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| Studies of adaptation focus on the many ways in which organisms respond to environmental stresses. Human adaptation is particularly interesting because humans not only adapt both biologically and culturally but also must deal with stresses from their physical and cultural environments. Biological adaptation includes physiologic responses and genetic adaptation (natural selection). Cultural adaptation includes aspects of technology, economics, and social structure. In any study of human adaptation, we must look at multiple stresses and multiple adaptive (or nonadaptive) mechanisms. Many studies of human adaptation have focused on cold and heat stress. Though as mammals, humans have the capacity for maintenance of body temperature, they must still cope with extremes in temperature. Physiologic responses of the human body to temperature stress include changes in peripheral blood flow and evaporation. Studies have shown that general relationships exist worldwide between body size and shape and temperature. These observed trends agree with the predictions of the Bergmann and Allen rules. In hot climates, small body size and linear body shape maximize heat loss. In cold climates, large body size and less linear body shape minimize heat loss. Cranial studies show that worldwide the shape of the skull also varies predictably, according to the principles of differential heat loss and the Bergmann and Allen rules. Although some of these biological features are the result of genetic adaptation, studies of children have revealed that response to temperature stress can affect growth. Cultural adaptations, especially those involving clothing, shelter, and physical activity, are also important in climatic adaptation. Millions of people around the world live at high altitudes. The major stresses of a high-altitude population are hypoxia (oxygen shortage) and cold stress. Many physiologic changes have been documented in high altitude peoples, including short-term physiologic responses and long-term increases in the size of the lungs and other components of the oxygen transport system. These changes are caused by hypoxic stress during the growth period, and their degree of change is related to the time spent living at high altitudes: the longer one has lived there as a child, the more adapted one is. Early studies of high-altitude populations also noted small body size that, along with delayed maturation, could be due to insufficient energy levels for body growth because of hypoxia and cold stress. More recent studies have shown that this is not always the case, because some high-altitude groups do not show this growth deficit. Instead, variation in diet appears to be the key factor. Humans have adapted culturally to basic nutritional demands in a variety of ways. For millions of years, human ancestors utilized different methods of hunting and gathering to feed themselves, and about 12,000 years ago, humans adopted agriculture as their primary means of subsistence. Although humans have a number of ways of adapting to their nutritional needs, there are some basic biological limits to adaptation, and an increasing number of children are malnourished. Modernization has altered traditional diets as well as other aspects of lifestyle, resulting in an increased biological cost in many cases. |