NOTES for WEEK 10 Human Variation I

HUMAN ECOLOGY AND HUMAN ADAPTABILITY

I. Human Adaptation (Harrison et al p 439)

Adaptive responses include a range of mechanisms, responses & traits that enable people to survive amidst environmental pressures. Humans (and their hominid ancestors) have adapted over millions of years to climate extremes, high altitude, seasons, food shortages, and disasters (McElroy & Townsend p 74). Adaptation can be viewed at several levels (McElroy p 74):

(1) Genetic changes

These are the slowest and tend to be irreversible. We associate these with mutation, natural selection and evolution. The sickle cell trait in a malarial environment is an example of genetic change.

(2) Physiologic response and developmental changes

These occur within a life span and are a focus in this unit. An example is tanning: it is an increase in the melanin content of the skin; it is a nearly universal response in our species occurring even in persons with the darkest skin colors. (see Harrison et al p 445)

(3) Cultural adaptation

Is at the population level. This is problem-solving, learning, and exchange of cultural information.

(4) Psychological adaptation This is at the personal level; it includes how an individual copes with acute and chronic disease.

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II. Human Ecology (Harrison et al p 440)

Culture is a mechanism for a population's survival in diverse environments. You can look at this the other way, also. Environment can shape the culture necessary for survival, as we shall see.

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A. Cultural Ecology (Harrison et al p 440)

Julian Steward said that the natural environment, through its effects on human subsistence behavior has a strong effect on the kinds of social and political structures which developed in societies using comparable natural habitats.

A slightly different way of understanding these ideas is the culture area concept developed by Alfred Kroeber and others to make sense of the lifeways of North American Indians.

For example: thirty-one distinct tribes lived on the Great Plains two hundred years ago. They came from different backgrounds, many spoke languages unintelligible to one another, yet they shared an adaptation to buffalo hunting, the horse, and the rifle. (see Garbarino and Sasso)

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B Evolutionary Ecology (p 441)

How people get their food strongly influences their social and political organization. The way a society gets its food predicts many other aspects of its culture--from community size and permanence to type of economy and degree of inequality and type of political system, even art styles, religious beliefs & practices (Ember and Ember p 94).

Cultural evolution has three facets: increase in complexity, increase in energy flow, and an increase in population. Each of these have a significant influence on health and disease. (McElroy and Townsend p 122)

There is a wide disparity between food foragers and people in industrialized countries in their view of the world and how they use the world around them.

C Biological Human Ecology (Harrison et al p 442)

Stated cleanly and simply--this is how the natural and cultural environment affected the biological characteristics of human populations. Another concept: persons have neurological, physiological, and behavioral mechanisms to maintain homeostasis. Vasoconstriction, shivering, and putting on warm clothing are responses to cold. (Do understand that many people have maladaptive responses. Excess alcoholic drink to warm up could be considered an maladaptive response.)

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III. Human Adaptability, p 445.

The Book of Mormon is unique in American literature for its explanation of American Indian origins It says that American Indians were cursed with a dark skin because they were 'Lamanites.' Their way out was conversion (to Mormonism). This explanation is tinged with racial prejudice.

It was the Greeks who suggested a more rational understanding of variations in skin color: it might relate to intensity of the sun. Skin color has long intrigued anthropologists. In the Old World there is a geographic gradient with darker skins in equatorial environments and lighter skins in the higher altitudes.

Gloger's Rule says that within a wide-ranging, warm-blooded polytypic species, those populations living in warmer, more humid regions tend to be more melanin-pigmented than those living in colder, drier regions.

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THE PHYSICAL ENVIRONMENT

(My comment: what value is there to knowing how people cope with extreme environments besides giving anthropologists something to do? Knowledge of human tolerance and adaptation to heat, cold and altitude is very important to the military.)

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I. The Potential Stressors (Harrison et al p 450)

Historically, most primates were limited to lowland tropical and warm weather environments. Our relative hairlessness and ability to function in a tropical setting (sweating, tanning, etc.) imply a tropical origin. Recently some macaque monkeys in Japan have adapted to regions with snow. Human intrusion into cold only took place with Homo erectus, Neandertals and modern humans.

Long ago our ancestors seem to have adapted to high daytime temperatures, cool evenings, high solar radiation, and high atmospheric O pressure. (Bunney Ch 2.2)

Although camels exceed our abilities, humans are among the few mammals that can remain moderately active during the day in the hottest regions of the world. The ability results from efficient sweating.

