Lecture II.4. Answers to the Questions.
1. (2 pts) Animals colonized the land during the Devonian
period. This was during the _________ era and the
__________ eon.
Paleozoic; Phanerozoic
2. (2 pts) The mother of all mass extinctions occurred at
the end of the _______ period and the ________ era.
Permian; Paleozoic.
3. (4 pts) 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.
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4. (2 pts) Which two mass extinction events were coincident with massive volcanic eruptions that left enormous
deposits of lava called traps?
Permo-Triassic (end-Permian) and CretaceousTertiary (end-Cretaceous).
5. (6 pts) 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.
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6. (4 pts) Why were stromatolites important in the evolution of the earth’s atmosphere?
Photosynthetic bacteria living in stromatolites
oxygenated atmosphere.
7. (4 pts) 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.
(8 pts) 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.
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3. The nucleobases (biological and nonbiological) detected in the meteorites can be
synthesized abiotically by reacting hydrogen
cyanide, ammonia and water.
9. (8 pts) 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:
𝟐𝑶𝟐 + 𝑪𝑯𝟒 → 𝑪𝑶𝟐 + 𝟐𝑯𝟐 𝑶
Water vapor, 𝑪𝑶𝟐 and methane are all greenhouse
gases. Of the three, methane is by far the most
effective in retarding re-radiation. Hence the re4
sult of increased atmospheric oxygen would have
been to cool the earth.
10. (4 pts) From a thermodynamic point of view, what is the
advantage of packaging biological molecules in a membrane?
One consequence of the 2nd 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. (6 pts) 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 symbiogene5
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
the chronocytes died out.
12. (6 pts) 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. (4 pts) 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.
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