CHAPTER 20 Test Your Knowledge
Driving Question 1
What contributes to human skin color, and why is there so much variation in skin color between different populations?
By answering the questions below and studying Infographics 20.1, 20.2, 20.3, 20.4, 20.9, and 20.10, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
What pigment molecule gives dark skin its color? What cell type produces this pigment?
melanin, which is produced by melanocytes.
In the course of human evolution, which of the following environmental factors likely influenced whether populations had mostly light-skinned individuals or mostly dark-skinned individuals?
a. average annual temperature
b. average annual rainfall
c. levels of UV light
d. the vitamin D content of the typical diet
e. mitochondrial DNA inheritance
c
As hypothesized by Jablonski and Chaplin, darker skin is advantageous in________________UV environments because darker skin_____________.
a. high-; reduces vitamin D production
b. high-; protects folate from degradation
c. high-; increases the rate of folate synthesis
d. low-; allows more vitamin D to be produced
e. low-; allows more folate to be produced
b
USE IT
If folate were not destroyed by UV radiation, predict the skin color you might find in populations living at the equator; in populations living in Greenland. Explain your answers.
Normally, folate is destroyed by UV exposure, and dark skin protects folate from destruction in high-UV environments. If folate were not destroyed by UV exposure, there would not be selection for dark skin to protect folate in populations at the equator, and thus skin color would be predicted to be lighter than it is in those populations. In Greenland, levels of UV are low enough that folate is destroyed, so skin is light. In this population, there would be no change from the current skin color if folate was not destroyed by UV.
Which of the following would help darker-skinned people who live in low-UV environments remain healthy?
a. folate supplementation
b. sunscreen
c. increased production of melanin
d. vitamin D supplementation
e. calcium supplements
d
What can you infer about the skin-color genotype and the geographic origins of the ancestors of a light-skinned person and a dark-skinned person?
A light-skinned person likely has alleles associated with lighter skin (e.g., variants in the enzymes that produce melanin or in proteins that determine the melanin-producing activity of melanocytes), and their ancestors likely originated in northern latitudes with low levels of UV. People with dark skin likely have alleles associated with darker skin (e.g., variants in the enzymes that produce melanin or in proteins that determine the melanin-producing activity of melanocytes), and their ancestors likely originated in a high UV environment near the equator.
Our closest primate relatives, chimpanzees, have light-colored skin yet live in tropical (high-UV) environments. How would the Jablonski-Chaplin hypothesis explain this observation?
a. Chimpanzees don’t need folate for successful reproduction.
b. Chimpanzees are not susceptible to skin cancer.
c. The hair of chimpanzees protects their light skin from UV light.
d. Chimpanzees require much higher levels of vitamin D than humans do.
e. In chimpanzees a light-colored pigment offers UV protection.
c
Vitiligo is a disease in which melanocytes are destroyed, with resulting loss of pigmentation. If a dark-skinned person develops vitiligo and therefore lighter-colored skin, would his or her race change? What factors have led people to classify (or misclassify) themselves or others as members of one race or another?
Biologically, people with vitiligo still has the same skin color genotype and the same ancestors as unaffected people in the population. That is, they have the same evolutionary history and genotype, even after the disease causes their skin to lighten. However, they may be perceived by others to be of a different race on the basis of their skin color. Society tends to determine race on the basis of skin color, as well as on geography and culture.
Driving Question 2
Where did the earliest humans evolve, and how do we know?
By answering the questions below and studying Infographics 20.5, 20.6, 20.7, and 20.8, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
What percentage of DNA sequences do all humans share?
a. 0%
b. 25%
c. 50%
d. 75%
e. >99%
e
Why is mtDNA a useful tool in the study of human evolution? (Think about how mitochondrial DNA is inherited.)
Mitochondrial DNA is inherited only from the mother and does not undergo recombination. Thus, it is transmitted intact from mothers to all their children and so can be used to trace a lineage. As mitochondrial DNA mutates at a constant rate, it is possible to use it as a molecular clock to estimate the time since different lineages diverged.
According to the “out of Africa” hypothesis of human origins and migration, which group of people should show the highest level of genetic diversity?
a. Africans
b. Europeans
c. Asians
d. South Americans
e. Australians
a
447
USE IT
Rank the levels of genetic diversity you would expect to find within the five populations listed in Question 11 from highest to lowest. Justify your ranking.
See Infographic 20.7. Africans have the highest level of genetic diversity. On the basis of migration patterns and dates, Asians would be predicted to have the next highest levels (having an early date of 57,000 years ago), then Australians (40,000–60,000 years ago), then Europeans (40,000 years ago). South Americans would be predicted to have the lowest levels of genetic diversity, on the basis of migration to that region 13,000 years ago.
If there were many human females living ~200,000 years ago, why do we find that the mitochondrial DNA in all living humans is all related to a single woman from that time?
What kind of evidence could you look for to test your explanation? (Think about all the human fossils that have been uncovered, and consider that it is possible to extract DNA from fossils.)
While there were many women alive 200,000 years ago, the mitochondrial lineages of all but one ended, either because the others did not have children at all, or because at some point there were only sons, who do not pass on their mitochondrial DNA to the next generation. Thus, between 200,000 years ago and now, there were presumably multiple mitochondrial DNA lineages, but only one continued until the present day. If ancient fossils had intact mitochondrial DNA that could be isolated and tested, we would expect to find mitochondrial DNA from lineages that are no longer present in the human population.
