregnancy and lactation. Fat deposits of some tropical peoples, restricted to the upper thighs and buttocks (steatopygia), provide resources of substantial fat storage with minimal insulating affect.

Production of sweat to cool the skin in a hot, dry environment requires an ample water supply since one can lose as much as 4 L/hr of water. A person becomes incapacitated when water loss is equivalent to 10% of body weight. As water is lost, the blood becomes more viscous, heart and respiration rates increase, body temperature rises, and the individual feels thirsty. Under moderate heat and working conditions, thirst is satiated before ingestion of a volume of water equivalent to the amount of water lost through evaporated sweat. This satiation of thirst produces a temporary weight loss of up to 4% of body weight even when ample drinking water is available. Sweat also contains sodium that must be replaced from dietary sources.

The ability to respond effectively to environmental stressors such as heat and cold is greatly affected by age, acclimation, physical fitness, body composition, and behavior (return to outline).

Solar Radiation

Though the atmosphere absorbs most of the sun's radiation, exposure to the ultraviolet rays that do penetrate it burns human skin and causes dangerous cancers. Radiation damage to skin cells must be repaired by the body's defenses. The immune system detects and destroys skin cancers, but repeated or serious radiation injuries to the skin weaken body defense mechanisms, making it more difficult to detect and control tumors.

In humans, vulnerability to skin cancer is affected by skin pigmentation. Lightly pigmented Europeans are most effected. The primary pigment for blocking solar radiation is melanin in the epidermis, the outer layer of skin. Exposure to radiation is increased at high elevations, where the atmospheric barrier is thinner, and in equatorial regions where the sun is directly overhead. Loss of some atmospheric components, such as ozone in the stratosphere, allows more damaging radiation to reach the surface. The geographic distribution of skin pigmentation supports the hypothesis that skin color is an adaptation to solar radiation.

However, it should be noted that selection for darker pigments due to skin cancer may not be very strong since cancers tend to appear after the primary reproductive years. A commonly offered explanation for the geographic distribution of skin pigmentation is the vitamin D hypothesis. Vitamin D is manufactured in skin by action of ultraviolet light on certain sterols. Because darkly pigmented skin reduces the amount of vitamin D that is formed, selection might favor lighter pigmentation at high latitudes. Darker pigments would prevent an accumulation of excess vitamin D in the tropics. A child wearing clothing in a northern climate may be vitamin D deficient and develop rickets, a deficiency in deposition of bone salts that distorts bone shape and reduces bone strength. Vitamin supplements in bread and milk virtually eliminates rickets. This was apparent when rickets reappeared in areas of Britain where school milk distribution was suspended (return to outline).

Altitude

There are numerous correlates of altitude, including temperature and changes in the biota, but this discussion will focus on the drop in partial pressure of oxygen that is a consequence of higher altitude. At sea level, alveolar partial pressure of oxygen (PO2) is about 100.0 mm Hg. At 3,000 m PO2 falls to about 60 mm Hg, stimulating increased ventilation. Humans generally do not live permanently above 5,500 m. Without oxygen supplement, a human would eventually lose consciousness at altitudes above 6,100 m. If one breathes 100% O2 from a mask, alveolar PO2 is about 100 mm Hg at 10,400 m. PO2 drops to 40 mm Hg at 13,700 m and loss of consciousness will occur even on 100% O2 at higher altitudes. Respiration above 13,700 m altitude requires pressurization. Sudden decompression of a plane above 16,000 m results in loss of consciousness in less than half a minute and death in less than five minutes from lack of oxygen (hypoxia). Exposed to an altitude of 19,200 m, human body fluids boil at body temperature and hypoxia would produce rapid death.

