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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
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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, 3rd edition.
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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.
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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.
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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
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.
http://www.virtualmedicalcentre.com/anatomy/skin-colour/9
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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.
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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 irreversibleNina Jabonski on human skin color.
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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.
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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.
• Over many generations natural selection has resulted in some populations being genetically more suited to stresses at high altitude.
• 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.
An Andean woman and Himalayan
man: Her red cheeks primarily due to
increased blood flow near the skin
surface. More red blood cells helps her
get oxygen to the cells
of her body.
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High altitude adaptation• Many anatomical &
physiological adjustments of high altitude natives are 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.
• Evolving Altitude Aptitude
• Hypoxia and adaptation to altitude:
Reading Between the Genes
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.
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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
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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