1

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

2

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.

3

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.

4

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.

5

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

6

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.

7

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.

8

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.

9

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.

10

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.

11

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

12

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