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

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.

Four conditions are required for fossilization to occur:

- Rapid burial- Presence of hard body parts- Absence of decay organisms- Long period of stability

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

Fossil DatingAbsolute dating: actual age of specimen in years

Relative dating: age relative to other fossils (older or younger)BP: Before Present

Absolute DatingPotassium-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)

Absolute DatingCarbon-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.

Absolute DatingCarbon-14 / Radiocarbon Dating

Absolute DatingCarbon-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

Absolute DatingAccelerator 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

Absolute DatingDendrochronology (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.

Absolute DatingDendrochronology (Tree ring Dating)

Relative DatingStratigraphy• 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

Relative DatingStratigraphy

Relative DatingFossilised 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.

Geological Time scale•Eras periodsepochs