chapter 20

Chapter 20What is Race?

Introduction

• First Black President, First Black Master’s Winner, First Black Actress to Win Oscar

• To group Barack Obama, Halle Barry, and Tiger Woods into one simple racial category is misleading. Why??? – What does the term “Black” really mean?

• Historically, racial categories were employed primarily by one group to maintain power over another to justify oppression and slavery

US• In US, racial categories were reinforced by

laws such as the “one drop” rule adopted by several states in 1920s– Any American with one drop of African blood was

to be considered black– People still invoke racial categories like “black”

and “white”= simple physical description• Regardless of the reason, it is increasingly

clear from a biological perspective racial categories are meaningless.

All humans are members of a single biological species

• Research on the evolution of humans increasingly shows that race is a social, not biological category.– Groups of people can and do share similar physical

characteristics , such as skin color and other features. • All humans are members of a single biological

species Homo sapiens.• The only thing skin color might accurately identify

is the geographical location where a person’s ancestors lived!!!!!

Race

• Why is there so much variation in human skin tone?

• How did the geographical variation come about?

• Answers lie in the evolution and migration of our earliest ancestors

• Human are recent species, first walking the earth 200,000 years ago.

All humans are members of a single biological species

• The physical differences we see among people today have all emerged in the very recent past. – While the physical differences may appear large,

genetic studies comparing regions of the human genome from person to person show that each person’s DNA is 99.9% identical to any other unrelated person.

• Only 0.1% difference hold clues that help explain how our varying skin tones and other physical traits evolved.

Skin Color

• Many traits helped our ancient ancestor survive the environments they encounters as they migrated the globe. – African ancestry, carry high frequency of an allele

for malarial resistance– North Europeans are able to digest milk better

than other populations. Digest milk for survival– Tibetans have alleles for low oxygen in high

altitude

Adaptations

• Skin color is another example of human evolution• Reason for adaptations in skin color became clearer

following the research by Nina Jablonksi, anthropologist

• She and her husband, George Chaplin (georgraphic specialist) studied the human population and skin color

The evolution of skin color

• Skin tone largely reflects the amount of melanin present in skin―

• A pigment produced by a specific type of skin cell that gives skin its color


• People naturally produce different levels of melanin, resulting in different skin tones.

• Skin also responds to sunlight by producing more melanin and becoming darker temporarily.

• Do you notice a change in your skin tone during different seasons?


• In general, skin tone correlates with geography: people from northern climates tend to be fair and those from areas close to the equator tend to have dark skin.

• Jablonski wanted to know if there was an evolutionary advantage of having light vs dark skin in different environments.


• 1978 study suggest a link with Folate• Folate, a B vitamin also known as folic acid, is an

essential nutrient that is necessary for basic bodily processes such as DNA replication and cell division.

• An hour of intense sunlight can halve the amount of folate in light-skinned people. Low folate levels can cause severe birth defects. – Spina bifida and anencephaly. What are these?– What did she find about tanning beds????– What about men?

The evolution of skin color• Research suggests that

people with light skin are more vulnerable to folate destruction than are darker-skinned people

• Presumably because melanin absorbs and dissipates UV light as heat.

• Evolution to darker skin to protect folate levels


• What about the advantage of lighter skin??– 1960, it was suggested that Vit. D plays a role in the

evolution of light skin color. • Further research suggests vitamin D, a fat-soluble

vitamin required to maintain a healthy immune system and to build healthy bones and teeth, might play a role in the evolution of skin color. – Help absorb calcium and deposit it into bones. – Important for normal pelvis for child bearing

• The production of vitamin D requires UV light.


• When scientists compared data on skin color in indigenous populations from more than 50 countries to levels of global UV light, they found a clear correlation --the weaker the UV light, the fairer the skin – suggesting that both dark and light skin are linked

to levels of global sunlight.


• Light skin evolved because it helped the body produce vitamin D in sun-poor parts of the world, whereas dark skin evolved because it helped protect the body’s folate stores in people who lived in sunny climates.– If skin color developed overtime, then at some point

in history, all humans likely had the same skin tone– Genetic studies suggest anatomically modern

humans first evolved from Africa

Out of Africa: All humans can trace their ancestry back to a single woman

• Mitochondrial DNA (mtDNA) is the DNA in mitochondira.

• Mitochondrial DNA is inherited solely from mothers.

• mtDNA has been used to construct an evolutionary tree of humanity.


• Rebecca Cann and Mark Stoneking• Scientists have determined that all humans

can trace their ancestry back to a single woman who lived in eastern Africa some 200,000 to 150,000 years ago― “mitochondrial Eve.”

Out of Africa

• If every person on the planet constructs a family tree that listed every relative they would all converge at a single common female ancestor and a single common male ancestor-

• Female - Eve


• Mitochondrial Eve was not the only female living at the time, but the lines of descendants of other females living at the time eventually died off.

• Only Eve’s descendants populate the earth today.


• Mitchondrial DNA is DNA located in the mitochondria in all our cells.

