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Geological time scale

measures vast amounts of time, from the creation of Earth to present time, shows the evolution of Earth


Relative age vs absolute age and techniques

Relative age- age based on chronological order of existence
• Stratigraphy- explains the age of a rock based on its superposition, and the age of fossils- found in lower strata are older than those found in overlying strata
• Law of fossil succession- uses index fossils for comparison between two sites (date rock layers relative to another, same fossils and characteristics can be correlated between two different locations)
Absolute age- exact age
• Radiometric dating technique (igneous only)- dates igneous rocks based on radioactive elements (half-life time taken for half the original radioactive isotope to decay) to determine the age of the rock layer in between two layers of volcanic ash
○ K-Ar dating determines age of igneous rocks through the amount of K-40 (isotope) found (half-life: 1.3 billion years )
○ C-14 dating determines age of organic matter (has C-12 and C-14, but once it dies C-14 decays to N-14, half-life: 5730 years, >6000 years)


Pleistocene epoch

Pleistocene epoch- megafauna
Cretaceous period- dinosaurs, large marine reptiles, some fur covered mammals
Ordovician period- small plants, marine invertebrates and animals (no terrestrial), first vertebrates
Archaean eon- atmosphere is anoxic, devoid of plants and animals, no ozone layer, prokaryotic microbes (stromatolites- layered rocks that build up when cyanobacteria trap sediments)


Types of fossils

macro and micro

Physical fossils (mineralised- bones, teeth or leaves, where its organic material has been replaced by minerals), trace fossils (footprints and burrows), and biosignatures (physical or chemical sign preserved in rocks/sediment that can be inferred to have cellular or metabolic origins (ratio of bio-essential elements that are similar to which microbes produced)

Macroscopic (seen by the naked eye) and microscopic (electron microscope) fossils


Steps in the fossilisation

• Rapid burial in sediment (sea floor, lake bed), burial in anoxic conditions, very cold environments (preserve through freezing)
• Covered by more sediment burying it deeper, compresses original sediment layer so that it becomes rock
• Exchange of minerals, carbon or minerals replace chitin, cellulose (organic material)
• Needs time and has to remain undisturbed


Mould vs cast

A mould is formed when the dead organism preserved in rock decays and leaves an impression, whereas a cast is formed later on, when sediment fills the mould (fine-grained sediment) allowing for a 3D representation of the organism


Evidence for biological change over time
-fossil record
-structural morphology
-developmental biology

Fossil record
- Organisms appear in a consistent order, the law of fossil succession (prokaryotes before eukaryotes etc.) ancestral species appear before the species that descend from them
- Transitional fossils relate an ancestral group with its descendants (archaeopteryx)
- Incomplete because not all fossils have been found, not every organism would have undergone fossilisation, some may have been damaged
• Structural morphology
- Mammals have a similar number of bones arranged in the same pattern (even if they are used for different functions) homologous- same structure, but have evolved to have a different function selection pressures
- Vestigial organs (reduced in use through evolution, whale hind limbs)
• Biogeography
- Native species in different isolated regions will be distinctive (evolved from different ancestor)
- Native species will be more similar to ancestors that lived in that region in the past than to modern species in a similar environment
- Same ecological niche but different isolated regions-> different species
Species living in an isolated region will be more similar to those organisms in that region, those that lived in the past than those who live in different areas (even if niche is similar)
• Developmental biology
- Similarity between the developmental stages of related organisms (embryos of vertebrate (pharyngeal arches and tails)


Transitional fossils

Transition fossils show an organism which has features of a modern group and those of an ancestral group


Divergent evolution (adaptive radiation)

Divergent evolution- similar ancestor, species become more dissimilar over time, different selection pressures
Adaptive radiation- evolution of varieties of species from one common ancestor, each having adapted (finches and marsupials)


Convergent evolution

Convergent- from two different ancestors, but similar in present time due to a similar way of life and selection pressures
Shark (fish) and dolphin (mammal) possess similar features because of the natural environment they live in



two species reciprocally affect each other's evolution



Global extinction- loss of taxonomic group
Local extinction- loss of taxonomic group in one region, but still alive somewhere else
Mass extinction- period during Earth's history characterised by the abnormal loss of high numbers of species