human bio unit 4 - evidence of evolution

homologous structure

same basic structure, adapted to different functions

analogous structures

different structure, same function, adapted to similar environments

vestigial structures

structures that have lost/changed its function

fossil

any preserved trace of an organism that lived long ago (e.g. bones, teeth, footprints)

4 conditions for fossils to form

1. hard tissues are present

2. absence of decay organisms

3. rapid burial

4. no disruptions

steps of fossilisation

1. death and decay

2. rapid burial

3. permineralisation

4. erosion / exposure

absolute dating

gives the actual age of a fossil (not exact) which allows scientists to establish a sequence of events that led to evolution and discover species that previously lived on earth

isotopes

same elements with different mass numbers/neutrons, they are considered “unstable” and radioactive

potassium-argon dating

measures the ratio of potassium-40 (radioactive) to argon-40 (decay product).

process of potassium-argon dating

1. AR is a noble gas. when rock is at liquid phase (lava/magma), any argon gas will escape

2. when rocks cool down, the minerals will crystallise and trap any newly formed AR-40

3. K-40 decays at a slow rate (half life ~1.25 billion years), producing more AR-40 and trapping it inside the rock

4. by comparing the amount of K-40 to the amount of trapped AR-40 , scientists can calculate when the rock last cooled & trap argon.

limitations of potassium-argon dating

• can only be used for volcanic rock

• can only date rocks older than 200,000 years

• reheating or weathering can melt the minerals/rock and let argon escape

radiocarbon dating

measures the ratio of carbon-14 to nitrogen-14

how it works

1. carbon-14 is produced in the upper atmosphere, due to cosmic radiation of nitrogen at roughly the same rate at which it decays.

2. In life, the ratio of C-14 to C-12 stays constant because we keep taking in new carbon

3. at death, C-14 starts to decay to

limitations of radiocarbon dating

• only works for 50,000-60,000 years, as older samples have too little C-14 left to measure

• only works on organic material (no rocks)

• atmospheric C-14 levels vary over time, so results will require calibration

• standard dating needs at least 3 grams of sample

relative dating

determines whether a fossil is older or younger than other samples where the rock is found. often used when absolute dating is not possible.

stratigraphy

refers to the study of layers, allowing the age of fossils to be compared

principle of superposition

sedimentary layers are deposited in a time sequence. in undisturbed rock layers:

• rocks found closer to the bottom layers = old

• rocks found closer to the top surface = young

correlation of rock strata

involves matching rock layers from different locations to establish a similar age range. if the same index fossil is found in two different places, we assume those layers are the same age.

index fossils

refers to fossils that were previously on earth for a short period. they are used to correlate strata from different locations. these fossils are easily recognisable and abundant.

limitations of stratigraphy

• folding; layers can be bent by pressure from tectonic forces

• cracks/faults in earth’s crust can shift rock layers around

• wind, water, or ice can erode rocks and wear away upper layers