Chapter 1. Chapter 20: Human Evolution

1.1 Introduction

Interactive Study Guide
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Polaris Trail

Welcome to the Interactive Study Guide for Chapter 20: Human Evolution! This Study Guide will help you master your understanding of the chapter's Driving Questions, using interactive Infographics and activities, as well as targeted assessment questions. Click "Next" to get started, or select a Driving Question from the drop-down menu to the right.

Skin Deep:

Science redefines the meaning of racial categories

DRIVING QUESTIONS

  • What contributes to human skin color and why is there so much variation in skin color among different populations?
  • Where did the earliest humans evolve and how do we know?
  • What can we learn about human evolution from the fossil record?

1.2 Driving Question 1:

Driving Question 1

What contributes to human skin color and why is there so much variation in skin color among different populations?

Why should you care?

Human skin color has been used for centuries as a way to separate people into groups, as a way to make value judgments about both individuals and groups, and as a basis for stereotyping and discrimination. Understanding the evolutionary significance of skin color allows us to see that a given skin color can’t be “good” or “bad.” Instead, skin color is a trait that is subject to natural selection; the colors we see all have conferred selective advantages in some place at some time.

Is your skin light or dark? Freckled or not? Do you tan easily or burn at the slightest exposure to the sun? These skin characteristics are a function of the type, amount, and distribution of melanin in your skin. Melanin is the pigment molecule found in the skin and hair of all mammals. It comes in two forms, one that produces brown-black colors and one that produces red and yellow colors. Variation in the kind, amount, and distribution of these molecules is responsible for variation in human skin color and tone around the world.

Adequate intake of folate (vitamin B9) and vitamin D is a crucial component of prenatal nutrition because both vitamins play critical roles in fetal development and maternal health. Levels of folate and vitamin D are affected by both skin pigmentation and sunlight. Selection for the appropriate balance of these two nutrients in different environments explains much of the variation in human skin color around the world.

Understanding the selection pressures that have been part of our past and that are with us now can help us understand simple physical traits like skin color and can also help us understand why genetic diseases such as cystic fibrosis and sickle-cell disease persist.

What should you know?

To fully answer this Driving Question, you should be able to:

  1. Explain how melanin influences skin color.
  2. Describe the types of birth defects related to folate deficiency and vitamin D deficiency.
  3. Illustrate and describe the relationship between sunlight (UVB rays), skin color, and folate levels in the body.
  4. Illustrate and describe the relationship between sunlight, skin color, and vitamin D levels in the body.
  5. Infer and discuss the combinations of sunlight and skin color that would maintain adequate levels of both folate and Vitamin D in the body.
  6. Describe the general correlation between skin color and geography (specifically distance from the equator).
  7. Compare and contrast positive, negative, and neutral selection.
  8. Diagram and describe the broad pattern of human skin color evolution using the folate–vitamin D hypothesis to explain selection acting on this trait.

Infographic Focus

The Infographics most pertinent to the Driving Question are 20.1 to 20.4, 20.9, and 20.10.

Explain how melanin influences skin color.

Question 1.1

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No, there are two kinds of melanin and people produce varying levels of the pigment.

Question 1.2

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Yes, skin responds to sunlight by producing more melanin (like when you get a tan when you are outside in the sun).

Describe the types of birth defects related to folate deficiency and Vitamin D deficiency.

Question 1.3

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Folate is necessary for DNA replication and cell division. It is most critical during periods of rapid cell division like embryonic and fetal developemtn. Thus two types of birth defects related to folate deficiency are spina bifida (where the spinal column does not close around the spinal cord before birth) and anencephaly (where the brain and skull are highly underdeveloped and is fatal).

Question 1.4

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Vitamin D helps the body absorb calcium and deposit it in bones. As such, a deficiency in Vitamin D can lead to reduced bone density in the pelvis of pregnant women and Rickets disease (which is the formation of abnormal bone) in children.

Illustrate and describe the relationship between sunlight (UVB rays), skin color, and folate levels in the body.

Question 1.5

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Strong Sun + Light Skin = ↓ Folate Levels
Strong Sun + Dark Skin = No change in Folate Levels
Faint Sun + Light Skin = No change in Folate Levels
Faint Sun + Dark Skin = No change in Folate Levels

Question 1.6

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UV light halves the body’s level of folate. Dark skin protects the body from the damaging UV light in strong sunlight while light skin is susceptible to folate loss in the same conditions.

