Introduction
Human populations occupy a wide range of habitats, from the icy wastelands of the Arctic to the equatorial deserts of North Africa. While most animal species have become adapted to relatively narrow niches, humans have survived in farranging and highly diverse niches. For the sake of discussion, two mechanisms of human adaptability may be differentiated: adaptation and adjustment. Adaptation refers to changes in gene frequencies resulting from selective pressures exerted by environmental factors on a population. Human species is able to adjust to a wide variety of environmental conditions without undergoing major micro evolutionary changes. Such changes are termed adjustments. However, it must be emphasised that the potentials for such non genetic adjustments are the end products of the evolutionary process. One of the major problems faced by researchers in this area is the determination of the relative importance of genetic and non genetic forms of adaptability, which we have termed here adaptation and adjustment, respectively. In fact, in most situations, both probably operate together.
ACCLAMATORY ADJUSTMENTS
Acclamatory refers to reversible physiological adjustments to environmental stress. Instances of acclimatisation have been observed in three environments: the arctic, the desert, and high altitudes. The Arctic, is perhaps one of the more stressful habitats occupied by humans. It was inhabited relatively late in human prehistory, and gradually human became acclimatised to the climate. The primary environmental stress in the arctic is very low temperatures. Normal human core body temperature is 370C (98.60F). When the core temperature falls below 34.40C (940F), hypothermia occurs and at 29.40C (850F), temperature regulating ability of the hypothalamus in the brain is impaired resulting in death. Cold exposure which leads to freezing of the tissues causes frostbite. It usually occurs in exposed area of the body parts such as fingers, toes, and earlobes. Perhaps the most important acclamatory adjustment is increase in the basal metabolic rate. The basal metabolic rate refers to minimum amount of energy required by the body to maintain body processes necessary for life and is measured as the amount of heat produced by the body per unit time at rest. This increase can be as much as 25 per cent in adults and 170 per cent in infants. The increased basal metabolic rate results in the production of additional body heat but it requires that individuals’ consumption of food, high in nutritive value. The native diet in these regions consists largely of protein and fat, which provides the required types of food. People living in arctic conditions for long periods of time are less affected by the cold.
A. ACCLIMATIZATION AND EXTREME COLD
The nude human body at rest begins to combat against hypothermia at the air temperature of approximately 310C (87.8°F). This temperature is known as the critical temperature. Consequently, the individual reduces the loss of heat from body to the environment and produces heat to increase body temperature.
A major mechanism for conserving heat is peripheral vasoconstriction. Constriction of the capillaries below the skin prevents warm blood from reaching to the surface of the skin, where much of the body’s heat would be lost to the air. Additional body heat is produced voluntarily by exercise and involuntarily by shivering. A high degree of muscle activity yields heat. However, exercise can be performed only for limited periods of time depending on the physical fitness of an individual. More important is shivering. Low body temperature causes the hypothalamus of the brain to stimulate increased muscle tone, which results in shivering. At the peak of shivering, the increased muscle metabolism can increase the rate of heat production five times than that of the normal.
B. ACCLIMATIZATION TO DESERT HABITATS
Acclimatising to desert life is not well understood, but within a few weeks of exposure to hot and arid climates individuals acquire some adjustments. Meanwhile, the sweat glands become sensitive and produce sweat. Sweat also contains salts, and much sodium is lost through sweating. With time, the concentration of salt in sweat is reduced, although the relatively high salt concentrations found in desert water easily compensate for salt loss. Urine volume also reduces, thus conserve water in the body. This same acclimatisation ability has been found in peoples from all parts of the world, so it appears to represent a basic ability of the human species instead of an adaptation of certain populations. In addition, there are short-term reactions to increased heat loads. The physiological response involves vasodilatation of the capillaries under the skin. In vasodilatation, the bloodstream brings heat to the body surface, and is lost to the environment by conduction, convection, radiation or evaporation. Acclimatisation escalates the ability to work longer and more efficiently in hot climates. People also adopt various behavioural adjustments to the hot climates. In desert regions people tend to reduce physical activity during the heat of the day, thereby reducing heat production by the body. Also, assume a relaxed body posture that increases the surface area of the body from which sweat may evaporate.
Adjustment to hot climates is aided by cultural factors such as clothing and shelter. Desert dwellers cover their bodies to protect the skin from ultraviolet radiation as well as to reduce the amount of heat from the sun which otherwise is absorbed by the body. Such clothing is designed to permit the free flow of air between the clothing and the body. This airflow is necessary to carry off the water vapor formed by the evaporation of sweat. Interestingly, the color of clothing does not seem to make much difference in hot climates. An experiment conducted on black and white Bedouin robes shows that black robes gain about 2 ½ times as much heat as white robes. Yet the temperature of the skin under black robes is the same as that under white robes. Most likely, the greater convection currents between the black robe and the skin are responsible for this phenomenon.
In addition to physiological and cultural factors, psychological elements appear to play major roles in adjustments to intense heat. Some Europeans adapt quickly to desert life, others never seem to adjust to changing conditions of life. In an emergency, the European, anxious to do something, is active, gets hotter and uses up his limited supply of water. In contrast, the nomad, secure in the will of Allah, tends to relax and behave more calmly.