MINI CASE
A mother with medium skin tone gives birth to a baby with darker skin than she has. Her lighter-skinned husband accuses her of infidelity.
a. How reasonable is this, given the genetics of skin color? (Hint: Refer to Infographic 12.9.)
b. Could an mtDNA analysis be used in a paternity test? Why or why not?
a: As skin color is determined by many genes, each with at least two alleles, its inheritance does not follow a simple dominant or recessive inheritance pattern. It is possible for a child to have darker skin than either of its parents, depending on which combination of alleles it inherits. As a hypothetical example, assume that dominant alleles contribute to darker skin. A mother with medium skin tone could have a genotype of: Aa Bb cc Dd ee (three dominant alleles) and her lighter-skinned husband could have a genotype of aa bb Cc dd Ee (two dominant alleles). It is this possible that the child would have darker skin, given a possible genotype of Aa Bb Cc Dd Ee (five dominant alleles). b: No. An mtDNA analysis would not be useful in a paternity test, as the father does not contribute mtDNA to the child. An mtDNA test would confirm only maternity.
Driving Question 3
What we can learn about human evolution from the fossil record?
By answering the questions below and studying Infographics 20.7 and 20.8, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
Of the following traits that are associated with being human, which evolved most recently?
a. upright walking
b. ability to control fire
c. social communication
d. tool use
e. big brain
e
Place the following ancestors in order of most ancient (1) to most recent (5).
______________Homo sapiens
_______________Last common ancestor of chimpanzees and humans
________________Australopithecus
______________Ardipithecus ramidus
__________________Homo erectus
(1) last common ancestor of chimpanzees and humans; (2) Ardipithecus ramidus; (3) Australopithecus; (4) Homo erectus; (5) Homo sapiens
USE IT
Where would the last common ancestor of gorillas and humans fit into the ordering in your answer to Question 16? Explain.
The last common ancestor of gorillas and humans would be even more ancient than the last common ancestor of chimpanzees and humans. Chimpanzees and humans are the most closely related of the living primates.
Why would individual Australopithicines who could make and use tools have had a selective advantage (that is, higher fitness) over individuals who could not make or use tools?
Tool use is important for hunting and preparing food. Those who could use tools would presumably have better nutrition and be less likely to starve. Better nutrition would contribute to higher fitness, especially when food was not easily available.
Ardi was partially arboreal (that is, the species could live in trees). The ability to move around in trees was facilitated by an opposable big toe that would help grip branches. Once ancient hominids moved permanently to a grounded lifestyle, would there have been any selective pressure to maintain an opposable big toe? Explain your answer.
If an opposable big toe conferred advantage on the ground, it would have been maintained by natural selection. As the descendants of these ancient hominids do not have an opposable big toe, there must not have been a selective advantage to maintain it in grounded populations.
Members of the genus Australopithecus walked upright, and their fossilized footprints show no evidence of an opposable big toe.
a. What foot structure and lifestyle might have been selected if early hominid evolution occurred in a forested environment? In a grasslands environment? Would you predict any differences because of the selective pressures in each environment? Why or why not?
b. What other traits would you expect to be favored in a forested environment? In open grasslands?
a: If early hominid evolution occurred in a forested environment, foot structure might still show evidence of an opposable big toe, to facilitate tree climbing and moving along branches, particularly if tree climbing provided access to food. In a grasslands environment, an opposable big toe would not be useful, as there are no trees to climb. Instead, the foot, leg, and pelvis would be adapted for walking and running. b: In a forested environment it would be helpful to have good vision in low-light conditions, a good sense of balance (for tree climbing), upper body strength (for tree climbing), and perhaps height (to see over the underbrush and saplings). In a grasslands environment, darker skin coloration or hair would be important to avoid excessive UV degradation of folate (in an environment with limited access to shade). Speed (in running) would be an advantage (to avoid predators and chase prey). Good distance vision would be useful, to see out across the landscape (to detect predators or prey).
INTERPRETING DATA
An extensive study of a hominid fossil dating from approximately 2 million years ago was published in 2013. For each of the features described below, consider whether they are closer to an ancestral state or closer to modern humans. On the basis of the features described, where would you place this fossil on the lineage between the chimpanzee–human ancestor and modern humans—what genus is it likely a member of?
The shoulder structure and very long arms suggest the ability to climb and perhaps hang or swing.
The spine and other skeletal features suggest an upright stance.
There is no opposable toe.
The heel is very narrow and pointed (not flat and wide).
The skeleton suggests that the gait would have been rolling, the feet rolling inward with each step.
The skull is very small.
The chest is not cylindrical but wider at the base and narrow at the shoulders, much like a triangle.
The shoulder structure appears to be more ancestral; the upright stance is closer to modern humans; the absence of an opposable big toe is closer to modern humans; the narrow and pointed heel is more ancestral; walking upright is more modern, but the rolling gait is not a modern gait; the small skull is ancestral; the triangular chest (wide at the base and narrow at the shoulders) is ancestral. The skeleton has a mixture of features. Overall, as an upright walker with arms that could still hang/swing, and with the small skull, and absence of an opposable big toe, this is most likely the skeleton of a member of the genus Australopithecus.
BRING IT HOME
The U.S. Census Bureau provides information on classifying race (http://www.census.gov/population/race/about/). What races does the U.S. Census Bureau recognize? What about people of mixed race? What about people who identify themselves as Hispanic? How easy is it for you to identify yourself with respect to race given the racial categories on the U.S. Census?
Answers will vary.