Lecture II.4. Answers to the Questions.

Transcription

1 Lecture II.4. Answers to the Questions. 1. Animals colonized the land during the Devonian period. This was during the era and the eon. Paleozoic; Phanerozoic 2. The mother of all mass extinctions occurred at the end of the period and the era. Permian; Paleozoic. 3. The lack of Precambrian fossils reflects which of the following factors? (There may be more than one correct answer.) a. Lack of Precambrian organisms. b. Lack of readily fossilized hard parts shells, skeletons, teeth, etc. c. Lack of fossil-bearing rock. d. Lack of scientific interest in studying Precambrian strata. e. None of the above. There are lots of Precambrian fossils. b., c. 1

2 4. Which two mass extinction events were coincident with massive volcanic eruptions that left enormous deposits of lava called traps? Permo-Triassic (end-permian) and Cretaceous- Tertiary (end-cretaceous). 5. You invent a time machine and travel back in time seven million years (early Miocene) to South America. You observe large, cat-like creatures stalking a herd of hoofed mammals. Are the predators marsupials or placentals? Explain your answer. Hint. When did the Panamanian land bridge form between North and South America? Marsupials. The Panamanian land bridge wouldn t form for another 4 million years (Pliocene), at which time placental carnivores invaded South America and indigenous marsupial predators went extinct. 2

3 6. Why were stromatolites important in the evolution of the earth s atmosphere? Photosynthetic bacteria living in stromatolites oxygenated atmosphere. 7. Regarding the origin of life, why is it important whether the primitive atmosphere was reducing or mildly oxidizing? Amino acids and other building blocks of life are more readily produced in a reducing environment. 8. In 2011, NASA biologists announced the detection of adenine and guanine in carbonaceous chondrites (meteorites with a high organic fraction). What evidence did the scientists give to support their claim that these nucleobases are truly of extraterrestrial origin, i.e., as opposed to having resulted from contamination? 1. The meteorites contained nucleobase-related compounds not found in terrestrial organisms. 2. The concentration of the nucleobases in the surrounding ice and soil was on the order of thousand times less than in the meteorites themselves. 3

4 3. The nucleobases (biological and nonbiological) detected in the meteorites can be synthesized abiotically by reacting hydrogen cyanide, ammonia and water. 9. Why could the removal of methane from the earth s atmosphere by accumulating concentrations of molecular oxygen have caused the climate to cool? Methane removal increased the rate at which energy from the sun was radiated back into space as heat. In greater detail: Earth s temperature is determined by a balance between incoming and outgoing energy, the former as shortwave radiation; the latter re-radiated as heat (long wave radiation). Greenhouse gases such as methane, water and carbon dioxide, retard re-radiation and thus warm the earth and its atmosphere. As noted in lecture, oxygen reacts with methane to yield water and carbon dioxide: 2O 2 + CH 4 CO 2 + 2H 2 O Water vapor, CO 2 and methane are all greenhouse gases. Of the three, methane is by far the most effective in retarding re-radiation. Hence the re- 4

5 sult of increased atmospheric oxygen would have been to cool the earth. 10. From a thermodynamic point of view, what is the advantage of packaging biological molecules in a membrane? One consequence of the 2 nd Law is that substances in solution move down the concentration gradient i.e., from regions of greater to lesser concentration. Packaging biological molecules, be it within a membrane, a mineral cell, or what have you, reduces their loss by diffusion. 11. The phylogenies at the right contrast two symbiogenic scenarios for the origin of eukaryotes. Assuming that a cytoskeleton is necessary for engulfing, but not fusion, discuss when the cytoskeleton may have evolved. Assume that once evolved, the cytoskeleton was never lost. If Ring of Life scenario correct, the cytoskeleton evolved during or just after symbiogene- 5

6 sis, i.e., if before, we would expect to find cytoskeleton genes in living Archaea or Bacteria; if after, living eukaryotes lacking cytoskeleton genes. If chronocyte scenario correct, cytoskeleton evolved before symbiogenesis, following which chronocytes became eukaryotes. 12. J.B.S. Haldane is reputed to have remarked that evolution was a falsifiable hypothesis; that it could easily be refuted, for example, by the discovery of elephants in the Cambrian. What did he mean by this? Elephants in the Cambrian would have existed long before their presumed ancestors, i.e., before the first mammals, before the first land vertebrates, before the fish that evolved into land vertebrates, etc. 13. Molecular evidence suggests that the major groups of animals diverged well before the Cambrian. If this was indeed the case, we must conclude that the Cambrian explosion. reflects the acquisition of fossilizable hard parts by lineages already in existence. 6