Many animals sweat, but the human system is the most effective for three reasons:

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II. Heat Tolerance (Harrison et al p 451)

There are two kinds of sweat glands (Overfield p 16):

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We are excluding here the sebaceous glands which open into the hair follicles. The sebaceous glands secrete an oily substance. The glands are poorly developed in small children. They become much more active at pubescence. Sebaceous glands are under endocrine control. (Poirier et al p 566)

The number of active sweat glands seems to be greater in persons exposed as youth to high heat loads. Sweat glands seem to be more active in darkly pigmented people. Sweat glands are under control of the autonomic nervous system. (Poirier et al p 566)

Apocrine and eccrine sweat glands vary in different groups, but sebaceous glands do not vary in different groups. (appocrine Gr = away plus Gr krinein = to separate; Gr ec = outside of; eccrine = producing a secretion without a loss of cytoplasm.

Appocrine glands become functional only after puberty (which explains the lack of underarm sweating and body odor in children.

There are fewer apocrine glands in Orientals and Native American Indians than in Blacks and Whites. Apocrine glands excrete fat and protein along with water (Poirier et al p 567).

Apocrine glands function to produce a characteristic odor. They develope close to the hair follicles. Interestingly, they appear everywhere on the body of the fetus at the 5th month IU, but then most appocrine glands regress except for the axilla, nipple, navel, anogenital area and the external canal of the ear. Wet, sticky earwax is more common in hot, moist environments (Overfield, p 17).

Apocrine glands respond to stress and sexual stimulation. In humans, the apocrine and eccrine glands work synergistically with bacteria to produce what you call body odor. We share the two glands of the armpit (the axillary organ) with the gorilla and chimpanzee. (My comment: odors are important in many primates for sexual attraction, marking territory and in lumurs--for 'stink fights'. Extracellular chemicals are extremely ancient--ranging from natural antibiotics to hormones and neurotransmitters.)

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The eccrine sweat glands are distributed all over the human body. In dogs, cats, and some other animals, eccrine glands are found on the digital pads and serve to enhance friction (we discussed that adaptation in our unit on dermatoglyphics.) They are most widespread in us where they have been modified for cooling. (Poirier et al p 568)

Eskimo seem to show an adaptation since they sweat less on their trunks and extremities but more on their faces. (This is advantageous to traditional Eskimo where moisture in clothing is a hazard.)

The amount of chloride excreted by sweat glands varies by race: Blacks have more chloride in sweat than do whites. Acclimatized Whites excrete less chloride than unacclimatized whites-- a useful adaptation (text 452). Water loss can be considerable: in extreme temperatures young males can loose 4 liters per hour. Thus, human ancestors in tropics must have always had ready access to water (see Overfield for many details).

They could not, like camels, store it for days. Fat persons will sweat significantly more than lean individuals. Hot dry climates produce total evaporation, but in hot wet climates, sweating of of little or no value. A lower air temperature therefore is important for cooling. Work in deep mines with saturated air requires frequent rest periods to cool off.

Sweat production acclimatizes in just a few days: sweating will increase and core temperature will stabilize at a lower temperature.

In extreme desert conditions, water loss to sweating exceeds water intake even if freely available. Dehyration is a risk without ample water or in extreme conditions. Other primates handle heat less well than we do. Our ability to cope with heat suggests our tropical evolutionary origins.

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The sweating response is much less effective in infants (Poirier et al). Children under two years sweat poorly and irregularly and are therefore unable to cope with heat as well as adults. Handling the heat is less effective in the elderly.

In World War II, death rates in heat were 9x more for persons just 10% overweight compared to persons of normal weight.

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Harrison et al says that various groups show major differences when first tested under hot working conditions, but these differences largely disappear after heat acclimatization. Typically, if individuals work in the heat for 7-10 days, the differences between them almost disappears. Heat acclimatization is a near universal capability.

During acclimatization, sweat rates rise and the sweat becomes less salty; body temperature rises less and strain on the cardiovascular system decreases. Acclimatization gives us more versatility in coping with various environments and may have resulted from our long history of mobility (Bunney p 48)

In principle, a small body will improve heat tolerance because of the larger surface area per unit volume compared to a big body. Long extremities will also be advantageous as these increase conductive and radiative cooling. The extremely small bodies of the pygmies in tropical Africa may have arisen from such selection but there is no experimental proof of this. People adapt behaviorally to heat in many ways. The most extreme example is air conditioning. Some of us now think AC is an entitlement.

"The human species is remarkably adaptable, and in the fullness of time, as the weather goes...so goes man."

