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Lecture 13

BIOLOGY 171 Lecture Notes - Lecture 13: Archean, Adaptive Radiation, Hadean

Course Code
Josephine Kurdziel

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Thursday, March 14, 2019
Phylogenies and the fossil records are the major tool that biologists use to study the history of
White clods of dover: primarily composed of fossilized phytoplankton mainly forams
Contain thousands of cosines of forams
Fossil records
The fossil record is the only source of direct evidence of what prehistoric organisms looked
like, where they lived, and when they lived
Fossils are necessary to calibrate phylogenies based on molecular data
They provide absolute dates for when a particular group originated
Fossils place evolutionary events in the context of earths dynamic environmental history
Process of fossilization
Requires two thing:
quick burial (by sentiments)
Lake bottoms or ocean bottoms are good places
no decomposition (none of microbes that decompose organic matter are present) or slow
decomposition ( very few microbes that decompose organic matter are present)
Two main types of fossils
Body fossils: direct evidence of organisms
Bones, teeth, skulls, vertebrae, and shells
Organisms persevere in ice (wooly mammoths)
Organism preserved in amber (plant resent that has been fossilized)
insects usually found in amber, also small lizards, and feathers of birds
Petrified wood (tree died and carried by sentiments, organism material in wood got filled in
by minerals)
Fossilized pollen (pollen grain have otter cover thats decay resistant)
Fossil jungle spores (most body of fungus doesn’t fossilize well, but the covering is decay
Trace fossils: indirect evidence off organism’s activities
footprints, tracts, burrows,
Coprolites (fossilized poop- we can infer what the organism ate)
Chemical fossils (lipids fossilize well, protein, DNA)
Paleontology: field that analysis the fossil record
Analysis of fossil record
Inference about how organisms lived
Not a complete record (fossilization is a rare event)

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Thursday, March 14, 2019
Some limitations of the fossil record are
Habitat bias
Better fossil record marina organisms that live in sentiment than compared to other
Organism that live in the bottom o ocean or bottom of lates have good fossil record
Top of mountains have a very bad fossil record because erosion happening
Taxonomic bias
Organisms that have hard parts are more likely to be fossilized than organism that are all
soft bodied
Organisms with bones and teeth and vertebra or hard shells (clams and scallops and
other mollusks) are more likely to be fossilized
Organisms that produce decay resistance molecules have a better fossil record
flowering plants and gymnosperm- pollen grains have coating that is decay
Fungi (soft body) covering is decay resistant so good fossil record through their
Fossil records for nematodes worms are all soft body- have terrible fossil records
Note: we do get soft body fossils occasionally but not often
Temporal bias
We have better fossil record for organisms that live closer to present compared to older
Earth is dynamic and the curst is being pulled down to mantel (hot) and destroys rock
Oldest rocks have destroyed because of subduction
Abundance bias
Species that were once abundance and wide spread will have a better fossil record than
species that were only found in one place and were less abundant
Fossilization is rare event, if there is more of something, there is a higher chance that
some were fossilized
How do scientists figure out how old a fossil is?
They use Radioactive decay and geological dating
radio active isotopes: isotopes that differ in how many neutrons they have
ex. Common isotope of carbon is carbon 12 (6 protons + 6 neutrons) stable.
Carbon 14 is an unstable (6 protons + 8 neurons) so undergoes radio active decay and
produces decay products nitrogen 14
Concept of a half life: half life is the amount of time it takes for a radio isotope to decay and
produce 50% of the daughter isotope
Table provided: Radio isotope= parent isotope, decay product= daughter isotope
Different radio isotopes have different ranges for
which they are good
ex. If you want to know how old the earth is
your wouldn’t use carbon14 its half life is too
short, you would use one of the uranium

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Thursday, March 14, 2019
CLICKER: You use Uranium235 to age a fossil that you found. Uranium235 decays to Lead207,
with a halflife of 710 million years.
You find 25% Uranium235 and 75% Lead207 in the sample. Based on this, how old is the
A. 177.5 million years
B. 355 million years
C. 710 million years
D. 1,420 million years
E. 2,130 million years
"Why? Half life is 710 million years
At 1 half life you
50% off parent isotope: Uranium 235 and 50% of the daughter isotope:
lead 207
At 2 half life:
You would have half off what you have at one half life
25% of Uranium 235 and 75% of lead 207
2 half lives have passed so 2x710= 1420 million years
NOTE: With each passing off a half life, half off what is present decays to
produce more of the daughter isotope
CLICKER: After 3.9 billion years, the ratio of 40K to 40Ar in a rock would be:
A. 0:100
B. 50:50
C. 25:75
D. 12.5:87.5
E. 100:0
Why? 3.9 billion years is 3 half lives
1 half life is 1.3 billion years
50% parent, 50% daughter
2 half lives is 2.6 billion years
25% parent, 75% daughter
3 half lives is 3.9 billion years
12.5% parent, 87.5% daughter
Radioactive decay and geological dating
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