lecture 14 human variation

Anthro 101: Human Biological Evolution

Lecture 14: Human Variation

Human Variation

• These charts tell us that Homo sapiens evolved relatively recently, and expanded rapidly

• In addition, they tell us that biological race does not exist…

But there seems to be variation between people in different parts of the world. How can we understand this?

Molecular data tell us that non-Africans are all descendants of people that moved out of Africa about 50 kya

Over the last 50ky humans have diversified• Human morphology varies

• Height and body proportions

• Skin color • Hair color and texture• Facial features

• Human physiology varies• Lactose absorption• Hemoglobin types• Susceptibility to diseases

• Human behavior varies• Subsistence strategies• Social organization• Beliefs, values, etc.

Variations among humans can be due to:genetic differences or environmental differences

• Genetic variation • Differences caused by variation in genes inherited from

parents

• Environmental variation• Differences caused by environmental factors• Differences caused by culture

• Both of these interact to generate our variation• Difficult to distinguish between these two causes because

parents and children share genes and environment

Natural selection contributes to some of this genetic variation between groups

• Different environments favor different traits (genes)

• Lactose absorption• Hemoglobin types• Genetic diseases• Sickle-cell anemia• Skin color• Body shape

Natural Selection & Heterozygote advantage can produce genetic variation between groups

• Heterozygotes for sickle cell allele (AS) are partially protected against malaria

• Heterozygotes more likely to survive than homozygotes of either type

• Selection maintains balanced polymorphism

• Variation in genetic composition of population will be maintained

Presence of sickling gene is associated with the rate of malaria in different parts of the world

Natural selection can shape differences in ability to digest milk between populations

• Mother’s milk contains large amounts of lactose (large sugar)

• Lactose too big to be absorbed through intestinal walls

• Enzyme lactase breaks lactose into two smaller sugars that can be absorbed

• Infants produce large amounts of lactase

• All normal mammalian infants can digest milk

Most mammals lose ability to digest lactose after weaning

• In most mammals, lactase production tapers to low levels after weaning

• Most humans are lactose intolerant as adults

• Drinking milk causes cramps, diarrhea

• can eat processed milk products like yoghurt & cheese

• But in some populations, adults CAN drink milk

• Controlled by dominant allele, LAC*P

Rate of Lactose Intolerance among Indigenous Peoples

E. Asia 80-100%

S.E. Asia 70-80%

China 70-80%

Greenland 72%

S. America 100%

40%

30-50%

12-20%

18%

20%

38%

61%

N. America50-70%

94%

17%

In most of world, ability to digest lactose associated with dairy production

• In Africa, pastoralists who use fresh milk have higher proportion of lactose tolerance than pastoralists who use fermented milk products

Orma women milk cows

Lactose absorption is also common in Northern Europe, but not because of dairy culture

• Calcium necessary for skeletal maintenance

• Vitamin D necessary to absorb calcium

• Sunlight passes through skin and produces Vitamin D

• In northern latitudes, little sunlight Vitamin D deficits

• Milk is an alternate source of calcium and Vitamin D

• Ability to digest milk may help prevent Vitamin D deficiency

Sami reindeer herder

ABO blood groups

• The proportion of each blood group follows a geographical cline• Type A & B more common where bacterial disease common• Type O more common where viral disease common

http://anthro.palomar.edu/vary/vary_3.htm

Type A

• 10 - 35% of people in most populations

• increased risk of small pox, some cancers

• absent in native Central & South Americans

ABO blood groups• The proportion of each blood group follows a geographical cline• Type A & B more common where bacterial disease common• Type O more common where viral disease common

http://anthro.palomar.edu/vary/vary_3.htm

Type B

• Rarest type

• 20 - 30 % in some Asian populations

• lowest in native populations of Americas & Australia

ABO blood groups• The proportion of each blood group follows a geographical cline• Type A & B more common where bacterial disease common• Type O more common where viral disease common

http://anthro.palomar.edu/vary/vary_3.htm

Type O

• most common type - universal donor

• Esp. in Native populations of the Americas

Skin color varies across the globe

Variation in skin color may reflect adaptations to local environment

• Chimpanzees have light-colored skin, covered by hair

• Pigment on faces and hands darkens with exposure to sun

• When H. ergaster moved out into the savanna, body proportions adapted to hot climate• may have lost hair and increased

sweat glands to keep cool• Like chimps, skin may have

darkened as it was exposed to sunlight

Sunlight is necessary, but potentially harmful

• Sunlight necessary to start production of Vitamin D• Vitamin D is necessary for absorption of calcium• Vitamin D deficiencies cause rickets and other skeletal

disorders

• Sunlight also has harmful effects• UV radiation damages DNA and causes

skin cancer• UV destroys Vitamin B folate

Melanin is a natural sunscreen

• Skin contains melanocytes that synthesize brown melanin pigment

• Melanin prevents and repairs damage from UV rays• Absorbs UV rays and causes them to lose energy• Neutralizes free radicals that form after damage by UV

rays

• Skin color may reflect adaptation to balance beneficial and harmful effects of sunlight

Skin color may be adaptation to preserve B vitamin folate levels

• Often suggested that high concentrations of melanin were favored as adaptation against skin cancer• But, serious forms of skin cancer are uncommon and occur

late in life• Selection against traits that occur after reproductive years

is weak

• Sunlight causes reduction in folate levels• Folate deficits in pregnant women linked to neural tube

defects, like spina bifida• Folate essential for DNA synthesis, new cell production,

and sperm production

• Where sun is strong, dark skin protects folate levels

Skin color is related to exposure to UV

• Map shows exposure to UV radiation (red = high, blue = low)• In Zone 1, people can have dark skin and still produce enough