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III. Cold Tolerance (Harrison p 457; see Overfield pp 137-142 for an excellent review)

A. Cold

Susceptibility to cold stress is influenced by the same factors as with heat stress--climactic experience, race, body build, physical fitness, age, and sex.

Without clothing, our cold tolerance is low, although subcutaneous fat does afford some insulation from the cold. A first defense is peripheral arterial vasoconstriction. A very heavy person can withstand more cooling than a thin one. In the inactive person, shivering begins as that person is chilled--it can increase the metabolic rate 100% above basal metabolism.

With prolonged exposure to cold, a reduction in brain temperature will lead to consciousness. Our acclimatization to cold is much less effective than it is to heat. There is some sensory acclimatization--after long exposure it bothers us less.

This is surprising since many human populations have a long history of exposure to cold. Modern humans lived in Europe and Asia during the last part of the Ice Age. Paleo-Siberians lived in some of the most extreme cold conditions; their successors persist as aboriginal peoples in Siberia and were predecessors to the Aleut-Eskimo peoples of the New World. They show some physical adaptation: flattened faces, fleshy cheeks, and epicanthic folds at the medial commissure of the eye.

Physically fit persons and ones with significant body fat do better with cold stress than the inactive and the very thin. Subcutaneous fat has low thermal conductivity and reduces loss of central body heat; the extent of protection is directly related to its thickness. Somewhat conflicting evidence indicates that fatter individuals, with more body insulation, conserve core temperature than thinner individuals. Both fat and thin persons experience similar subjective feelings of cold. There is no short term acclimatization to cold.

There is an inverse relationship between body weight and average annual temperature.

Fat is particularly beneficial in cold water. Successful swimmers of the English Channel (and more recently Cuba to Florida) tend to be women with thick fat layers.

Australian Aborigines can sleep without shelter or clothing at near freezing temperatures. This means that, while sleeping conditions cold enough to raise the metabolic heat production of Europeans by 15%, Australian Aborigines remain at basal metabolic levels. Their skin temperatures fall, too, thus decreasing heat loss.

B. Cold Adaptation (Overfield p 137)

Our first response to cold is behavior, such as putting on more clothing or a warm place.

If this is inadequate, peripheral blood vessels constrict to conserve heat. A curious phenomenon occurs: there is alternating vasodilation-vasoconstriction called variously the hunting reaction or the Lewis Curve. It is discussed in section IV below.

Shivering is the next line of defense against cold stress. It begins when skin temperature cools to 28 to 30 C. Shivering increases heat production through muscle contraction. When people are subjected to cold over several weeks, shivering decreases because a more efficient hunting reaction ensues.

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IV. Extremity Cooling (p 462)

When an adult immerses a finger in freezing water, there is an immediate stoppage of blood flow in that finger.

When only the hand or foot is exposed to severe cold, a phenomenon called the Lewis Wave (or Lewis Curve) is observed. There are cycles of alternating vasoconstriction and vasodilation.

This response is variable. For some, finger temperature drops and stays there. Men of African ancestry show much more cooling and less cycling. Europeans and Japanese have only an intermediate response. Eskimo and high altitude Peruvian American Indians have the best cold acclimatization.

With repeated exposures, the vasoconstriction response declines, rise in response to cold shock declines, and these responses are limited to the extremity exposed--all this is called habituation. Cold-acclimatized individuals develop efficient hunting reactions which protect surface tissues from cold injury, but prevent excessive heat loss from the body core.

Fisherman in Maine therefore maintain warmer hands in cold sea water than non-fishermen--a useful acclimitization. Repeated immersion in cold water diminishes the sensation of cold-induced pain.

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V. Population Differences in Response to Cold Stress, p 464. (also see Overfield)

(1) Total body cooling p 464.

With overnite exposure to cold, it seems that Australian Aborigines responded with lower skin temperatures, lower core temperatures, and lower metabolic rates. Other studies show that under some conditions resting metabolism and the metabolic response to cold stress may be increased. (My comment: studies of persons who have suffered extreme cultural isolation grow up insensitive to cold.)

Darkly pigmented skin, whether that of a human or some experimental animal appears to be more susceptible to frostbite than does lightly colored skin (Poirier et al p 564. Frostbite was more common in darker skinned soldiers than lighter skinned ones in both World Wars and the Korean War. Genetics seems to be the underlying factor.

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(2) Extremity cold stress p 466. (see Overfield pp 137-142)

When just a single extremity is chilled, vasoconstriction and rewarming was less in Blacks. In Korea, the incidence and severity of frostbite was much greater for Blacks.

