Lecture 6 – Dating the Major Events of LifeDating the History of Life on Earth- Fossil record- Molecular phylogenetics- These two go hand in hand to reveal what we know about when and how life evolvedo What types of organisms are presento When those organisms aroseAge of the Earth- Prevailing Western views to Darwin, Lyello Earth is very young (few thousand years)o Catastrophism: shape of earth’s landscape from few key events occurring all across globeo Species are immutable- During 1800’s uniformitarianism replaces catastrophismo Earth must be very oldo Geologic process act for very long time, making gradual changesFossil – Assumptions- Fossils are deposited in layerso Oldest layers are deepesto Newer layers are above- Age of organisms determined by age of surrounding rockso No mixing- Fossil pose problem – why are there fossilized animal forms that don’t exist today?Relative Dating- Geostratigraphyo Use strata (or layers) to determine the sequence of geological eventso Actual layers are not always trivial to interpret - Biostratigraphyo Help to correlate layerso Date rocks based on animals found in certain layerso Best for animals living around the globe so you can correlate the layers with one anothero Best also if animals only lived during a short period of time so that their presence indicates a particular time periodFossils take many forms- Intact fossil- Compression fossil- Cast fossil- Permineralized FossilAbsolute Dating – why is this important?- Places evolutionary events in an appropriate physical/geological context- Tests plausibility of evolutionary change- Allows us to determine rate of important eventso Did life arise quickly?o How quickly did complex life forms evolve?o Has the diversification of life/major groups proceeded at a constant rate or varying rates?Gradualism vs. Punctuated Equilibrium - Darwin proposed evolutionary change occurred gradually- Eldridge and Gould studied fossil record and found otherwiseo Suggested animals stay same for long periods of time (stasis) then change rapidly (Punctuated Equilibrium)Stasis in Large Population may be affected (Snails)- Sea level change causes isolation of small populationo Change is more likely in a small population- Rapid change in small populationo This might occur because of drift where a random allele of a gene forcone shape gets picked and cone sizes get larger- Sea level rise reconnects populationso Now there is competition between large and small shelled species- Long shelled species might out-compete the small shelled versions- Stasis in large population againFossil record shows rapid change Absolute Dating- Radioactive carbon produced in atmosphere o Gets incorporated into living organisms Ratio of natural carbon isotopes-12C 98.89%-13C 1.11%-14C / 12C = 1.18 x 10-12 - When animal dies, no longer intake 14Co Ratio decreases as 14C decaysAbsolute Dating 14C 12N- Half life = time for 50% of 14C to decay
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