Further Thought

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Further Thought

Use the questions at the end of the chapter to explore concepts and connections in greater depth through application and synthesis.

1. The genesis of life is sometimes said to have required four things: energy, concentration, protection, and catalysis (for example, Cowen 1995). Explain why each of these four things was necessary for the generation of the primordial form. [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

2. It has often been thought that life could develop on Earth only because Earth is just the right distance from the sun. Any closer, and Earth would have been too hot (like Mercury or Venus); any farther away, there would not have been sufficient solar energy for the evolution of living things. Recently, communities of organisms have been found in deep-sea vents on Earth. These communities seem to get all of their energy from the vents rather than the sun. That is, the vent communities derive energy from the inner heat of Earth (which is provided ultimately by radioactivity). How does this discovery inform consideration of whether life might exist on other planets or moons that are not at "the right distance" from the sun? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

3. Recently, it has been proposed that the conditions present in our solar system are exceedingly rare in the universe at large and that life has a very poor chance of evolving past a microbial-like state, even on other planets in other solar systems (Ward and Brownlee 2000). For one thing, our solar system lies in a region of the galaxy that experiences relatively little bombardment from solid debris and a minimum of ionizing radiation. In fact, the presence of the massive planet Jupiter helps to absorb and eject much of the potentially destructive material that enters our solar system. Using the definitions of life given in this chapter, propose a living system that is based on a chemistry that is more likely to withstand these threats.
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

4. Many people have attempted to calculate the chance that "advanced" extraterrestrial life exists on other planets. By "advanced," they typically mean highly intelligent life forms that have a technological civilization with radio communication, such as has existed on Earth during the past century. One of the fundamental uncertainties in these calculations is the probability that any life at all will evolve on a planet. On Earth, how soon after Earth became habitable did life appear? How long did it take until eukaryotes appeared? How long until intelligent life appeared? In your opinion, do the answers indicate that the evolution of life (of any kind) on other Earth-like planets is probable or improbable? How about the evolution of intelligent life? How about advanced civilization? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

5. Leslie Orgel admitted to John Horgan (1991) that he and Crick intended their directed panspermia hypothesis as "sort of a joke." In their 1973 paper, however, Crick and Orgel treat the idea seriously enough to consider biological patterns that might serve as evidence. They point out, for example, that

It is a little surprising that organisms with somewhat different [genetic] codes do not exist. The universality of the code follows naturally from an "infective" theory of the origins of life. Life on Earth would represent a clone derived from a single extraterrestrial organism.

Since 1973, biologists have discovered that the genetic code is not universal and that organisms with "somewhat different codes" do, in fact, exist. Our mitochondria, for example, use a code slightly different from that used by our nuclei (see Knight et al. 1999). Many ciliates and other organisms also have slightly deviant codes (see Osawa et al. 1992). How strongly does the discovery that the genetic code is not universal refute the directed panspermia hypothesis? How strongly does it refute other versions of panspermia? Explain your reasoning. Can you think of other kinds of evidence that could (or do) either support or refute some version of panspermia? Does the fact that there are organisms with somewhat different genetic codes refute the panspermia hypotheses?” [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

6. Examine closely Figure 16.5 (page 623 in your textbook). What fraction of the 140 randomized nucleotide positions did not mutate by the ninth round of selection and reproduction? Why did these positions show no mutations? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

7. In the experiment diagrammed in Figure 16.6 (page 625 in your textbook), why was it important for the researchers to include a tag on the end of the small substrate RNAs? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

8. In the entire chain of events leading from the abiotic synthesis of biological building blocks to the evolution of eukaryotes, which transition appears to be the least characterized? Why do you think this is the case? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

9. Imagine an extremely primitive organism that has very primitive ribosomes with no proteins. Would it be possible to place this organism on the phylogenies in Figure 16.18 or Figure 16.21 (pages 641 and 645 in your textbook)? Why or why not? How about an organism with no ribosomes? (Can you think of such an organism?) Is it conceivable that there are some as-yet-undiscovered primitive organisms that cannot be placed on these phylogenies? How would the discovery of such organisms affect our reconstruction of the cenancestor? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

10. When biologists worked out the details of DNA replication in bacteria and eukaryotes, many researchers were surprised to discover that there are several different DNA polymerases, each with a different role. The machinery for replication seemed enormously complex, and every piece seemed essential if the whole system was to function at all. Many people found it hard to imagine how such a complex system of interdependent parts could have evolved by natural selection. Does the discovery of organisms with only one DNA polymerase (such as Methanococcus jannaschii) offer new insight into the evolution of replication? Why or why not? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

11. Suppose you are trekking through remote Greenland on a day off from your summer job at a scientific camp and you find an unusual layer of sedimentary rock that is not mapped on your geological charts. You suspect this rock might be even older than the 3.7-billion-year-old rocks from Isua. What would you do to determine whether these rocks have any evidence of ancient life? What results would show that life was indeed present before 3.7 billion years ago? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

12. How do we know that mitochondria and chloroplasts are descended from free-living bacteria? What would the phylogeny in Figure 16.30 have looked like if, instead, these organelles had simply evolved as internal structures of eukaryotic cells? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.