Native Americans tend to maintain a higher hand or foot temperature during cold exposure than do Europeans. The Inuit maintain the highest peripheral skin temperture than any group tested. Is this a genetic adaptation? There is no clear answer.

Concluding remarks: adaptation to cold is less than it is for heat. (My note: except for cultural adaptation, our biological defenses against cold are limited.)

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(3) Growth and Development (see Overfield)

Does a cold climate make a difference? We said earlier it didn't in rats. There are apparently some permanent modifications in children exposed to cold. This cold response developes throughout childhood. The hunting reaction becomes more efficient. Children may increase their metabolic rates in response to cold stress more than adults.

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(4) Sex Differences (Overfield)

Most studies have been done on males. The few comparative studies suggest that under cold stress, women experience cooler skin differences. Strangely, over age 60, men die more frequently of excessive cold than do women over 60 years. This may result from more homelessness in older men.

(5) Maladaptation to Cold Stress (Overfield)

Frostbite and hypothermia are he severe consequences of cold exposure. Chilblains and immersion to (trenchfoot) are less severe consequences. Other short-term effects of exposure to cold include diuresis, loss of manual dexterity, loss of muscle power and speed, and extremity pain.

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VI. Cultural Adaptations to Cold and Biological Adaptability (see Harrison et al p 470)

The most basic material cultural adaptations are fire, shelter, and clothing. (My comment: we have a clear mindset to modify our environment, a product of our exploitive world view.)

Of all aboriginals, the Inuit were the most creative in devising clothing to cope with extreme cold.

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VII. Altitude Tolerance (see Harrison et al p 471)

At higher altitudes, air temperature and water vapor decline in linear fashion; radiation increases and air pressure declines. The decline in oxygen is especially stressful for humans. At 15,000 feet the pressure is much less than at sea level. The reduced level of oxygen is the main culprit causing stress to people.

Remarkably, some 25 million people live in high altitude zones in Ethiopia, South America and Tibet. There is some evidence to suggest that historically, Tibetans and Ethiopians have had a much longer exposure to high altitude.

(1) Lack of Oxygen/hypoxia Harrison et al p 472) Rapid ascent is especially stressful for lowlanders. Vigorous exercise has serious consequences with fluid accumulation in the lungs.

While red blood cell numbers increase in high altitude, it is believed to be more of a stress response than anything else. There is no evidence that a red-cell count increase increases aerobic capacity.

For those from low altitudes, the immediate response is more rapid breathing and an increased heart rate. Symptoms of distress tend to disappear with acclimatization.

The ability to cope with sustained work load is reduced in high altitude. That reduced capacity recovers with a return to low altitude.

Growing up at a high altitude seems to be critical for acquiring high altitude work capacity at sea level values. When the Olympics were in Mexico City (altitude 7,000 feet), sprints got faster but long endurance events required more time. Some athletes tried injections of red blood cells prior to events to boost their blood oxygenation effectiveness (see Poirier et al Ch 27). Muscle metabolism apparently increases in efficiency after acclimatization to high altitude (Overfield p 143).

A short-term adaptation to high altitude is hyperventilation. If this is excessive, it alters the blood chemistry with more CO2 lost than normal. The blood becomes more alkaline. Haven't you experienced lightheadedness when you hyperventilate?

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(2) High-altitude People (see Harrison et al p 475)

The author of the text seems to minimize body change in response to altitude; other writers are very emphatic about it.

High altitude doesn't seem to increase-or decrease life expectancy.

Pregnancy at high altitude seems to result in lower birth weights (Overfield p 47 and pp 142-143). At high altitudes, male and female birth weights are about the same. At low altitudes, males weigh more than females. Thus, altitude has less affect on female birth weight.

While birth weights decline, the weight of the placenta increases. High altitude results in lower birth rates in all races. (My note: when I visited Cuzco, Peru in May, 1998, the guide mentioned that Spanish women who came with the conquest experienced reduced fertility, frequent stillbirths, and viable babies with low birth weights.) Chronic mountain sickness is more common in men than women (Overfield p 145). Smokers suffer more than non-smokeres.

Newborns at high altitude are shorter and have smaller head diameters. Tibetan newborns seem to weigh more resulting from many generations of selection. Himalayan groups have lived at high altitudes for over 25,000 years, while Andeans have been there less than 10,000 years.

Child growth rate is decreased at high altitude during infancy and adolescence. Interestingly, their growth period can be extended into their early twenties (adult stature is attained by age 20 in females and age 22 in males. Skeletal age is significantly delayed. (Poirier et al p 612. This extended maturation enables many children to eventually achieve heights of their low altitude cousins. Menarche is delayed by about a year among Peruvian Indians, but this apparently is not so for Tibetans.