Vitamin D• In Zone 2, people need lighter skin during some parts of the year

to produce sufficient Vitamin D, and people’s skin color changes seasonally

• In Zone 3, people need light skin year-round to absorb enough Vitamin D, and do not tan

Zone 1

Zone 2

Zone 3

Zone 3

Zone 2

Some exceptions to general pattern are informative

• Inuit are darker than expected based on their location• Migrated to N. America fairly recently,

and/or• Diet very rich in Vitamin D from fish

• Peoples on western side of Red Sea have very dark skin, while people on the eastern side have lighter skin• Peoples on the western side have been there

longer than peoples on the eastern side • Peoples on the eastern side have adapted

culturally (garments that cover bodies, tents for shelter from sun)

Variation within groups Women’s basketball team

Range = 5’2” to 6’7”Mean = 5’10”

Women’s gymnastics teamRange = 5’0” to 5’6”

Mean = 5’2”

Variation between groups

Sources of variation within & among groups are often different

Waters, weeds, fertilizes

Watches TV, studies

Variation within each lawn = geneticVariation between lawns = environmental

Children’s height correlated with Parent’s height

People living in colder climates tend to be bigger

Mean annual temperature

20 40 60 80

50

40

60

70

Weight (kg)

What does this tell us about the variation between groups?

• Much of the variation in height within populations is genetic

• The variation in height between populations is more likely to be adaptive (but not always!)

• You might conclude that variation between groups is due to genetic variation between groups

• This argument is wrong!

Environment can explain differences between populations more often than genetics

• In early 20th century, Japanese migrated to Hawaii to work on sugar plantations

• Migrants were similar to those who stayed in Japan (in height)• The children of immigrants were 2.5% taller than their parents• Rapid change must reflect environmental variation (diet, disease, etc)

Environment affects height differences between groups more strongly than genetics• Compared the height of Mayan children, Mayan-American

children, and white-American children.• Mayan-American children were more similar to white-

American children

B. Bogin & L. Rios 2003

Mayan-American kids

Mayan kids

American kids

What about variation that we associate with different races?• Long history of classification• mostly used for racist purposes

• Linnaeus’ 5 subspecies or races of Homo sapiens (NOT valid)• Afer Ferus• Americanus Europaeus• Asiaticus

The five human racial divisions proposed in Carleton Coon's The Origin of Races (1962)

Unpacking the definition of racism

• Racism = prejudicial belief that members of one ethnic group are superior to those of another

• Assumes that important qualities of an individual: • Intellect• Physical ability• Temperament

are biologically determined by his or her racial group= stereotyping

Assumption 1: People fall naturally into a small number of distinct groups

• Americans tend to classify people into 3 groups• People of European ancestry • People of Asian ancestry• People of African ancestry

• These schemes ignore the complexity of human variation

Classifications based on morphology link together people from Africa, Australia, & New Guinea

Morphology Genes

Africa

New Guinea

Morphology Genes

Similarity based on morphology doesn’t reflect similarity based on genes

But I can see races…However, these discontinuities are an illusion• Suppose you parachute

into Stockholm• Immediately know you

are not in Nairobi

Discontinuities are an illusion

• Suppose you parachute into Stockholm• Immediately know you

are not in Nairobi• So, you’d think races

are real• But imagine you bicycle

from Nairobi to Stockholm• Boundaries would be

impossible to detect

Discontinuities are an illusion

• Suppose you parachute into Stockholm• Immediately know you

are not in Nairobi• So, you’d think races

are real• But imagine you bicycle

from Nairobi to Stockholm• Boundaries would be

impossible to detect

Assumption 2: Differences between races are due to biological heritage

• Implies members of each group are more similar to one another than to members of other groups

• Differences within local human groups account for 85-95% of total variation

• Differences between local human groups account for 5-15% of total variation

• If we wiped out everyone on the planet, except the Inuit, the Aché, and the Efe, we would maintain about 85-95% of the total variation in the human species

Racial classification schemes explain little of the genetic variation among humans

• There is more variation within each racial category than between them

Amerindians, West Africans, & Portuguese in Brazil

• Brazilians define “color” by• Skin color• Hair type• Nose shape• Lip shape

• Brazilians identify many categories of color• (mestiço, mulato, caboclo, cafuzo, moreno, pardo)• based on phenotype not descent • Siblings can be classified differently

• Do phenotypic categories match genotypic traits?

Racial categories are not natural biological entities

• Recall that we argued that biological species are natural units• They have discrete boundaries• There are no (rarely) intermediate cases

• Racial groups are not natural units• They are based on an arbitrary set of traits • There are intermediate cases• Categorizations based on different traits produce different

groupings…lots of overlap between populations• Blood type• Skin color• Eye shape• Body proportions

Assumption 3: Race tells us something important about a person

• Is race important in diagnosis and treatment of disease?

• African Americans higher rates of hypertension than White Americans

• FDA approved specific drug that’s more effective in African Americans

Race is not a meaningful biological category, but racism exists

Brown vs Topeka Board of Education US Supreme Court 1954

“Separate educational facilities are inherently unequal”

Housing discrimination in the USA

USA, 1993Sidanius & Pratto 1999

Retail discrimination in USA

Price differential for Buying New Car, USA 1990’s

Sidanius & Pratto 1999

These kinds of social experiences could contribute to differences in health

• People act on perceptions about race

• These have real consequences for individuals

For example:• Discrimination

• education & job opportunities • Socioeconomic status • dietary options & preferences

stress hypertension

So, we have to deal with a contradiction between the biology and the culture of rac(ism)

• There is more genetic variation within human groups than between them

• Racial categories are based on a small number of superficial traits

• Phenotype is poor measure of ancestry

• Racial and ethnic categories are culturally constructed

• Racism and ethnic prejudice have real consequences on people’s lives