When individuals first arrive at a high altitude, they are stimulated by hypoxia to hyperventilate. This rapid breathing does enhance oxygen exchange in the alveoli, but it also depresses alveolar PCO2. It is this carbon dioxide in blood plasma that forms part of the buffer that keeps human blood pH at 7.4. Loss of PCO2 through rapid breathing results in a decrease in plasma HCO3 and an increase in blood pH to 7.5 or 7.6, a dangerous condition known as alkalosis. The kidneys respond by increasing the rate of bicarbonate excretion. At lower partial pressures of O2, a greater proportion of hemoglobin is saturated with oxygen. Not only is there less oxygen, the hemoglobin system for transporting oxygen is not as efficient. At sea level, 97% of the hemoglobin in arterial blood is combined with oxygen. At 3,000 m, only about 90% of the hemoglobin is saturated with oxygen. As PCO2 decreases, hemoglobin becomes less capable of transporting oxygen, compounding the danger of hypoxia.

Other changes associated with altitude hypoxia include an increase in resting heart rate, dilation of retinal arteries and veins, decreased visual acuity, short-term memory impairment, and numerous endocrine changes. Exercise under altitude stress can produce "mountain sickness," a temporary illness characterized by breathlessness, headache, insomnia, irritability, nausea, and vomiting. Extreme exercise that sustains hypoxic stress and hyperventilation for a prolonged period may damage the kidneys or produce kidney failure.

Immediately upon exposure to high altitude environments, the body begins to adjust to the new circumstances. Erythropoietic activity increases, raising the concentration of red cells in blood. Renal circulation increases for a few days and then stabilizes with vessels dilated. As the increased hemoglobin assists the oxygen transport system, most altitude problems subside. Athletes sometimes choose high altitude training sites to benefit from changes in vascularization, size of striated and cardiac muscles, and red cell concentration. The effects of altitude on athletic events were studied during the 1955 Pan American Games and the 1968 Olympic Games, both of which were held in Mexico City (alt. 2,380 m). There were improvements in short-distance races (100 m to 400 m) due to decreased wind resistance - such events are too brief for enhanced breathing to supply excess oxygen during the activity. Athletes from higher altitudes had advantages over those from sea level countries at long-distance events. However, long-distance events that required endurance and taxed oxygen transport systems posted slow winning times. The greater distance the event, the slower time compared to records held from lower altitude games.

Abnormal increases in atmospheric pressure have serious biological consequences. Since water is much heavier than air, ambient pressure increases by 1 atmosphere for every 10 m of depth in sea water and every 10.4 m depth in fresh water. This means that divers who breathe pressurized gases face special risks. SCUBA gear, which supplies breathable air at a pressure that matches the atmospheric pressure outside the body (ambient pressure), allows a diver to breathe pressurized air comfortably under water. A vital component is the fragile human lung, which can only sustain a pressure differential of less than 0.13 atmospheres above the outside ambient atmospheric pressure. This means that a diver who takes a breath of air at ambient pressure can rise only about a meter under water without bursting a lung. Divers avoid this problem by exhaling when rising, allowing air to escape through the nose and mouth to keep lungs at ambient pressure.

Divers face many other problems. Pressurized atmospheric gases become toxic to humans. At pressures of 4 to 5 atmospheres (depths of 30 to 40 m), nitrogen produces nitrogen narcosis, a euphoria that impairs mental functions. Both CO2 and O2 are toxic at high pressures. At 1 atm, 100% O2 is somewhat toxic; at 4 atm, dizziness, convulsions, and coma may develop within 30 minutes. Divers who breathe pressurized air also risk dangers of decompression. Gases enter the bloodstream and body tissues under high partial pressures during a dive. If a diver ascends rapidly, these gases leave body tissues and fluids faster than they can pass from alveoli and be exhaled. Thus bubbles can form in blood and body tissues, a condition called decompression sickness (the bends, caisson disease), and they can obstruct circulation and damage tissue. Interestingly, passengers on an aircraft pressurized at sea level who later experience rapid decompression above 8,550 m face the same decompression dangers as from an ascent from 20 m of sea water (return to outline).