Hemoglobin values increase at high altitude. Cerebral blood flow is disturbed at high altitudes. Acute mountain sickness and pulmonary edema are two syndromes of altitude exposure. The text suggests that malnutrition may contribute to growth depression sometimes ascribed to high altitude(see Overfield for a nice section p 144).

It has long been observed that lung capacity and chest size are greater in persons living at high altitudes. (McElroy & Townsend pp 92-94) In the Andes, lifelong residents tend to be short legged, to grow slowly, and to have a large thoracic volume. They also have more red bone marrow. The more viscous blood due to more red blood cells leads to an enlarged heart.

Above 14,750 feet, a non-native cannot achieve the functional work limits that the native-born can achieve.

Andean Indians have increased muscularization of the arterioles, a response to hypoxia.

High altitude people have more red blood cells with a possible increase in blood viscosity. High altitude people seem to be well adapted compared to the experiences of short term visitors. Highland Peruvian Indians have unusually large chests and lungs (see McElroy & Townsend).

Here are some remarks on the 'high altitude phenotype' from Poirier et al, p 608: The high altitude native has the typical altitude thorax-barrel chested with relatively short stature. The enlargement accomodates expanded lung capacity. The lungs are large within extremely large capillary bed; the diaphragm is large and powerful.

The symptoms of 'mountain sickness' are nausea, shortness of breath and headaches. (My note: I experienced this in Cuzco, Peru in May, 1998.) Some people acclimatize well and others do not. The best candidates to climb Mt. Everest may be those persons from high altitude regions of the earth.

Physiological plasticity and the evolution of culture has greatly extended our ability to adapt to diverse climates--even to those of a space station such as Mir.

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A curious cultural adaptation in the high Andies is chewing leaves of the coca plant. The leaves are not addictive and users do not experience withdrawal. Men use it more than women. It was a religious plant in Inca precontact days. The plant aleviates fatigue, hunger and cold. It produces vasoconstriction, which conserves core body heat (McElroy and Townsend pp 229-230).

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(3) Monge's disease (see Poirier p 612)

One oddity not mentioned often is that some--not all--some natives in high altitude seem to lose their adaptation in adulthood. These symptoms are seen as an overadaptation of adaptive mechanisms. The red blood cell count goes astronomically high. Arterial blood pressure is twice normal value with the result that hemorrhages can be seen under the fingernails. The right ventricular hypertrophy that characterizes the native is pronounced. Often, both ventricles become enlarged. The patient becomes cyanotic. There are neuromuscular disorders that affect the thoracic cage, deformities of the spine, and emphasema. (My note: as I prepare this unit, I am reminded of Selye's general adaptation syndrome where a long period of resistance is followed by physiological collapse. Does extreme adaaptation have a price?) Who is Monge? He was a prominent Peruvian researcher earlier in this century.

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VIII. Other Stressors (Harrison et al p 478)

There is exposure to many other stressors in the environment. Nutrition and infectious disease affects every one of us. Smoking and its health consequences is in the news frequently. The influence of electrical transmission lines of people is an ongoing debate. Others could be cited and certainly merit concern such air pollution, preservatives in food, human crowding in large urban centers--something utterly unknown in human history prior to the rise of cities five millenia ago.

Nothing in our evolutionary past has prepared us for living in dense, crowded yet lonely urban environments.

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Resources

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Garbarino, M. and Sasso, R. Native American Heritage 3rd ed. Prospect Heights: Waveland Press, Inc., 1994.

Jamison, C. Bioanthropology B200: Bioanthropology. a learning guide. Bloomington: Indiana University Study Program, 1992.

McElroy, A. and Townsend, P. Medical Anthropology in Ecological Perspective 3rd ed. Boulder: Westview Press, 1996.

Mascie-Taylor, C. and Bogin, B. Human Variability and Plasticity. New York: Cambridge University Press, 1995.

Overfield, T. Biologic Variation in Health and Illness 2nd ed. Boca Raton: CRC Press, 1995.

Poirier, F., Stini, W., and Wreden, L. In Search of Ourselves 5th ed. Englewood Cliffs: Prentice Hall, 1994.

Relethford, J. The Human Species 3rd ed. Mountain View: Mayfield Publishing Company, 1996.

Sinclair, D. Human Growth after Birth 5th ed. Oxford: Oxford University Press, 1985.