Chapter 24 Learning Curve

What continent was the starting point for the origin of humans?

Africa

Which of the following statements reflects why cultural evolution is more rapid than biological evolution?

Culture can be transmitted through multiple, unrelated individuals at the same time. Biological evolution occurs through traits passed only from parent to offspring.

FOXP2 is the name of a gene that may be important to language, as mutations in this gene are implicated in many speech pathologies. Why does FOXP2 have such a broad impact on the development of speech?

FOXP2 is a gene encoding a transcription factor that controls the expression of many genes involved in the development of speech.

African Origins

From the wealth of fossil evidence, it's clear that modern humans first evolved in Africa. But for a while the timing of this event was unclear. Can all modern humans, Homo sapiens, date their ancestry to early hominins in Africa that migrated out and spread around the world about 2 million years ago? Or did the groups that left Africa ultimately go extinct, so that modern humans evolved from a group of hominins in Africa that migrated out much more recently? Molecular studies helped to answer this question. Page 491

Which of the following statements about great apes is correct?

Gorillas are more closely related to humans than to orangutans.

Humans are included in which of the following groups?

Great apes

Of the taxa listed below, which of the following nonmodern humans was the last to go extinct?

Homo floresiensis

There is a phenomenon called parental leakage, in which a small amount of the mitochondrial DNA from the father is found in the offspring. If parental leakage is assumed to occur in most matings, how would it change our interpretation of studies of mitochondrial DNA in human ancestors and in groups of different modern humans?

If it is truly a small amount, then those deviations shouldn't influence the conclusions generated from the study.

There is a phenomenon called parental leakage, in which a small amount of the mitochondrial DNA (mtDNA) from the father is found in the offspring. If parental leakage is assumed to have occurred throughout human history, how does it change our interpretation of Matthias Krings and Svante Pääbo's studies of mitochondrial DNA in specimens of Neanderthals and modern humans?

If paternal mtDNA with Neanderthal sequences did show up in the offspring alongside maternal mtDNA, these would have immediately indicated that the two groups interbred.

The mitochondrial DNA study indicated that there was no genetic input from Neanderthals in modern humans, but the whole-genome study suggests that in fact there was. One suggestion to explain this discrepancy is that the Neanderthal input into modern human populations was from males only. Which of the following strategies could be employed to test this hypothesis?

Look at the Y chromosome because it is paternally inherited.

Distinct Features of Our Species

Many extraordinary changes in anatomy and behavior occurred in the 5-7 million years since our lineage split from the lineage that gave rise to the chimpanzees. Fossils tell us a great deal about those changes, especially when high-quality material such as Lucy or Ardi is available, but in general this is an area in which fossils are hard to come by and there is a lot of speculation. Speculation is especially common when we try to explain the reasons behind the evolution of a particular trait. Why, for example, did language evolve? It is easy enough to think of a scenario in which natural selection favors some ability to communicate—maybe language arose to facilitate group hunting. There are plenty of plausible ideas on the subject, but, in most cases, no evidence, so it is impossible to distinguish among competing hypotheses. We can, however, be confident that the events that produced language occurred in Africa, and, through paleontological studies of past environments, we can conclude that humans evolved in an environment similar in many ways to today's East African savanna. Page 495

There are seemingly conflicting studies that have taken a look at whether Neanderthals interbred with human ancestors. One study of mitochondrial DNA suggests that the two groups did not interbreed. Another study, of the whole genome, suggests that they did. Which of the following could explain these results?

Mitochondrial DNA is maternally inherited, so if Neanderthal females did not interbreed with our ancestors, Neanderthal mitochondrial DNA would never have entered the modern human gene pool.

Which of the following statements is true regarding cultural change? Cultural change occurs at the same place or rate as biological evolution. Cultural change always occurs independently of biological evolution. Only human populations experience cultural change.

None of the answer options is correct.

Which of the following characteristics separates primates from other mammals?

Primates have nails instead of claws.

Which of the following explains why the human population has so many phenotypic differences despite relatively low numbers of nucleotide differences from individual to individual?

Sexual reproduction allows for new genetic combinations of phenotypic variants.

Human Genetic Variation

So far, we have treated humans as all alike, and in many ways, of course, we are. But there are also many differences from one person to the next. Those differences ultimately have two sources: genetic variation and differences in environment (Chapter 18). A person may be born with dark skin, or a person born with pale skin may acquire darker skin—a tan—in response to exposure to sun. The differences we see from one person to the next are deceptive. Despite appearances, ours is not the most genetically variable species on Earth. While it is certainly true that everyone alive today (except for identical twins) is genetically unique, our species is actually rather low in overall amounts of genetic variation. Modern estimates based on comparisons of many human DNA sequences indicate that, on average, about 1 in every 1000 base pairs differs among individuals (that is, our level of DNA variation is 0.1%). That's about 10 times less genetic variation than in fruit flies (which nevertheless all look the same to us) and about two to three times less than in Adélie penguins, which look strikingly similar to one another (see Fig. 21.1). Why, then, are we all so phenotypically different if there is so little genetic variation in our species? Given the large size of our genome, a level of variation of 0.1% translates into a great many genetic differences. Our genome consists of approximately 3 billion base pairs, so 0.1% variation means that 3 million bp differ between any two people chosen at random. Many of those differences are in noncoding DNA, but some fall in regions of DNA that encode proteins and therefore influence the phenotype. When those mutations are reshuffled by recombination, we get the vast array of genetic combinations present in the human population. Page 499

Which of the following is the most likely explanation for why the human brain is capable of so much more than what would be required for evolutionary fitness?