Undernutrition

Starvation is classified into three categories based on caloric intake over time (Frisancho, 1981):

1. Acute - less than 600 calories/day for less than two weeks.

2. Semiacute starvation - less than 1100 calories/day for less than 30 days.

3. Moderate semistarvation - less than 1600 calories/day for as long as 24 weeks.

Initial consequences of starvation are apathy, muscle weakness, and reduction of activity. Body weight loss is progressive. The body first extracts energy from carbohydrate stored in the liver and glycogen in muscles. Then the body burns fat to get needed energy for about two weeks. After that point both fat and protein are used. After 24 weeks there is a loss of physical work performance. Individuals with a daily diet intake of less than 2,000 calories per day have impaired physical work capacity (return to outline).

HOMEOSTASIS

Homeostasis is the ability to maintain a stable internal environment regardless of external environmental fluctuations. This ability usually is accomplished through feedback loops or antagonistic controls. The central nervous system is usually described as employing three separate control systems - neuromuscular control of the voluntary nervous system, neurovisceral control of the autonomic nervous system, and neuroendocrine control of hormone-producing glands.

Neural feedback loops and reflex arcs are vital components of the maintenance of posture and coordinated movements. The autonomic nervous system consists of two separate nerve tracts, the parasympathetic and the sympathetic nervous systems. They are distinguished by the chemical neurotransmitter between the terminal neuron axon and the target organ. The parasympathetic system produces acetylcholine, the usual neurotransmitter within ganglions. Sympathetic system terminal synapse produces either adrenalin or noradrenalin. Acetylcholine has the opposite effect on target organs as adrenaline or noradrenalin, so the alternate paths of the autonomic nervous system have opposite effects on the target organ; one stimulates, the other inhibits. Generally the parasympathetic system stimulates normal body functions and the sympathetic system interrupts these functions to prepare the body for fight or flight. Neuroendocrine activity is partly controlled by the hypothalamus, a structure at the base of the brain which has access both to information about the body's internal states (appetite, temperature, reproductive status,...) and external stimuli (emotion, fear, pain,....) (return to outline).

Hormones and Energetics

An example of neuroendocrine homeostasis is the regulation of glucose, the body's primary source of energy. Glucose is stored in the body as glycogen. When blood glucose levels fall below normal, two different organ systems react. The islets of Langerhans, a specialized area of the pancreas, releases the hormone glucagon into the bloodstream. At the same time, cells in the adrenal medulla respond to low glucose levels and release adrenaline into the bloodstream. Together, glucagon and adrenaline speed the rate of conversion of glycogen to glucose. If blood glucose levels are higher than normal, the islets of Langerhans produce insulin, which stimulates the formation of glycogen from blood glucose. Individuals with impaired abilities to make insulin have a defect in this feedback loop that allows excessively high levels of glucose in the blood, a disorder known as diabetes (return to outline).

Respiration

Gas exchange in the lungs is governed by diffusion, that is, gases pass from regions of high pressure to regions of low pressure. The approximate composition of three important gases are:

We inspire air by pulling our ribs upward and contracting the diaphragm to expand the volume of the chest cavity. Air is drawn into the lungs, and oxygen moves by diffusion across alveoli into the bloodstream. At the same time, carbon dioxide and nitrogen diffuse from alveoli to the atmosphere. As partial pressures of oxygen in inspired air approaches 0.12 atm, diffusion can no longer maintain tissue oxygen levels. We quickly become helpless and are unconscious by the time inspired oxygen levels reach 0.10 atm.

The average human consumes about 0.25 liters of oxygen per minute for their basal metabolism - just staying alive. Any activity markedly increases consumption. Heavy work may consume 4.0 liters per minute. The work capacity of a human is assessed by measuring the maximum amount of oxygen that can be transferred from lungs to muscle and consumed, a parameter called maximum aerobic capacity or VO2 max.

