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Fossil Record & First Life Notes

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Biology (Biological Sciences)
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January 25, 2013 – Lecture #6: Fossil Record & First Life Fossil Records  Fossil is a preserved remnant/evidence of organisms that lived in the past o Fossilis = dug up  Typically associated w/ sedimentary rocks - Rocks formed through accumulation of mud, silt or sand - Distinct layers of rock are called strata  Whether an organisim is fossilized depends on chance o More likely if hard than if soft-bodies o More likely if aquatic than terrestrial o Morel ikely if inshore marine than offshore o More likely if decomposing organisms are absent  Therefore, our understanding of diversity and distribution of past life is biased and incomplete  Many types of fossils o Trace fossils record evidence of behaviour:  Tracks, burrows, faees  Study ofr trace fossils = ichnology o Ichnos = track, trail  A cast forms when minerals fill space in sediment where organisms decayed after having been buried  Petrified fossils have had their tissues replaced by minerals o petra = rock  sometimes original organic material is retained o as thing carbon films o in amber o in tar or peat o frozen  fossil vs. sub-fossil o fossil has most organic parts replaced by minerals o sub-fossils has a high percentage organic Dating Fossils  relative dating is done via sedimentary stratigraphy o stratum = layer o graph = write, record  we usually cant tell how long ago fossils were created, but we can tell which fossil came first, second, third etc. o the lower the fossil was in the stratigraphy the older the fossil, which allows us to see the order of fossils created  it is common to have gaps in sedimentary sequence o erosion o temporary halt in sediment deposition  widespread, common index (=indicator) fossils can help to “read” incomplete or scrambled layers Geological Time Scale  created based on the occurrence and disappearance of major taxa o including index fossils  oheg. Disappearance of non-avian dinosaurs at the end of the cretaceous  …are correlated with chagTable 25.1below: only need toch know the order of them, and only if bolded in notes • created based on the occurrence, and Whdisappearance, of major taxa – iincluding index fossils • thus it is not astonishing that major event ts… – e.g. disappearance of non-avian dinosaurs at heeendd ofthe Crreta • …are correlated with changes in eon, erra, peeriodd or eppocch – see Mastering Biology activity “A Scrolling Geolog ic Record” for chap 25 – I will not be asking you for any dates of heese imee periods, orfor theirna unless they are in bold in the notes Dating..  how to determine absolute dating  radiometric dating o involved measurement of radioactive isotopes in fossils or rocks o certain elements decay from one isotopic form to another at a constant rate  eg. Carbon occurs at both Carbon-12 and Carbon-14 o differ in the number of neutrons (6 vs. 8) o C-12 is very common o C-14 rare result of cosmin rays crashing into Nitrogen-14 in atmosphere  Carbon-14 unstable, decays to Nitrogen-14 o Half-life is when 50% of atoms in a given amount of carbon of have decayed  Plants take up C-12 and C-14 during photosynthesis  Animals get same raito of c-12 and c-14 through eating plants or herbivores  Once organism is dad, stops taking up carbon  C-12 stays, but C-14 decays o Half-life is 5730 years  Increasing ratio of C-12: C-14 in fossils allows fossils to be dated  Carbon only good for young fossils (only up to ~75 000 years old)  Uranium isotopes better for older fossils o Half life of U-238 is 4.5 billion years o Created in molten volcanic rock o Fossils sandwich btwn layers of volcanic rock can be dated this way  Other isotope good for intermediate ages Continental Drift  Location of fossils in place as well as time important to understand history of Earth  Land-asses are not constant in location o Drift around on plates o Tectonic boundaries sites of earthquakes and volcanoes  Relative location of land-masses have changed over time  Fossils provided first evidence for continental drift  But continental drift has also resulted in mass extinctions o Eg. When Antarctica drifted to pole and froze solid  So continental drift has been good and bad for diversification Mass Extinctions  Two of the most famous events are the end-Permian extinction and the end- Cretaceous extinction  End-Permian o occurred 245 million years ago  know