The additional capabilities are by-products of what was necessary for evolutionary fitness.

Which of the following explains how chimpanzees and humans can be so different with such similar genomes?

The genetic differences could have occurred in genes responsible for development.

Why is it that there is less variation among individuals in any one non-African population than among individuals in the contemporary African population?

The individuals that left Africa were a relatively small sample of the total amount of variation present.

The first DNA comparison studies between humans and chimpanzees relied on which of the following?

The more complementary two strands of DNA are, the higher the melting point.

Culture, Language, and Consciousness

The most remarkable outcome of the evolutionary process described in this chapter is the human brain. This allows us to do extraordinary things, like appreciate Bach's music, read books, and build skyscrapers. But does this wonderful brain make us qualitatively different from other organisms? Does it in some way take us out of nature? Traditionally, the answer to these questions would have been a resounding yes. However, research into the capabilities of other species is questioning this conclusion: The human brain is certainly remarkable, but, in essence, what we can do is merely an extension of what other animals can do. Page 502

There is some debate about whether Homo floresiensis is a separate species or an aberrant form of Homo sapiens. Regardless, what we do know is that H. floresiensis were only found on an Indonesian island and adults were only 3 feet tall. What factors would have selected for this distinct phenotype on an island habitat, specifically?

The smaller body size was advantageous for living on an island, as food is often limited on islands.

"Hominins" refers to members of all the different species in the lineage leading to humans.

True

Consider the following hypothetical hominin lineage: Homo ergaster → Homo heidelbergensis → Homo sapiens. This is currently the accepted hypothesis because H. ergaster evolved into H. heidelbergensis.

True

Phenotypic differences between humans and chimpanzees may be explained by the time during development when genes are turned on in humans and chimpanzees.

True

Population genetic studies of humans show a significant amount of variation across all humans.

True

Sometimes cultural change can drive biological evolution.

True

The Great Apes

We can approach the question of our place in the tree of life in three different ways: through comparative anatomy, through molecular analysis, and through the fossil record. In this section, we use data from all three sources as we apply the standard methods of phylogenetic reconstruction (Chapter 23) to figure out the evolutionary relationships between humans and other mammals. Page 486

When Rebecca Cann decided to conduct research on the evolution of humans, she chose to study mitochondrial DNA. Which of the following explains why she chose this type of DNA?

because mitochondrial DNA is more abundant and therefore easier to extract

A young student is trying to figure out the order in which certain traits evolved in humans. Which of the arrangements below is most likely correct?

bipedalism → more dexterous thumbs → human jaw → language

A high degree of genetic variability among individuals of the same species means that these individuals will always appear morphologically different. For example, as all Adelie penguins look alike, this species must have a low degree of genetic variation.

false

All of the genetic differences between humans and chimpanzees are responsible for making us human.

false

The hemoglobin gene is the only gene that is under selection for resistance to malaria.

false

Tool use is unique to humans.

false

The commonly heard statement, "Humans evolved from chimpanzees" is inaccurate. In reality, the evolutionary relationship between chimpanzees and humans is that:

humans and chimpanzees share a relatively recent common ancestor.

Which of the following, if true, would force us to reevaluate the findings that Cann discovered with regard to the human family tree?

if early human material was discovered in Asia that had DNA completely different from the DNA found in early Homo sapiens in Africa

You are working in a lab and come across a gene, gene X, that you think might explain some of the differences seen between chimpanzees and humans. Which of the following results would support your hypothesis?

if gene X was active in humans, juvenile chimpanzees and inactive in adult chimpanzees.

Humans are neotenous because:

in their adult state they resemble juvenile chimpanzees.

Neoteny refers to the case when a sexually mature organism:

retains characteristics of the juvenile form.

Cann's studies involving mitochondrial DNA showed that the two deepest branches of the human family tree are African. Which of the following observations explains this statement?

that all humans evolved in Africa and only later migrated to other places

The tree of human evolution is shallow. What does that mean?

that all humans, no matter how distantly related they are, share a recent common ancestor

Studies of the denaturation temperatures of hybrids of human and different apes' DNA provided us with what new information, after morphological studies fell short of resolving the primate phylogenetic tree?

that the chimpanzees are more closely related to humans than gorillas are

Which of the following anatomical changes accompanied the transition (in humans) from partial to full bipedalism?

the development of a "basin-shaped" pelvis the narrowing of the human foot the repositioning of the foramen magnum in the skull the development of longer legs All of these choices are correct.

Culture is defined as the ability to:

transmit learned behavior.