Anthropology 150

 Both the “Out of Africa” model and the “Multi- regional” model accept that, for whatever reason, Neandertals were extinct as a distinct group by about 25,000 B.P.

 For the first time in over 1.8 million years, there was a single hominid group inhabiting the planet

 Paradoxically, while biological diversity was greatly reduced after 25,000 B.P. cultural diversity greatly increased, as Homo sapiens sapiens spread into new regions of the world

 Adaptive radiation  Selective pressures

 Climate  Effects skin color, nose shape, body shape, head shape

 Bergman’s ruler: within a species, the body mass increases with latitude and colder climate.

 Allen’s rule: within the same species of warm-blooded animals, populations having less massive individuals are more often found in warm climates near the equator, while those with greater bulk, or mass, are found further from the equator in colder regions.

 Food sources  Need to use milk in adulthood lead to selection for mutation enabling lactose

digestion after childhood in pastoralist societies  Predators and parasites

 Example: Sickle-cell trait in West African Populations

 Drift  Isolation of populations inhibits gene flow  Neutral traits can develop under conditions of isolation as well as adaptive

traits

 •Darker skinned people tend to inhabit areas nearer the equator •However, many exceptions to this especially in Asia and the New World

 •Several selective pressures probably had an important influence distribution of skin color seen in human populations. •Many theories have been proposed

Heat exchange Vitamin D regulation Radiation Protection Frostbite susceptibility Sexual Selection

 The problem of dark skin in environments in which heat load through direct exposure to sunlight is a problem. ØDark skin absorbs more radiant energy than light skin (black car seats get hotter than white car seats when exposed to the sun). ØThis increases the heat load, dark skinned people in such environments. •Heat absorbed by dark skin may be an adaptation for replacing body heat lost during the night in desert environments in which it gets cold at night, but doesn’t freeze.

 Ultraviolet light is known to be harmful if it penetrates the skin. It causes sunburn and may induce cancer through its mutagenic effects •Darker skinned people have an advantage when exposed to high levels of ultraviolet light •The late onset of skin cancer may reduce its significance as a selective pressure

 •Vitamin D is synthesized in the epidermis through the action of ultraviolet light on a steroid precursor •Low levels of vitamin D produce rickets and skeletal malformations that can result in death during childbirth •Too much vitamin D results in kidney stones and calcification of soft tissues •As people moved from the tropics to temperate areas, reduced sunlight and the necessity of wearing clothes would select for depigmentation to regulate vitamin D synthesis

 •Body proportions are to some extent a reflection of developmental plasticity

 •Children who develop in conditions of heat stress tend to have smaller trunk size and longer limbs of smaller girth than controls.

 •The body proportions of some groups of people may be explained by natural selection for thermoregulation.

 The appendages of widely distributed species tend to be shorter in individuals that inhabit the colder areas of their range

 􀂾 Developmental plasticity may explain some variation in limb proportions, but probably not all

Longer appendages

increase the surface area

available for heat

radiation

•This increases heat loss

in warm environments

 •The members of widely distributed species tend to have a larger body size in the colder areas of their range

 •Examples

 􀂾 Possums

 􀂾 Bears

 􀂾 Deer

•Heat conductance depends on the surface

area of the body

•With increase in body size, volume

increases at a faster rate (x3) than surface

area (x2)

•Thus heat retention increases with

increased body size

•Heat conductance depends on the surface area of the body

•With increase in body size, volume increases at a faster rate (x3) than surface

area (x2)

•Thus heat retention increases with increased body size

Side=1

Surface=6

Volume=1

Surface/Volume=6

Side=3

Surface=32x 6= 54

Volume= 33=27

Surface/Volume=2

Side =2

Surface = 2x2x6 = 24

Volume = 2x2x2=8

Surface/volume=3

 •Best known example is that of sickle- cell trait in humans

 People homozygous for the trait (ss) are severely anemic

 Normal people (SS) are susceptible to malaria

 Heterozygous people are slightly anemic but have the advantage of malaria resistance

 Human modification of the environment may be reducing the advantage of having the sickle cell trait

 •30-50 million Americans are lactose intolerant (have reduced capacity to produce the enzyme that breaks down milk sugar (lactose) as adults

 •The disorder affects some populations more than others:  􀂾 Seventy-five percent of all African-Americans and Native Americans

are lactose intolerant.  􀂾 Ninety percent of Asian-Americans are lactose intolerant.  􀂾 Lactose intolerance is least common among people with a northern

European heritage.  •Milk dependence and the geographical distribution of lactase deficiency

is correlated with the availability of dairy products  •Explanations  􀂾 Developmental plasticity?  􀂾 Genetics?  􀂾 Both?

 Clinal variation

 Defined as gradual variation with a species

 A cline is roughly synonymous with “population”

 Human populations exhibit continuous variation, rather than rigid boundaries

 Race is not a valid biological concept

 “Race” is often used a synonym for “sub- species”

 There is only one human species (and sub- species) on the planet: Homo sapiens sapiens, which shows clinal (not racial) variation across the globe. Humans are polymorphic, but not polytypic.

 Richard Lewontin – Harvard Geneticist

 Studied variation for 16 genetic traits in 7 populations that he defined along using North American views of “race.”

 1) Caucasians

 2) Black Africans

 3) Mongolids

 4) South Asian Aborigines

 5) Native Americans

 6) Oceanians

 7) Australian Aborigines

 Only 6.3% of all variation can be accounted for at the level of these geographic groups

 Approximately 94% of all genetic variation is within groups

 Differences within the geographically defined groups is about 15 times the differences between them

 Race is not a valid biological concept, but it does exist as a socially constructed means of classification. Race is a social construct in search of a biological justification.  Race is a folk classification

 Race is a cultural scheme of classification  Different cultures have different schemes of racial classification

 Compare the U.S.A, Brasil, and Colonial Central America

 Different cultural traditions have, at different times, used varying schemes of classification  U.S.A. census classifications

 All human societies classify people  “Race” provides an easy to use package of superficial traits (skin

color, eye shape, hair color, etc.) that make classification simple.

 In the absence of such traits, human societies often generate other means of easy classification: clothing, hair styles, body marking.

 Race is the lazy person’s way to classify human biological diversity

“Racial beliefs constitute myths about the diversity in

the human species and about the abilities and

behavior of people homogenized into "racial"

categories. The myths fused behavior and physical

features together in the public mind, impeding our

comprehension of both biological variations and

cultural behavior, implying that both are genetically

determined. Racial myths bear no relationship to the

reality of human capabilities or behavior. Scientists

today find that reliance on such folk beliefs about

human differences in research has led to countless

errors. “

http://www.aaanet.org/stmts/racepp.htm