Bipedal locomotion is the hallmark of the hominin family. Both the morphology and the orientation of bones and joints must be extensively altered from the ancestral pongid condition in order to accommodate bipedality. These alterations affect the foot, leg, pelvis, and vertebral column. Extensive biomechanical analysis of bipedalism has been conducted in living humans. This analysis demonstrates that there is very little electrical activity in muscles when subjects walk at a normal pace and are unencumbered by burdens. Hence, although it is slow, bipedalism is a very energy efficient mode of locomotion. A human walking at a normal speed uses only about
87% of the energy used by a similarly sized, generalized quadrupedal mammal moving at the same speed.
Normal humans carry large amounts of subcutaneous fat. This is peculiar for terrestrial mammals, which typically accumulate fat only before breeding, migrating, or hibernating. Unlike marine mammals, humans do not need this fat for maintaining the core temperature of the body. Furthermore, given constant supplies of abundant food and little physical activity, humans can quickly increase their store of subcutaneous fat. The most likely explanation for this human peculiarity is that it evolved to allow humans to survive periods of starvation or near-starvation. Indeed, seasonal calorie restriction is documented today for hunter-gatherers, as well as for agriculturalists. Many contemporary humans experience famine, if rainfall is low, or inadequate emergency stores of food have been set aside. There may be no extra food to cache for emergencies. Seasonal want appears to be the norm for humans, and thus natural selection has provided a built-in reserve of fat to tide humans over the inevitable lean period.
The surface of the human body is virtually hairless. With high ambient temperatures, sweat evaporates from this hairless skin. The temperature of the human body surface is thus lowered through evaporative cooling. This physiological adaptation is seen in all humans. It is a species-specific trait, because it is based on the presence of eccrine sweat glands on the surface of the skin. Hairlessness promotes evaporation.
The abundance and density of eccrine sweat glands are unique to humans among other mammals. These glands are mainly restricted to the bottoms of the paws and adjacent regions in other mammals. Eccrine sweat glands do not produce the fatty secretions that are associated with scent and scent-marking in mammals. Instead, eccrine glands produce abundant watery secretions that contain salt, potassium, and calcium. The human sweating response is entirely dependent upon access to abundant fresh water, because any water lost through sweating must be quickly replaced. If this water is not replaced, death, caused by shock through loss of blood volume and heat stroke, can occur within a single day. A normal human sweating rate is 0.5-1 liter/hour, but this can be increased to 2 or sometimes 3 liters/hour in working humans accustomed to high temperatures. This rate, however, cannot be sustained.
Human body build shows climatic adaptation to extremes of temperature. This has been noted since the nineteenth century, and confirmed in many studies during the twentieth century. In 1847, Bergmann observed that endothermic animals had heavier bodies in cold climates, and lighter body builds in hot climates. In 1877, Allen observed that endothermic animals had shorter extremities in cold climates, and longer extremities in hot climates. Humans conform to Bergmann's and Allen's rules. In 1994, Ruff established that human pelvic breadth, which is a good proxy for body width, is correlated with temperature. Pelvic breadth is wide in cold climates, and narrow in hot climates.
High altitude also affects humans, principally through low oxygen pressure. However, cold temperature, high winds, rough terrain, poor soils, and impoverished ecosystems also exercise a profound affect on humans living at high altitudes.
Humans entering high altitude areas from the lowlands gradually increase the number of red blood cells in their body. These cells carry hemoglobin, which binds to oxygen, and transports it through the system. This response is caused by reduced oxygen at higher elevations. Humans born at high altitudes have a larger heart and lungs, and grow more slowly. Human populations that adapt to high altitude through evolutionary time have larger placentas, and consequently develop better contact between the blood supply of fetus and mother. Newborns of these populations have a higher birth weight, and greater survivorship than infants from other groups that are new migrants to the region. Native people in Tibet, who may have evolved high altitude adaptations through the longest time, have a genetically based variant hemoglobin that has enhanced oxygen binding properties.
Human populations differ in skin pigmentation. Since the 1930s, a relationship has been documented between skin pigmentation and latitude. Darker skin occurs at low latitudes, and lighter skin at high latitudes. The pigment melanin, produced by melanocytes in deep layers of the skin, is responsible for variation in human skin color. The skin also synthesizes vitamin D when it is exposed to sunlight. Vitamin D has an important role in calcium metabolism, which affects not only skeletal density, but also proper functioning of the nervous system. The adaptive significance of melanin in the skin appears to involve maintaining critical amounts of vitamins—vitamin D synthesis and the preservation of adequate amounts of folate, necessary for normal development of the fetal nervous system. Light skin allows more vitamin D to be synthesized in high latitudes where sunlight is weak. Dark skin decreases vitamin D synthesis and preserves folate in low latitudes where sunlight is intense.
Human adaptation to extreme climates is dependent upon culture and technology. Culture and technology allow humans to create pleasant or balmy microhabitats in which to live. Fire, clothing, shelter, transportation, food acquisition, processing, and caching, water storage, and complex behavioral adaptations underlie and complement human morphology and physiological response.
Since the early 1950s, physical anthropologists have studied human populations, not races. This reflects an understanding of the importance of variation within populations, and an overriding interest in natural selection, adaptation, and other evolutionary processes. The earlier approach, defining races and human types, was typological in nature. It categorized humans, devised schemes for human classification, and was relatively indifferent to evolution. In 2003, race is prin-
cipally used by forensic anthropologists in the analysis of human DNA and skeletal and soft-tissue traits, where the ancestry of forensic material needs to be ascertained.
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