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Lecture 2

BIOC50H3 Lecture Notes - Lecture 2: Absolute Dating, Cyanobacteria, Lead

Biological Sciences
Course Code
Jason Weir

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Lecture 2 and 3 – Macroevolution
Many tools to decipher evolutionary ideas
Another idea – Geological strata
- Strata – layers of rock/soil with internally consistent characteristics that distinguish them
from other layers; strata tells us what was happening to the earth at the time
- There is a period formation table that should be memorized – the time periods correlate
with the strata (ex: Mesozoic = era, Jurassic = period, 200 = mya)
- Usually a mass extinction event separates each period
- The geological column will only have a subset of what happened in the time period we
are analyzing
- Ex: grand canyon – different parts of the grand canyon will have different strata in order
– it will always go up based on the period
- Fossils – formed from dead organisms – very rapid; typically have to be buried rapidly
and in areas without o2 – amber is a type of form for a fossil to be made
- Some fossils can be mineralized or made using collagen and salt; able to be used for
protein sequencing
- Fossils and the geological column – 4D history (a period in which some period of time is
lost) (4th dimension is time itself) – expectation is that with each fossil going up, it can
represent the time period in tandem
- Turbidite – underwater avalanches which form fossils or volcanic eruptions can make
Geological timing
- Radioactive age dating – absolute ages of rocks in the geological column are measured
using naturally occurring radioactive elements in the rocks
- Uranium 235 = 143 neutrons; Uranium 236 = 146 neutrons
- 235 decays to Pb 207 – half-life = 704 my; 238 decays to Pb 206 – half-life = 4.47 by
- Half-life = the amount of time it takes for one half of the atoms of the radioactive element
to decay
- C14 decays to N14 in 5730 years
- Carbon is incorporated into living tissue; with a ratio of c12 and c14; by figuring out the
ratio in a certain organism, we can determine its age
- C14 dating is useful for dating bones, ash or wood up to 50,000 years back (anything
past, the amount of c14 is lost)
- It does a pretty good job of dating but it has limitations (50,000 year limit)

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- Potassium-argon dating and uranium-lead dating are commonly used to date igneous
- Igneous rock forms when molten rock solidifies
- Potassium-argon and uranium-lead dating measure the time when igneous rock
- Dating the igneous layers = can place upper and lower bound ages on non-igneous
layers between them
- Why might radioactive dating not work well on sedimentary rocks = they are smaller
sediments of rock from different layers that have been compressed to make up a new
layer of rock – if dated, we would be sating when the original layer formed and not when
the layer of interest was formed
Plate tectonics
- Lithosphere – outer layer of the earth – comprises of the continental crust and the
oceanic crust below the oceans
- Mountains run parallel along where the plate collide (buckling)
- The plates move around time – continental drift
- Current position is not the position they occurred 100 million years ago or before that
- Pangaea – 220 million years ago; eventually broke up and slowly drifted to current
- Evidence – the distribution of fossils (fossil record); it makes no sense that a certain
fossil was formed in current Africa when it was found in South America
- During the Permian (~ 250 Mya) all the major continents formed the supercontinent
- During the Jurassic (200 to 145 Mya) Pangea broke up into a northern supercontinent
Laurasia and a southern supercontinent Gondwana
- Laurasia split into North America and Eurasia
- Gondwana split into Antarctica, South America, Africa, India, Australia and New Zealand.
~ 130 Mya South America drifted apart and the breakup was complete by ~ 80 Mya
Evolution of the Universe
- 13.7 billion years ago – the big bang; all the energy in the universe compressed into a
small point then it exploded and inflated for a while
- There was a dark ages – where hydrogen gas clouds existed and the universe was dark
for a while
Origin of life
- Planets formed 4570 mya from dust and gas (heavy bombardment by asteroid probably
occurred for the first few hundreds of millions of years)
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