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Fossil Evidence for Evolution

Fossil Evidence for Evolution. Fossil : any preserved trace left by an organism E.g. footprints, burrows, faeces, bones shells, teeth, impressions of an organism Artefacts : objects deliberately made by humans E.g stone tools, beads, carvings, charcoal from cooking, cave paintings

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Fossil Evidence for Evolution

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  1. Fossil Evidence for Evolution

  2. Fossil: any preserved trace left by an organism • E.g. footprints, burrows, faeces, bones shells, teeth, impressions of an organism • Artefacts: objects deliberately made by humans • E.g stone tools, beads, carvings, charcoal from cooking, cave paintings Fossil Formation • Generally organisms are decayed by other micro organisms • Organisms may be fossilized if it’s buried rapidly by drifting sand, mud deposited by rivers, volcanic ash, and intentional burial. • Wet, acidic soil  minerals in bone are dissolved • No O2, (peat) preservation of soft tissue and bones may occur. • Alkaline soils are ideal petrification occurs: new minerals (lime/iron oxide) are deposited in pores of bone, replacing organic matter  becoming a rock.

  3. Four conditions are required for fossilization to occur: • Rapid burial • Presence of hard body parts • Absence of decay organisms • Long period of stability

  4. Fossil Discovery • sometimes found by chance(due to erosion) but generally result of excavation • surface discoveries often lead to excavations • Steps: • Area surveyed and sectioned • Small hand tools to gently remove soil • Removed soil is sieved • Photographs taken at every stage • Items retrieved are labeled and catalogued • In lab, fossils are scraped clean and repaired • Plaster casts/latex moulds are made

  5. Fossil Dating • Absolute dating: actual age of specimen in years • Relative dating: age relative to other fossils (older or younger) • BP: Before Present

  6. Absolute Dating • Potassium-Argon Dating • Dating of a rock sample (assuming there is a fossil in/near the rock of the same age) • Decay of K  Ca + Ar • Isotopes of Potassium are K-39, k-40, K-41. (diff number of neutrons, same protons) • K-40 is radioactive and decays into Ar-40 + Ca-40. • Decay is slow but constant • Comparing K-40 to Ar-40 can provide an age for a rock. • Used to date rocks 100,000 years and above • Dates before 100,000 produce to little Ar-40 (0.0053%) • Half life: time taken for any given quantity of a radioactive substance to half • K  Ar Half life = 1.25 x 109 years. (1,250,000,000)

  7. Absolute Dating • Carbon-14 / Radiocarbon Dating • Dating of actual organic sample (must contain Carbon) • Requires minimum of 3g organic material • Used on samples up to 60,000 years old • Decay of radioisotope C-14 into Nitrogen • C-14 is produced in upper atmosphere by cosmic radiation converting it from Nitrogen. • In atmosphere ratio of C-14 to C-12 is 1: 1012 (one trillion) • Green plants use atmospheric CO2 to photosynthesise, ratio is maintained • When animal eats plant, ratio is maintained • Animal dies, C-14 continues to decay • Comparing C-14 to C-12 can provide an age for a sample.

  8. Absolute Dating • Carbon-14 / Radiocarbon Dating

  9. Absolute Dating • Carbon-14 / Radiocarbon Dating • Half life: time taken for any given quantity of a radioactive substance to half • Ratio between C-14 to C-12 is 1:1012 • Ratio decline to 0.5:1012 after 5730+40 years. • Ratio of N-14 to C-14 in atmosphere appears to fluctuate so results may vary

  10. Absolute Dating • Accelerator Mass Spectrometry (AMS) Radio carbon Dating • Used on samples as small as 100 micrograms. • Breaks sample into atoms so they can be individually accounted for. • E.g. cave paintings

  11. Absolute Dating • Dendrochronology (Tree ring Dating) • Each ring on the surface of a cut tree trunk represents one years growth (due to seasonal changes) • Width of ring demonstrates how favourable the period of growth was. • Living trees can be dated by drilling a small core from the trunk and counting the rings • Marker rings: rings which reveal years of exceptional growth • Correlation of marker rings allows different samples to be compared and dated. • E.g Bristle cone pine (US): living trees dated at over 4500 years old • Correlation of marker rings in dead pines date them back as early as 8600 years • Used up to 9000 years.

  12. Absolute Dating Dendrochronology (Tree ring Dating)

  13. Relative Dating • Stratigraphy • Study of layers (strata) • Principle of superposition: layers at the top are younger than those at the bottom • Distortions of Earth’s crust may occur  turn layers upside down • Specimens may have been deliberately buried (younger than surrounding Earth) • Correlation of rock strata: matching layers from different areas • Index fossils: fossils which were widely distributed and only existed for a brief period of time

  14. Relative Dating • Stratigraphy

  15. Relative Dating • Fossilised pollen grains • Indicates amount and type of vegetation existing at specific time. • Can be used to support or refute relative dates from other sources • Fluorine Dating • When bone is left in soil, F- from water, diffuse into bone. (depends on original F- level in soil) • Older fossils will contain more Fluorine ions • Fossils from same specimen should have similar Fluorine levels.

  16. Geological Time scale • Erasperiodsepochs

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