• Sperm do not contribute their mitochondria to the newly formed zygote, so mtDNA passes from mothers to offspring essentially unchanged.


• Like nuclear DNA, mtDNA mutates at a fairly regular rate, although it appears to mutate faster than nuclear DNA.

• A mother with a mutation in her mtDNA will pass it to all her children, and her daughters will pass it to their children in turn.


• Because mutations in mtDNA pass down without being combined and rearranged with paternal mtDNA

• mtDNA is a powerful tool by which to track human ancestry back through hundreds of generations.


• Collected mtDNA from 147 individuals• Using mtDNA sequence patterns from

contemporary individuals from Africa, Asia, Australia, Europe, and New Guinea, researchers created an evolutionary tree. – Branches of the tree from all five areas could be

traced back to Eve.


• The tree had two major evolutionary branches: – one that included the ancestors of populations

now living in Asia, Australia, Europe, and New Guinea,

– and one that included the ancestors of modern-day Africans.


• The mtDNA of people on the African branch had acquired twice as many mutations as the mtDNA of people on the rest of the tree.

• The most likely interpretation of these data was that the African mtDNA had had more time to accumulate mutations, and was consequently older, evolutionarily speaking.


• This genetic evidence suggests that humans likely originated in Africa, where they formed several ancestral populations.

• After some period of time, one group of Africans left the continent, and their descendants continued to migrate to other continents, eventually becoming the ancestors of modern-day Asians, Australians, and Europeans.

Wilson’s study

• Additional evidence continues to back the “out of Africa” hypothesis

• Fossil discoveries in Ethiopia in 2003 and 2005 represent the oldest know fossils of modern humans– 160,000 (Herto fossil) and 195,000 (Omo fossils)

• Date back to the time Eve lived in Africa• In 2008, Myers found less and less genetic variation

in people the farther away from African they lived- small group broke away. What effect?

Becoming Human

• Eve represents one branch on the evolutionary tree that includes our species

• The tree has several other branches representing other hominid species that came before her.

Becoming human

• A hominid is any member of the biological family Hominidae, which includes living and extinct humans and apes.

• Humans and apes are grouped together because the fossil evidence shows that modern humans and present-day apes evolved from a common ancestor that lived 13 million years ago.– Humans and chimpanzees are the most closely related

but 6 million years since their common ancestor lived.

Becoming human

• Of the living primates, humans and chimpanzees are the most closely related, although it has been more than 6 million years since their ancestor lived.

• Humans and chimps have undergone tremendous amount of evolutionary change• Why humans look and behave differently than chimps

• October, 2009, fossil remains of 4.4 million year old hominid, Ardipithecus ramidus, Ardi- oldest discovered

• Oldest hominid fossil discovered

Becoming human

• Scientists haven’t yet discovered fossil remains of the last common ancestor between chimps and humans, but the discovery of early hominid fossils gives clues to early human origins.

Becoming human

• Among the defining characteristics of Homo sapiens are the ability to walk upright and a big brain.

• An upright gait meant the hands were free to make and use tools.

• A big brain enabled Homo sapiens to develop complex language.

Defining features of Homo Sapiens

• Walk-upright• Large brain• Complex language• Hands free to make and use tools• Control fire• Large brain

Ardi

• Ardi helped scientists discover that the ability to walk upright evolved first

• Ardi had small brain (could not use complex language)– Ardi could maneuver on all fours in trees, but

could walk upright without dragging its knuckles – Planet of Apes Movie

Fossil Record

• Small tools found to be about 2.6 million years old.

• First tool users genus “Australopithecus”– They walked upright and lost the ability to live in trees– Another milestone was the ability to use fire by Homo

erectus to cook meat and bone marrow, to stay warm, fight off predators

– Between 800,000 and 200,000 years ago, hominid brain size began to expand- Homo sapiens

Selection for Skin Color

• Anatomically modern humans evolved in Africa suggests that the first humans likely had dark skin– How might varying skin tones later developed?

• Environmental factors played a role in the evolution of different skin colors

• New alleles due to mutations

Selection for skin color• The environment alone doesn’t cause

evolution. • The environment acts on traits, or

phenotypes, increasing or decreasing the frequency of alleles in a population by natural selection.

Selection for skin color

• Each time a cell replicates, mutations can occur.

• If these mutations occur in germ cells during meiosis, they will permanently change the genome of the next generation.

• This process continually introduces new alleles into the population.

Selection for skin color

• Sometimes alleles that are harmful in one environmental context may be beneficial in another.

• Different environments will select for different traits, and therefore different alleles.

• Survival advantage– Cystic fibrosis (heterozygous) beneficial for diarrhea

(cholera)– Homozygous serious pulmonary disease

Selection for skin color• Varying levels of UV light have selected for a

range of skin tones around the globe.

chapter 20

Documents

dark skin

human skin tone

thing skin color

evolution of skin colorpeople

skin tone correlates

different skin tones

skin colormany traits

varying skin tones