this in relative to other time periods o occurred at the same time as formation of the supercontinent: Pangaea  this increased volcanism, oceanic anoxia (lack of oxygen) and reduced amount of shallow shoreline  end-Cretaceous o occurred 65 million years ago o at Cretaceous-Tertiary (KT) boundary  note: ‘tertiary’ is not recognized anymore, its time span now covered by Paleogene, Neogene, and part of Quaternary) o only 20% of all families went extinct  but these included the non-avian dinosaurs  and some of the most common and diverse marine animals  what caused mass extinctions? o Paleontologists argue that an asteroid or comet strike caused massive fires and smoke.debris that blocked sunlight for months o Others, state that decline was more gradual and caused by climate change  Asteroid = the last straw Early Earth  Planet formed ~4.6 billion years ago  For several 100 millon years, earth was bombarded by large chunks of rock o This ended about 3.0 BYA First Life: Prokaryotes  Oldest fossil evidence of life = 3.5 BYA  Single-celled proks o Lack membrane-bound organelles o DNA not contained n ncleaus  Proks = the only evidence of lfe on earth for 1.5 BY First Life: First Communities  (?) Many of the oldest proks. Fossils found in stromatolites o Stroma = sheet o Lithe = rock  Sediments accumulate on top of species of proks living together on substrate  Proks grow up through the sediment o Creating banded rocks Oxygen Makers  Little oxygen in early atmosphere  Only began accumulating ~2.7 BYA when photosynthetic cyanobacteria started osiAdding of H to CO2 to make carbohydratesogen and oxygen o Cyanobacteria also called blue-green algae - even though it isn’t algae  They are photoautotrophic  First oxygen released through photosynthesis probabl reacted with the iron dissolved in oceans to make iron oxide o Iron oxide precipitated out and resulted in banded iron rock found today o Eventually all iron precipitated o And oxygen gas began to accumulate o However, oxygen was toxic to most early life  It attacked chemical bonds  Species sensitive to O2 could only live where there was very little O2 o There descendants are still present today  Obligate anaerobes  Other proks adapted to the oxygen rich atmosphere nad began respiring aerobically How Did Life Originate?  How did proks even arise? o No one knows for sure o Until 1800s, people believed in spontaneous generation  Pasteur disproved this theory  Early •arterly Earth very differents now o It hclimaate from nnoowa and methane in atmosphere, frequent lightning o Sc• nti 1950’s, experiments triedthese condto repllcate coondiiionssent   thought to exist BYA amino acids - But amino acids don’t behave like – hhotseas  Spontaneo– ammonia and methane inipid vesicles suatmosphereearly cell-like structure oayThey have a bi-layer structure similar to living cell membranes o Experiments show that vesicles form faster in the presence of a type of volcanic clay believed to be common 4 BYA Fig. 25.2) amino acids and Fig. 4.2 other products First Nucleic Acids  RNA (ribonucleic acid) o Thought to be the first genetic material o Single-stranded  Rather fragile o Could take on many shapes depending on length and constituents  DNA (deoxyribonucleic acid) o Makes up all genetic material of living things o Double helix, paired strands twisted around each other o Stable nature  DNA transcribed into RNA which is translated into proteins Where Did Life Originate?  How cells arose = still up to debate  Scientists thought cells arose in shallow water bodies exposed to the atmosphere  However, now many think it arose around hot, mineral-rich deep-sea vents o Where many of the earliest proks (archaea) still live  Others suggest the first prok came from space o Panspermia hypothesis  Pan = everywhere  Sperm = seed  This gained support when in 1990s nanobes – structures looking like bacteria, were found inside meteorites  But, recently shown that these structures can arise from mineral crystallization  However, A.A have been found inside meteorites  If life can evolve form non-living materials, then they should arise wherever the conditions are right o The most important is free water January 25, 2013 – Lecture 7: Prokaryotes Proks: First Life  First fossil evidence of life on earth are of proks o Approx.. 3.5 BYA  2 Domains: Bacteria & Archaea  this was the only form of life for 1.5 BY  still dominate the biosphere  live wherever there are euks. and in many places they cannot live in  d
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