Breathing is regulated by the partial pressure of blood carbon dioxide detected by the medulla oblongata in the brain. Small increases in carbon dioxide result in increased rates of breathing. Though oxygen-sensitive chemoreceptors stimulate more rapid respiration at high altitudes, the carbon dioxide sensing system is the main controller and there are situations where it can allow fatal failures in the stimulation to breathe. Swimmers often hyperventilate to allow them to swim underwater longer than normal. Hyperventilation (excessive rapid deep breathing) can lower carbon dioxide levels in alveoli and related tissues. This allows a longer period before rising carbon dioxide levels result in an irresistible urge to breathe. Unfortunately this alters tissue carbon dioxide levels far more than it affects oxygen partial pressure. During underwater swimming, oxygen levels in body tissues drop as carbon dioxide levels rise in the bloodstream. If the initial concentration of carbon dioxide is artificially lowered by hyperventilation, a person can experience a drop in oxygen level severe enough to produce sudden unconsciousness before carbon dioxide levels become high enough to compel breathing. In this situation, the unconscious person then drowns when brain mechanisms finally compel breathing (return to outline).

Immunology

The main homeostatic mechanism that aids body tissues challenged by invading organisms, toxins, and enzymes, is the immune system. Stem blood cells (called hematopoietic cells) provide a self-renewing source of blood cells. Erythrocytes (red cells) are produced from erythroblasts in red bone marrow. The human erythrocyte has lost most of the machinery of cell metabolism, including nucleus, ribosomes, and mitochondria, and has become a capsule packed with hemoglobin. Lacking a nucleus, the erythrocyte cannot replicate, and has a functional life expectancy of about four months. In contrast, leukocytes (white blood cells), retain their nuclei. The five types of leukocytes are grouped according to their staining properties:

A. Granulocytes

1. Neutrophil - primarily are phagocytes (cells that ingest microorganisms, damaged body cells, or foreign particles)

2. Eosinophil - reacts to foreign proteins

3. Basophil - carries heparin, histamine, and seratonin

B. Agranulocytes

4. Lymphocyte - undifferentiated; can be transformed into other types

5. Monocyte - primarily are macrophages (large phagocytic cells)

After a foreign tissue penetrates the body's skin, the next line of defense are phagocytes. Systems of fixed phagocytes, the reticuloendothelial system, are found in many organs (such as the liver or the spleen) where they destroy trapped cellular debris. Other phagocytes circulate with the blood. Granulocytes carry enzymes and substances that are toxic or damaging to the more common bacteria and fungi. When the skin is cut or ruptured, leukocytes contribute heparin and histamine to increase capillary blood circulation into the area, producing inflammation. Neutrophils migrate to the inflamed area in great quantity to engulf foreign proteins. Dead neutrophils produce much of the "pus" seen at a minor infection site. If the invading tissue is a virus, leukocytes produce proteins called interferon that inhibit the ability of viral agents to multiply in body cells.

Exposure to a foreign agent can stimulate specific immune responses based upon lymphocytes. B cells are lymphocytes produced by bone marrow that differentiate into plasma cells which, usually under stimulation by T cells, produce proteins called antibodies. The best known antibody is immunoglobulin G (IgG), which consists of four polypeptide chains, two long chains and two short chains configured as a "Y". Antibodies coat surfaces of invading organisms, assisting phagocytes, impeding activity of the foreign proteins, and sometimes rupturing the invader's cell membranes. T Cells, lymphocytes that mature in the thymus, provide cell-mediated immunity. Some T cells have antibodies on their cell surfaces and make direct cell-to-cell contact. Others interact with B cells to enhance antibody production. T cells can bind, lyse, or phagocytize other cells. They attack host cells that are viral infected or cancerous. If this function gets out of control, T cells may attack normal tissues to produce autoimmune diseases. Once a particular antibody has been stimulated, some lymphocytes remember the configuration, making second responses more rapid. This memory and rapid second response is the basis for immunization against diseases (return to outline).

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