Evolution and Human Adaptation
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Evolution and human adaptation ANTH 306/Medical Anthropology Evolution and human adaptation • Readings : • Chapter 3 of McElroy and Townsend – Genes, Culture and Adaptation , pages 82-129. • S. Boyd Eaton, Marjorie Shostak, Melvin Konner: Stone Agers in the Fast Lane • Wenda Trevathan: Evolutionary Medicine: An Overview . • Highly recommended: • Human Biological Adaptability: An Introduction to Human Responses to Common Environmental Stresses 1 Adaptation • Change & variation developing over time in response to the pressures of a given environment (M&T page 83). • The core concept of medical ecology – approach emphasizing study of health & disease in environmental context. • Health viewed as a measure of adaptation to environment. Human adaptation • Humans can adapt or adjust in several ways: 1. Biological Genetic Physiological 2. Cultural Individual/psychological • Group Taken from McElroy & Townsend’s text, 3 rd edition. 2 Distribution of Human Diversity • Physical traits that are often thought of as clustering together among particular peoples often have much broader distributions. • Continue well outside of the geographic areas in which a "race" is stereotypically supposed to exist. • An example is dark skin: • usually thought of as key trait in distinguishing sub- Saharan Africans from people elsewhere in world. • but dark brown skin is also found in southern Asia, Australia, New Guinea and on nearby islands of Melanesia, as well as in much of the Americas. Human skin • Body’s largest organ. • Comprised of 2 major layers: • Thin outer layer, epidermis. • Thicker, inner layer, dermis. • Skins functions in many ways: • Thermoregulation • Protection from physical & chemical injury. • Protection from invasion by microorganisms. • Aids in synthesis of vitamin D, an essential nutrient. 3 Skin color & adaptation • As one of most conspicuous of human physical traits, skin color has attracted a lot more attention than any other aspect of human variability. • Has served as a primary feature in most systems of racial classification, regardless of dubious scientific basis of these schemes. 4 Biological Determinants of Skin Color • Human skin color has wide spectrum of colors. • Pigments Carotene , Hemoglobin , & Melanin involved in determining skin color. • Primary determinant of variability in human skin color is amount, density, & distribution http://www.virtualmedicalcentre.com/anatomy/skin-colour/9 of melanin – which helps protect us from ultraviolet light. • Generally, amount of melanin in skin depends on a combination of inherited factors & degree of light exposure. 5 Distribution of human skin color prior to 1500 The distribution of human skin color before A.D. 1400 and the average amount of ultraviolet radiation in watt-seconds per square kilometer. 6 The evolution of human skin coloration. Nina G. Jablonski and George Chaplin, Journal of Human Evolution (2000) 39, 57–106. • Skin coloration in humans is highly adaptive and has evolved to accommodate the physiological needs of humans as they have dispersed to regions of widely varying annual UVMED . The dual selective pressures of photoprotection and vitamin D3 synthesis have created two clines of skin pigmentation. • The first cline, from the equator to the poles, is defined by the significantly greater need for photoprotection at the equator in particular and within the tropics in general. Deeply melanized skin protects against folate photolysis and helps to prevent UV-induced injury to sweat glands (and subsequent disruption of thermoregulation). • The second cline, from approximately 30 N to the North Pole, is defined by the greater need in high latitudes to accommodate as much previtamin D3 synthesis as possible in areas of low annual UVMED. Humans inhabiting regions at the intersection of these clines demonstrate a potential for developing varying degrees of facultative pigmentation (tanning) (Quevedo et al., 1975). Moderately melanized skin would appear to be at risk of vitamin D3 deficiency and rickets under conditions where UV radiation is restricted as a result of latitude, cultural practices or both. Genetic change • Skin color typifies adaptation through genetic change because it: • occurs at population level • is based on inherited traits • is irreversible Nina Jabonski on human skin color. 7 Physiological changes • Take place at individual level, occur within a lifetime. • Can be reversible or irreversible. 1. Homeostatic responses • immediate and aimed at restoring balance. • are reversible . • examples: sweating to cool, shivering to stay warm, constriction of pupil in excess light. 2. Developmental adjustments • take longer to develop. • are irreversible . • Example: physiological adaptations made to hypoxia by high altitude populations. High altitude adaptation • Two major kinds of environmental stresses at high altitude for humans. 1. Alternating daily extremes of temperature • often range from hot, sun-burning days to freezing nights. • winds often strong & humidity low; rapid dehydration occurs. 2. Lower air pressure. • Air pressure decreases as altitude increases • Usually is most significant limiting factor in high mountain regions. 8 High altitude • At high altitudes, body initially develops inefficient, stressful physiological responses. • Breathing & heart rate double even while resting. • Pulse rate & blood pressure ↑ as heart pumps harder to get more oxygen to cells. • More efficient response normally develops later as acclimatization takes place. • More red blood cells & capillaries are produced to carry more oxygen. • Lungs increase in size to facilitate osmosis of oxygen & carbon dioxide. • Increase in vascular network of muscles enhances transfer of gases. • But successful acclimatization rarely results in same level of physical & mental fitness that was typical of lower altitudes. High altitude adaptation • Great variability exists in human ability to adjust to high mountainous regions. An Andean woman and Himalayan • Over many generations natural man : Her red cheeks primarily due to increased blood flow near the skin selection has resulted in some surface. More red blood cells helps her populations being genetically more get oxygen to the cells suited to stresses at high altitude. of her body. • Most successful populations are those whose ancestors have lived at high altitudes for thousands of years. • Examples : • Native peoples of Andes Mountains of South America. • Populations of the Himalayan Mountains and high plateau of South Asia. 9 High altitude adaptation Evolving Altitude Aptitude • Many anatomical & • physiological adjustments of • Hypoxia and adaptation to altitude: high altitude natives are Reading Between the Genes developmental , occurring before birth & during childhood. • Some high altitude populations bodies’ produce more oxygen carrying hemoglobin in blood. • Lung expansion capability is usually greater. • Lower birth weights. • Slower growth rates. Cultural adaptation • Even though they are learned, cultural adaptations still have certain biological bases, such as: • large/complex brain • bipedal locomotion • digital dexterity • Advantages of cultural adaptation: • Can occur very rapidly . • Flexibility enables adjustment to wide range of conditions. • Learning means individuals benefit from knowledge, skills, ideas of many different people within one’s group & through time. 10 Cultural adaptation • McElroy & Townsend note that a danger of functionalist hypotheses is tendency to assume that all customs have some adaptive value or function. • Many examples contradict this assumption. • Numerous health risks posed by female circumcision (AKA female genital mutilation): • tetanus, hemorrhage, shock, urinary tract infections, incontinence, chronic pelvic infections, obstructed labor in childbirth, sterility, etc. Psychological adaptation • M & T discuss individual coping mechanisms used to deal with chronic or progressive diseases or a disability. • One example is Robert Murphy’s autobiographical account The Body Silent . • A major component of adapting to physical impairment can involve redefinition of the meaning of that impairment. • 'Fresh Air' at 20: Anthropologist Robert Murphy 11 Psychological adaptation • Gay Becker’s study of deaf community (M &T page 126). • Long-term support networks of deaf are adaptive in old age. Interdependence developed in adapting to hearing loss is “pre-adaptive” to old age. • Adaptation to old age (from Growing Old in Silence) • Aging in Today's World: Conversations Between an Anthropologist and a Physician • Gay Becker Memorial Fund 12.