C. ACCLIMATIZATION ADJUSTMENTS TO HIGH ALTITUDES
When a person travels into the mountains on vacation, he or she may experience high-altitude or mountain sickness. The symptoms include shortness of breath, respiratory distress, physical and mental fatigue, rapid pulse rate, interrupted sleep, and headaches intensified by activity. Slight digestive disorders and in some cases a marked loss of weight may also occur. In other cases the individual may feel dyspnea, nausea, and vomiting. Although most people eventually become acclimatised to high altitude, many do not. They will continue to suffer from chronic mountain sickness as long as they remain at high altitude. Less than 1 percent of the world’s population lives at high altitude, yet these populations are of great interest to anthropologists. High-altitude environments exert multiple-stress on human population. These stresses include low oxygen pressure, intense solar radiation, cold and dry wind, rough terrain, and relatively limited plant and animal life.
High-Altitude Hypoxia: As already known, the arctic and desert environments lay emphasis on culture as an efficient means of adjustment to stressful environments. Culture plays a pivotal role in high altitudes as well except in hypoxia. Hypoxia refers to low oxygen pressure, which occurs when relatively low levels of oxygen are supplied to the tissues of the body. Hypoxia may result from disease as well as environmental factors. High-altitude hypoxia represents one of the few environmental stresses that cannot be adjusted to by some cultural means. Although the use of oxygen tanks provides limited adjustment, this solution is available only in high-technology cultures and is practical only for short periods of time. The earth’s atmosphere exerts an average of 1 kilograms of pressure on every square centimeter (14.7 pounds per square inch) of surface area at sea level. This pressure is able to raise a column of mercury (which has the chemical symbol Hg) in a closed tube to an average height of 760 millimeters (29.92 inches). Therefore, we say that the average air pressure at sea level is 760 millimeters (29.92 inches) of mercury.
The atmosphere is composed of many gases and approximately 21 per cent of air is oxygen. The portion of the total atmospheric pressure due to oxygen is the partial pressure of oxygen, which measures 159 millimeters (6.26 inches) of mercury at sea level. At altitude, the partial pressure of oxygen decreases. At 4500 meters (14,765 feet) the partial pressure of oxygen is decreased by as much as 40 per cent, thus substantially reducing the amount of oxygen that can reach the tissues of the body. The oxygen enters into the bloodstream via approximately 300 million alveoli of the lungs. The alveoli are small air sacs that are richly endowed with blood capillaries. Although the partial pressure of oxygen at sea level is 159 millimeters (6.26 inches) of mercury, the partial pressure of oxygen in the alveoli at sea level is 104 millimeters (4.16 inches) of mercury. This is due to the fact that not all the air in the lungs is replaced with each breath. The partial pressure of oxygen in the arteries and capillaries of the circulatory system is 95 millimeters (3.80 inches) of mercury, and in the tissues it is 40 millimeters (1.60 inches) of mercury.
As gas moves from higher to lower partial pressure, oxygen diffuses from the blood to the tissues. At high altitudes, the partial pressure of oxygen in the blood would be too low to permit diffusion of oxygen from the blood to the tissues unless certain physiological adjustments take place. These adjustments make high-altitude environments a possible human habitation. When an individual inhabiting near sea level migrates to high mountains, he or she will probably notice an increase in the breathing rate, which may reach twice that of at sea level. The increased breathing rate brings more oxygen in the alveoli, and increases the partial pressure of oxygen in the blood. This hyperventilation, or increased breathing rate, eventually reduces, to normal level as the person becomes acclimatised to the high altitude. About 97 per cent of the oxygen in the blood is carried in chemical combinations with hemoglobin in the red blood cells; the other 3 per cent is dissolved in the plasma and may be ignored. The chemical association of oxygen and hemoglobin is loose and reversible. When the partial pressure of oxygen in the alveoli of the lungs is higher than that of blood, oxygen diffuses into blood vessels and combines with hemoglobin. When the hemoglobin molecule reaches the capillaries, oxygen diffuses into the cells where the partial pressure of oxygen is lower than blood.
When the blood leaves the lungs, the hemoglobin is about 97 percent saturated with oxygen. Some of the oxygen is then utilised by the tissues. As a result, the hemoglobin in the veins returning to the heart and lungs is only about 70 per cent saturated. At high altitudes, several factors operate to alter these percentages, thereby permitting the hemoglobin molecules to carry more oxygen to the tissues. As a result of hyperventilation, the concentration of carbon dioxide in the blood is decreased, thus altering the blood chemistry in a way to increase the amount of oxygen carried in the blood.
Other acclamatory changes include increase in the number of blood capillaries, thereby improves the diffusion of oxygen by shortening the distance between the cell and capillary. Increased number of red blood cells enhances oxygen carrying capacity due to elevated amount of hemoglobin. Therefore, although the partial pressure of oxygen as it enters the lungs differs at sea level and at high altitude, the partial pressure of oxygen in the blood is not very different by the time it reaches the capillaries. Many changes also occur at the cellular level that enables cells to carry out their metabolic functions at lower oxygen levels.
While the factors discussed above and many more, permit humans to live at high altitudes, people cannot overcome all the negative biological effects of highaltitude. For example, high altitude affects reproduction; birth weights are lower and infant mortality is higher. In addition, the growth and development of children are slower. A good example of developmental adjustment is found among the children of high altitudes. During growth the native develops greater chest circumferences than do those growing up at lower elevations.