Illustrate and describe the relationship between sunlight, skin color, and Vitamin D levels in the body.

Question 1.7

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Strong Sun + Light Skin = ↑ Vitamin D Levels
Strong Sun + Dark Skin = ↑ Vitamin D Levels
Faint Sun + Light Skin = ↑ Vitamin D Levels
Faint Sun + Dark Skin = ↓ levels of Vitamin D

Question 1.8

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Sunlight causes the skin to produce Vitamin D, so if a person does not have a lot of melanin (which blocks UV light), in strong or faint sunlight they would be able to produce of Vitamin D. People with more melanin, darker skin, would have a hard time producing Vitamin D in faint sun because not enough UV light is getting through to cause Vitamin D production, so these people may have a deficiency.

Infer and discuss the combination(s) of sunlight and skin color that would maintain adequate levels of both folate and Vitamin D in the body.

Question 1.9

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Strong Sun + Light Skin = ↓ Folate Levels
Strong Sun + Dark Skin = No change in Folate Levels
Faint Sun + Light Skin = No change in Folate Levels
Faint Sun + Dark Skin = No change in Folate Levels

Strong Sun + Light Skin = ↑ Vitamin D Levels
Strong Sun + Dark Skin = ↑ Vitamin D Levels
Faint Sun + Light Skin = ↑ Vitamin D Levels
Faint Sun + Dark Skin = ↓ levels of Vitamin D

Based on the predictions of the illustrations above, the best combinations of skin color and sunlight levels that provide the best balance of folate and Vitamin D are:

Faint Sun + Light Skin = No change in Folate Levels + ↑ Vitamin D Levels
Strong Sun + Dark Skin = No change in Folate Levels + ↑ Vitamin D Levels

Yes, these combinations are found in modern human populations.

Describe the general correlation between skin color and geography (specifically distance north and south of the equator).

Question 1.10

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Skin color generally gets lighter as you move from the equator to the poles. Because of the tilt of the Earth, the areas around the equator receive the most sunlight each year, while the areas towards the poles receive less. It would make sense that since people who live around the equator are exposed to more UV light, they would have darker skin, while people who live around the poles and are exposed to less UV light would have lighter skin.

Compare and contrast positive, negative, and “neutral” selection.

Question 1.11

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Neutral selection for a trait means that it does not influence reproductive success in a particular environment.

Question 1.12

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They both affect the reproductive success of a population and they both change the frequencies of alleles in the populations.

Question 1.13

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Positive selection increases the ability of individuals with the trait to have reproductive success, thus increasing the frequency of that allele in the population. Negative selection decreases the ability of individuals with the trait to have reproductive success, thus decreasing the frequency of that allele in the population.

Question 1.14

Use examples from the text to give a realistic scenario, based on natural selection, for each of the following:

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Dark skin becomes more common in a population: The population is likely exposed to high levels of UV light, causing individuals with darker skin to lose less folate, have offspring with fewer birth defects and thus have better reproductive success.

Dark skin becomes less common in a population: The population is likely exposed to low levels of UV light, causing individuals with lighter skin to absorb more UV light and produce more Vitamin D, have offspring with fewer birth defects and thus have better reproductive success.

Light skin becomes more common in a population: The population is likely exposed to low levels of UV light, causing individuals with lighter skin to absorb more UV light and produce more Vitamin D, have offspring with fewer birth defects and thus have better reproductive success.

Light skin becomes less common in a population: The population is likely exposed to high levels of UV light, causing individuals with darker skin to lose less folate, have offspring with fewer birth defects and thus have better reproductive success.

Diagram and describe the broad pattern of human skin color evolution using the folate/Vitamin D hypothesis to explain selection acting on this trait.

Question 1.15

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It is thought that humans originated in Africa. Our earliest ancestors likely had hair covering their entire body and thus probably had light skin (because the hair was efficient for blocking excess UV light). As our ancestors began to move around more and expend more energy, they lost their hair covering because it was too hot. This left their skin exposed to the sunlight of Africa. Since darker skin would protect individuals from folate loss and give them a reproductive advantage, dark skin became more common in the population.

