Chapter 1. Chapter 20: Human Evolution

1.1 Introduction

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.

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:

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.

1.3 Driving Question 2:

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.

1.4 Driving Question 3:

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.

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