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Chapter 6

ASTR 5 Chapter Notes - Chapter 6: Late Heavy Bombardment, Technological Evolution, Cell Nucleus

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Astro5: Life in the Universe
Dr. Schwartz
T. 6:45-9:50pm
Chapter 6: The Origin and Evolution of Life on Earth
Review Questions:
1. What are the three lines of fossil evidence that point to an early origin of life on Earth?
Discuss each line and what it tells us about when life arose. What are the implications of
an early origin for the possibility of life elsewhere?
- Stromatolites: rocks thought to be fossils made by ancient microbes. The similar structure
between the ancient and modern stromatolites implies that stromatolites are fossils remnants
of early life.
- Microfossils: individual fossilized cells. Dating the rocks where these cells were found can
tell us about the origin of these organisms, and studies on their structures.
- Isotopic Evidence: Living organisms have a lower ratio of C13 to C12 than inorganic
organism. Evidence on an island off the coast of Greenland shows ratios similar to those of
organic organisms in rocks dating 3.85bya, suggesting remnants of life from that time.
- If life arose early in our Earth’s history, it means that life might me quite common elsewhere
because we expect many other worlds to have conditions similar to those that prevailed
young Earth.
2. How do studies of DNA sequence allow us to reconstruct evolutionary history of life?
What living organisms appear to be most closely related to the common ancestor of all
present life?
- By comparing the genome of different organisms, we should be able to reconstruct the
evolutionary history of much life on Earth two organisms with similar DNA sequence
diverged more recently than two organisms with a less similar DNA sequence, in
evolutionary history. Besides extremophiles, recent research found that non-extreme living
Achaea that are genetically similar might be closer to the root of the tree of life.
3. Based on current evidence, what locations on Earth seem likely for the origin of life?
What locations can we rule out?
- The origin of life seem most likely to have risen in deep-sea or underground environments,
which would have been protected from high-energy radiation, and where volcanic vents offer
plenty of chemical energy to fuel reactions leading to life. Land surface is an unlike
environment for life to rise because of the lack of protection from high radiation our early
atmosphere offered. A shallow pond is another environment that can be ruled out because of
the lack of protection from radiation in shallow water. Even if life arose in the surface, the
late heavy bombardment would have allowed the survival only of life that migrated to deep-
sea or underground environments.
4. What was the Miller-Urey experiment, and how did it work? Why is its relevance now
subject to scientific debate? How else might Earth have obtained the organic building
blocks of life?
- The Miller-Urey experiment puts to test the hypothesis that Earth’s early atmosphere was
oxygen free, and so chemical reactions were sunlight-fueled, leading to the spontaneous
creation of organic molecules. The experiment used glass flasks to simulate chemical
conditions of the early Earth. One was filled with water (represents sea) and heated to
produce vapor water. A mixture of gaseous methane and ammonia that were added to the
water vapor represented the atmosphere. These gasses flowed to a second flask where electric
sparks provided energy for chemical reactions. The gas was cooled and then condensed
(represents rain) into water. Scientific debate is placed in these hypotheses due to the
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Astro5: Life in the Universe
Dr. Schwartz
T. 6:45-9:50pm
uncertainty of the composition of the early atmosphere (e.g. Hydrogen can play a major role
in the production of organic chemicals, but its content in early atmosphere is debatable).
- Other potential sources of organic molecules exist on Earth. 1. Chemical reactions near deep-
sea vents. 2. Material from space shown from analyses done to meteorites that contain
organic molecules within them.
5. What do we mean by “RNA world,” and why do scientists suggest that such a world
preceded the current “DNA world”?
- The RNA world is the idea that RNA molecules serve as both, as genes and as chemical
catalysts for copying and expressing these genes. Because DNA is to complex, is though that
RNA was the first molecule to rise. It can store hereditary information and replicate just like
DNA. After natural selection took over, fitter RNA molecules replicated and the weaker ones
died out. DNA evolved from RNA, becoming our hereditary storage due to extra protection
the double helix offers and because is a better mechanism to correct errors of replication.
6. Summarize current ideas about the sequence of events through which life may have
originated on Earth. What role(s) might clay or other inorganic materials have played?
- 1. Combination of atmosphere chemistry, chemistry near deep-sea vents, and outer space
molecules, the early Earth had some localized areas with significant amounts of organic
molecules that served as building blocks of more complex molecules. 2. More complex
molecules (including short strands of RNA) grew with reactions catalyzed by clay minerals.
These minerals also helped catalyze microscopic pre-cells, in which RNA and other organic
chemicals became enclosed. 3. The concentration of RNA facilitated reactions that led to
self-replication, at which point natural selection took over, favoring fitter molecules. 4.
Gradually, the complexity of these grew, leading these structures to become true living
organisms. 5. DNA evolved from RNA, and its advantage made it the proffered hereditary
7. Discuss the possibility that life migrated to Earth. Also discuss the possibility that Earth
life might have migrated to other worlds, and the implications of migration to the
search for life elsewhere.
- Life might have migrated to Earth on meteorites from other planets. Analyzed meteorites
from Mars and the Moon have shown organic material inside of them that had survived the
harsh space environment. On the other hand, Earth has also suffered impacts that might have
sent organisms to journey outside of Earth. These migrations cause us to think: If we find life
in Mars, did it originate on Earth or in mars? This could be known only if the biochemistry of
a Martian organism is too different to allow a common ancestor. Also, migration among
planets could make us wonder if life is indigenous to Earth, or from somewhere else in the
8. Why do we think that evolution would have proceeded rapidly at first, and what fossil
evidence supports this conclusion?
- Early organisms (with a limited set of enzymes) probably experienced many more errors in
DNA copying, which lead to higher mutations and a rapid evolution. Stromatolites suggest
the presence of organisms that obtained energy by photosynthesis some 3.5bya, and because
of its complex metabolic pathway, its early emergence indicates rapid early evolution.
9. Discuss the early evolution of life, from the first organisms to the development of
photosynthesis and oxygen production.
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