Question 1.16

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As our ancestor migrated away from the equator, the amount of UV light they were exposed to decreased. People with darker skin were not able to obtain enough UV light to produce needed levels of Vitamin D, which resulted in both birth defects and maternal issues, thus they were at a reproductive disadvantage. People with lighter skin, however, were able to absorb enough UV light to produce sufficient levels of Vitamin D and, since the UV light was weaker, they did not experience folate loss. This put them at a reproductive advantage and thus lighter skin became more common in the population.

Question 1.17

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Yes, as populations of humans migrated to different parts of the world and then migrated again, the frequency of alleles was ever changing in the populations. Going from dark to light skin color for example was possible through either new mutations in DNA that created a light skin allele, or the original light skin allele (that was still present in the population) was selected for once again. This likely happened many times, and is still happening today as humans are constantly evolving.

Thought Question: In modern society, people of all skin colors live on every continent, generally without folate or Vitamin D problems. Why do you think this is?

Review Questions

Question 1.18

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2
Try again.
Correct.
Incorrect.

Question 1.19

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2
Try again.
Correct.
Incorrect.

Question 1.20

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2
Try again.
Correct.
Incorrect.

1.3 Driving Question 2:

Driving Question 2

Where did the earliest humans evolve and how do we know?

Why should you care?

Scientists have been trying to understand human evolutionary history at least since the time of Darwin. One of the many lively debates in the field has been about when and how often anatomically modern humans (early Homo sapiens) migrated out of Africa to colonize the rest of the globe. The Mitochondrial Eve study was one piece of evidence supporting the hypothesis that all human populations share a single African origin and that all non-African populations arose from a single migration 150,000 to 200,000 years ago. The evidence comes from a study of mitochondrial DNA (mtDNA), which is maternally inherited. Mitochondrial DNA is a powerful tool in studying human evolution, not only because it is maternally inherited, but also because its relatively small genome has a relatively high mutation rate compared to nuclear DNA. Using samples from 147 people from around the world, investigators constructed a family tree of mtDNA; that tree converged on an African ancestor—more specifically, the most recentwoman from whom we have all inherited our mtDNA. Then investigators calculated the amount of variation in the mtDNA. By comparing it to known rates of change, they were able to estimate when that ancestor lived.

What should you know?

To fully answer this Driving Question, you should be able to:

  1. Explain the mechanism behind maternal inheritance of mtDNA.
  2. Compare and contrast nuclear DNA ancestry with mtDNA ancestry.
  3. Explain who Mitochondrial Eve was and why she is significant to understanding human evolutionary history.

Infographic Focus

The Infographics most pertinent to the Driving Question are 20.5 to 20.8.

Explain the mechanism behind maternal inheritance of mtDNA.

Question 1.21

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Yes, both eggs and sperm have mitochondria.

Question 1.22

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Because it is the mitochondria from the egg cell that passes to the next generation. The mitochondria from the sperm cells are not passed on to the newly formed zygote.

Compare and contrast “nuclear DNA ancestry” with “mtDNA ancestry.”

Question 1.23

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Question 1.24

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Eight ancestors

Question 1.25

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Three ancestors

Question 1.26

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Yes

Question 1.27

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You inherit your nuclear DNA from all ancestors both male and female. This is different from the inheritance pattern for mtDNA where you inherit your mtDNA from your mother, who inherited hers from her mother (your grandmother), who in turn inherited hers from her mother (your great-grandmother). So only the female ancestors on your mother’s side would contribute to your mtDNA ancestry.

Explain who Mitochondrial Eve was and why she is significant to understanding human evolutionary history.

Question 1.28

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Since we only inherit our mitochondrial DNA from our mothers, and that DNA undergoes a specific rate of mutation, we can trace back our maternal lineage all the way to our first common female ancestor, Mitochondrial Eve.

Question 1.29

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It supported the idea that humans originated in Africa, are all related and that one group migrated out of Africa and eventually spread to all parts of the globe.

Review Questions

Question 1.30

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Question 1.31

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Question 1.32

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1.4 Driving Question 3:

Driving Question 3

What can we learn about human evolution from the fossil record?

Why should you care?

