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 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