1.	The genesis of life is sometimes said to have required four things: energy, concentration, protection, and catalysis (for example, Cowen 1995). Explain why each of these four things was necessary for the generation of the primordial form. [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



2.	It has often been thought that life could develop on Earth only because Earth is just the right distance from the sun. Any closer, and Earth would have been too hot (like Mercury or Venus); any farther away, there would not have been sufficient solar energy for the evolution of living things. Recently, communities of organisms have been found in deep-sea vents on Earth. These communities seem to get all of their energy from the vents rather than the sun. That is, the vent communities derive energy from the inner heat of Earth (which is provided ultimately by radioactivity). How does this discovery inform consideration of whether life might exist on other planets or moons that are not at "the right distance" from the sun? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



3.	Recently, it has been proposed that the conditions present in our solar system are exceedingly rare in the universe at large and that life has a very poor chance of evolving past a microbial-like state, even on other planets in other solar systems (Ward and Brownlee 2000). For one thing, our solar system lies in a region of the galaxy that experiences relatively little bombardment from solid debris and a minimum of ionizing radiation. In fact, the presence of the massive planet Jupiter helps to absorb and eject much of the potentially destructive material that enters our solar system. Using the definitions of life given in this chapter, propose a living system that is based on a chemistry that is more likely to withstand these threats. To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



4.	Many people have attempted to calculate the chance that "advanced" extraterrestrial life exists on other planets. By "advanced," they typically mean highly intelligent life forms that have a technological civilization with radio communication, such as has existed on Earth during the past century. One of the fundamental uncertainties in these calculations is the probability that any life at all will evolve on a planet. On Earth, how soon after Earth became habitable did life appear? How long did it take until eukaryotes appeared? How long until intelligent life appeared? In your opinion, do the answers indicate that the evolution of life (of any kind) on other Earth-like planets is probable or improbable? How about the evolution of intelligent life? How about advanced civilization? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



5.	Leslie Orgel admitted to John Horgan (1991) that he and Crick intended their directed panspermia hypothesis as "sort of a joke." In their 1973 paper, however, Crick and Orgel treat the idea seriously enough to consider biological patterns that might serve as evidence. They point out, for example, that It is a little surprising that organisms with somewhat different [genetic] codes do not exist. The universality of the code follows naturally from an "infective" theory of the origins of life. Life on Earth would represent a clone derived from a single extraterrestrial organism. Since 1973, biologists have discovered that the genetic code is not universal and that organisms with "somewhat different codes" do, in fact, exist. Our mitochondria, for example, use a code slightly different from that used by our nuclei (see Knight et al. 1999). Many ciliates and other organisms also have slightly deviant codes (see Osawa et al. 1992). How strongly does the discovery that the genetic code is not universal refute the directed panspermia hypothesis? How strongly does it refute other versions of panspermia? Explain your reasoning. Can you think of other kinds of evidence that could (or do) either support or refute some version of panspermia? Does the fact that there are organisms with somewhat different genetic codes refute the panspermia hypotheses?” [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



6.	Examine closely Figure 16.5 (page 623 in your textbook). What fraction of the 140 randomized nucleotide positions did not mutate by the ninth round of selection and reproduction? Why did these positions show no mutations? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



7.	In the experiment diagrammed in Figure 16.6 (page 625 in your textbook), why was it important for the researchers to include a tag on the end of the small substrate RNAs? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



8.	In the entire chain of events leading from the abiotic synthesis of biological building blocks to the evolution of eukaryotes, which transition appears to be the least characterized? Why do you think this is the case? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



9.	Imagine an extremely primitive organism that has very primitive ribosomes with no proteins. Would it be possible to place this organism on the phylogenies in Figure 16.18 or Figure 16.21 (pages 641 and 645 in your textbook)? Why or why not? How about an organism with no ribosomes? (Can you think of such an organism?) Is it conceivable that there are some as-yet-undiscovered primitive organisms that cannot be placed on these phylogenies? How would the discovery of such organisms affect our reconstruction of the cenancestor? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



10.	When biologists worked out the details of DNA replication in bacteria and eukaryotes, many researchers were surprised to discover that there are several different DNA polymerases, each with a different role. The machinery for replication seemed enormously complex, and every piece seemed essential if the whole system was to function at all. Many people found it hard to imagine how such a complex system of interdependent parts could have evolved by natural selection. Does the discovery of organisms with only one DNA polymerase (such as Methanococcus jannaschii) offer new insight into the evolution of replication? Why or why not? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



11.	Suppose you are trekking through remote Greenland on a day off from your summer job at a scientific camp and you find an unusual layer of sedimentary rock that is not mapped on your geological charts. You suspect this rock might be even older than the 3.7-billion-year-old rocks from Isua. What would you do to determine whether these rocks have any evidence of ancient life? What results would show that life was indeed present before 3.7 billion years ago? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



12.	How do we know that mitochondria and chloroplasts are descended from free-living bacteria? What would the phylogeny in Figure 16.30 have looked like if, instead, these organelles had simply evolved as internal structures of eukaryotic cells? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.

Chapter 16: The Origin of Life and the Precambrian Evolution

Further Thought