One of the most common misconceptions about human evolution is that we are the product of a single straight branch of evolution rather than the sole remaining twig of an evolutionary tree that is at least 5 million years old. The more fossils we discover and study, the more we are learning about the sequence of evolutionary events that led to the suite of characteristics that define us as a species.

For years, scientists have been using a number of tools, including studies of mtDNA and fossils, to clarify when and how humans left Africa and colonized the rest of the globe. Our understanding of these patterns in turn can be used to help us understand patterns of natural selection and cultural evolution, including the evolution of language.

What should you know?

To fully answer this Driving Question, you should be able to:

  1. Describe the migration of humans out of Africa and use the pattern of dispersal to answer questions about human evolution.
  2. Interpret a human phylogeny to answer questions about human evolution.

Infographic Focus

The infographics most pertinent to the Driving Question are 20.7 and 20.8.

Question Test Your Vocabulary

Choose the correct term for each of the following definitions:

Term Definition
/mnCHrXqohtTKH8hBytdlRoBSxJ+m84KJlufGSbGYxC5kO24w8xQAysV8ozcp+ImBM4FenWuwFaO/P5ZI7NDng== A fat-soluble vitamin required to maintain a healthy immune system and to build healthy bones and teeth. The human body produces vitamin D when skin is exposed to ultraviolet light.
CP/iA8GpOXx8KJ2EBxuCe/XKT9A6g1FmkJBe8E/CsYxMgyIs/15MrIQ2gywMSJDBx78bn8GVNm8GoGgxGEM9HA== Pigment produced by a specific type of skin cell that gives skin its color.
MpTE6EWd1eXQdWoa+Xcuz6QYPifQVZs/Q3+rmLUW1aW1pLwze+I7irxiBLfCWQ+Qjw2tQ/mg8KZVZpO10+lBsg== Any living or extinct member of the family Hominidae, the great apes— humans, orangutans, gorillas, chimpanzees, and bonobos.
GVFlUpYVb4KRjq3zuk9D/kPc4O0MdG5Od9vQoiHVig0GybYJPO+F+nC2io2XY0U8KZZbp+hcE+rOYHZOzEgT6w== A B vitamin also known as folic acid. Folate is an essential nutrient, necessary for basic bodily processes such as DNA replication and cell division.
EFoLTdf9tTjtW4ojA9goQKzCwZxqx40WA+DtddVtq74CWUsdN9ZJpBC39TPBFubNkLEshOgUcrKr1EsJfDH79g== The DNA in mitochondria that is inherited solely from mothers.
Table
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Describe the migration of humans out of Africa and use the pattern of dispersal to answer questions about human evolution.

Question 1.33

Approximately when did humans reach each of the following regions, based on the information in Infographic 20.7?

Far eastern Asia: WYCxiN3Q6tJCdU6I
Western Europe: WYCxiN3Q6tJCdU6I
North America: WYCxiN3Q6tJCdU6I
Australia: WYCxiN3Q6tJCdU6I

Far eastern Asia: 67,000 years ago
Western Europe: 40,000 years ago
North America: 20,000 years ago
Australia: 60,000 - 40,000 years ago

Question 1.34

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Africa, because humans originated there and thus that population has had more time to accumulate mutations in their DNA.

Question 1.35

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South America populations would have the least genetic variation because they are the most recent population of humans (migrated about 13,000 years ago) and have thus not had a lot of time to accumulate mutations. The population with the most genetic variation is in Africa because that population has had around 200,000 years to accumulate mutations.

Interpret a human phylogeny to answer questions about human evolution.

Question 1.36

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About 4.4 million years ago. The fossil’s name is Ardipithecus ramidus (Ardi).

Question 1.37

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4 branches. Three differed from Homo habilis by walking and running on two feet, increasing hunting and migration, having less body hair and having dark skin. The remaining three members differed in brain size and complex language and problem solving ability.

Question 1.38

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We likely had the greatest diversity of hominids in Africa about 2 – 3 million years ago. This time period saw species that could walk upright and could use simple tools but were not yet migrating out of Africa.

Question 1.39

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No, not necessarily. Since modern day humans and Neanderthals share a recent common ancestor, about 1.5 million years ago, it is likely that we still harbor DNA that was in common with Neanderthal DNA.

Review Questions

Question 1.40

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Question 1.41

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Question 1.42

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