BIL 160 Chapter 25: Chapter 25 History of Life on Earth

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Macroevolution: the broad pattern of evolution above the species level
Conditions on early Earth made the origin of life possible
A. Four main stages for the production of simple cells:
a. The abiotic synthesis of small organic molecules, such as amino acids
b. The joining of these small molecules into macromolecules, such as proteins
c. The packaging of these molecules into protocells, droplets with membranes that
maintained an internal chemistry different from that of their surroundings
d. The origin of self-replicating molecules that make inheritance possible
B. Synthesis of Organic Compounds on Early Earth
a. First atmosphere had little oxygen and was thick with water vapor, nitrogen,
carbon dioxide, methane, ammonia, and nitrogen
b. Oparin and Halden’s hypothesis
i. Earth’s early atmosphere was a reducing environment
ii. Energy for synthesis could have come from lightning and UV radiation
c. Another theory suggests that the atmosphere was neutral
d. Another that the first organic compounds were produced in hydrothermal vents,
areas of the sea floor where heated water and minerals gush from the Earth’s
interior into the ocean
e. Meteorites
C. Abiotic Synthesis of Macromolecules
a. Abiotic syntheses of RNA monomers can occur spontaneously from simple
precursor molecules
i. Could have acted as a weak catalyst for a variety of other chemical
D. Protocells
a. Vesicles have a selectively permeable bilayer and can perform metabolic
reactions using external sources of reagents
E. Self-Replicating RNA
a. Ribozymes: RNAs that function as catalysts
b. RNA molecules with certain nucleotide sequences may have shapes that enable
them to replicate faster and with fewer errors than others
i. A copying error sometimes produced an even after replication
c. If a vesicle with self-replicating, catalytic RNA could grow and split, it could pass
its RNA to its daughters
d. RNA in protocells would have provided the template for DNA
The fossil record documents the history of early life
A. The Fossil Record
a. Based primarily on the sequence in which fossils have accumulated in
sedimentary rock layers
b. Shows there have been great changes in Earth’s organisms at different points in
c. Biased towards species that existed for a long time, were abundant and
widespread, and had shells or skeletons that facilitated fossilization
B. How Rocks and Fossils Are Dated
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a. Radiometric dating: based on the decay of radioactive isotopes, in this process a
radioactive “parent” isotope decays to a “daughter” isotope at a characteristic rate
i. Half-life: the time required for 50% of the parent isotope to decay
b. Fossils contain isotopes that accumulated in organism when they were alive
c. By measuring ratio of carbon-14 to carbon-12, you can determine fossil age
C. The Origin of New Organisms
a. Origin of Mammals
i. Synapsid (300 mya)
1. Had multiple bones in lower jaw and single-pointed teeth
2. Had an opening behind the eye socket through where the muscles
for closing the jaws passed through
ii. Therapsid (280 mya)
1. Had large dentary bones, long faces, and large canine teeth
iii. Early cynodont (260 mya)
1. Dentary was the largest bone in lower jaw
2. Temporal fenestra was large and positioned forward of the jaw
3. Teeth with several cusps appeared
iv. Later cynodont (220 mya)
1. Had teeth with complex cusp patterns
2. Lower and upper jaw hinged in two locations
v. Very late cynodont (195 mya)
1. Dentary-squamosal was the only hinge
2. Articular and quadrate bones migrated to ear (eventually became
hammer and anvil)
Key events in life’s history include the origins of unicellular and multicellular organisms and the
colonization of land
A. Geologic record: a standard time scale that divides Earth’s history into four eons and
further subdivisions
a. Hadean
b. Archaean
c. Proterozoic
d. Phanerozoic (half a billion years)
i. Paleozoic
ii. Mesozoic
iii. Cenozoic
B. The First Single-Celled Organisms
a. Stromatolites: layered rocks that form when certain prokaryotes bind thin films of
sediment together
i. Earth’s sole inhabitants for more than 1.5 billion years
b. Photosynthesis and the Oxygen Revolution
i. When oxygenic photosynthesis first evolved, free O2 probably dissolved in
ii. Soon it was able to react with elements in water, like iron, forming iron
oxide, which forms rock containing iron oxide
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