I. HUMAN ADAPTATION
The way in which humans meet the challenges of the environment is through a general process called adaptation. In evolutionary biology, we look at it as natural selection and survival of the fittest. Over many generations, it means that a change of gene frequencies in the population. This is the process at work in the selection for the sickle cell allele in a malarial environment.
On the other hand, the human as an individual can respond to environmental stress with physiological and growth adjustments. These changes are rapid and reversible as in the accommodation of the eye to light or darkness. Some are slower, such as an increase in the number of red blood cells in adjustment to a high altitude. Some long term changes as those that occur during childhood growth are irreversible.
II. PHYSICAL GROWTH IN HIGH ALTITUDE POPULATIONS
Early studies of adaptation to a high altitude had a sinister motive: they were studies by Europeans who sought to exploit people for the extraction of mineral resources in highland South America.
Studies of highland Indian populations in Peru have shown that chest dimensions and lung volume are greater at all ages in the people studied. The high altitude populations were also shorter at most ages than low altitude populations. The shorter stature is related to delayed maturation, whereas the increase in chest size is a functional adaptation to a high altitude. (My note: in spite of these statements, some authors minimize the effect of altitude whereas others are very emphatic about the effect of altitude.)
High altitude results in lower birth rates in all races. Newborns are shorter and have smaller head diameters. However, Tibetan newborns seem to weigh relatively more, a possible consequence of a longer history of occupation in highlands. This is a result of natural selection. While birth weights decline, the weight of the placenta increases . . . a compensatory response to reduced oxygen levels.
High altitude doesn't seem to increase--or decrease life expectancy.
Child growth rate is decreased at high altitude during infancy and adolescence. Interestingly, at high altitudes, adolescent growth can extend into their early twenties. (My note: adult stature is attained by age 20 in females and age 22 in males. This is delayed maturation by our standard.) Skeletal age is significantly delayed. This extended maturation enables many children to achieve the heights of their low altitude cousins.
Menarche is delayed by about a year among Peruvian Indians, but this apparently is not so for Tibetans. This is an intriguing issue: it appears that Tibetans have had a much longer adaptation to high altitude than have Peruvians.
Hemoglobin values increase and cerebral blood flow is reduced at high altitude. For lowland people, acute mountain sickness and pulmonary edema are two syndromes of altitude exposure (My note: mountain sickness symptoms are nausea, shortness of breath, and headaches. I know; I experienced them in Cuzco, Peru.) Malnutrition also seems to contribute to growth depression of some Peruvians at high altitude.
It has long been observed that lung capacity and chest size are greater for persons at high altitude. Life long residents of the high Andes tend to be short legged, usually grow slowly, and to have a larger thoracic volume. They also have more red bone marrow.
Above 14,750 feet, a non-native cannot achieve the functional work limits that the native-born can achieve. One implication: the best candidates to climb Mt. Everest may be those persons from high altitude regions of the Earth.
The irreversible changes reflect human genome plasticity, the ability to make adaptational changes during growth.
III. LIGHT AND SOLAR RADIATION
Exposure to sunlight varies enormously at different latitudes. In response to this environmental stress the body uses a mechanism that alters the skin pigment. Melanin, a biologically complex compound, absorbs ultraviolet light. Melanin is produced in the basal layers of the skin by specialized cells called melanocytes. Populations differ in skin color not so much because of the number of melanocytes, but in the way these cells are bunched and the size and number of the melanin granules produced.
Only melanin can project against the harmful effects of solar radiation. One result to overexposure to the sun is skin cancer. The body can respond to solar radiation by increasing the number of melanin granules, causing what we call a tan. All human populations can tan in response to exposure to solar radiation, but the effects are obviously more noticeable in the fair skinned individual.
Light-colored skin favors production of vitamin D, especially in northern latitudes. Also, light skin is less susceptible to cold injury. This was shown in the Korean War wherein black soldiers had an increased incidence of frostbite.
In the Old World before Columbus, populations in northern latitudes had fair skins, those in equatorial regions had dark skins. This may be the result of natural selection over long periods of time--selection in favor of solar radiation protection in low latitudes, and vitamin D production in high altitudes.
In the New World, there is no such distribution of skin color. Why? Probably it is because Native Americans may be recent immigrants to the New World and there has not been enough time for natural selection to occur.
IV. BODY SIZE AND SHAPE
ALLEN in 1877 said that 'in warm blooded species the body size usually increases with habitat cold. It applies to Homo sapiens as well as any other polytypic species. Good examples of extremes are the Inuit (Eskimo) and the Nuer (East Africans).
BERGMANN in 1827 said that 'within a polytypic warm-blooded species, the body size usually increases with habitat cold. This applies with validity to any species including humans. It is the advantageous for polar mammals to be large and round. The ice--is not a nice place for mice!
V. HEAT STRESS
We have more effective body physiology to cope with heat that we do with cold. We have specialized sweat glands which can produce large amounts of water for cooling; this also makes us continuously dependent upon water availability. At high temperatures, young males can loose 4 liters of water per hour. In extreme desert conditions, water loss can exceed the ability to take in fluid even if ample water is available. In World War II, the death rate for persons 10& overweight in heat stress was nine times that of persons of normal weight.
Children exposed to high heat loads will have a greater number of sweat glands as adults. All people can acclimatize to heat: after a few days of work at high temperatures, the body suffers less stress, cools more effectively, and can do more work than at the outset of the experiment. This is important for military strategy in hot climates.
VI. COLD STRESS
Without clothing, our cold tolerance is low, although subcutaneous fat does help. The first defense is peripheral arterial vasoconstriction. We've mentioned the 'Lewis Curve' in relation to extremity cooling. Shivering affords limited warming. For all this, however, we have little acclimatization to cold. Humans depend upon clothing and housing to survive.
Physically fit persons do better in cold than the inactive or the unfit. Subcutaneous fat is particularly beneficial in cold water. Successful swimmers of the English Channel (or more recently Cuba to Florida) tend to be women with thick fat layers. Central Australian Aborigines can sleep in the nude unperturbed at temperatures close to freezing. Knowledge of cold stress is a serious matter for the military.
Bunney, S. ed. The Cambridge Encyclopedia of Human Evolution. New York: Cambridge University Press, 1994. (My note: This is a superb resource with a British perspective.)
Harrison et al Human Biology 3rd ed. New York: Oxford University Press, 1993. (My note: This text is recommended for advanced study in physical anthropology.)
Jermain et al Introduction to Physical Anthropology 7th ed. Belmont: Wadsworth Publishing Company, 1997. (My note: a useful resource for physical anthropology.)
Overfield, T. Biologic Variation in Health and Illness: Race, Age, and Sex Differences. 2nd ed. Boca Raton: CRC Press, 1995. (My note: this obscure and expensive book is a superb reference for medical anthropology.)
Relethford, J. The Human Species 3rd ed. Mountain View: Mayfield Publishing Company, 1997. (My note: this text has useful sections on physical and medical anthropology.)