Introduction
The history of life on Earth is revealed through the fossil record and other evidence. Over millions of years, species change, adapt, and sometimes split into new species through processes such as isolation and genetic divergence.
Key Concepts
Fossil Record Evidence
- Faunal succession – fossil species appear in a predictable order, with older forms in deeper layers and more recent forms in upper layers.
- Index fossils – species that lived for a short, known period and were widespread, useful for dating.
- Transitional fossils – show intermediate forms between ancestral and modern species.
Dating Fossils
- Relative dating – compares the positions of fossils in rock strata. The deeper the layer, the older it is. This method gives an order of events but not an exact age (e.g., fossil A is older than fossil B).
- Absolute dating – uses radioactive isotopes to calculate the actual numerical age of a fossil or rock.
- Carbon-14 dating – suitable for dating organic (once-living) material up to about 50,000 years old. Living organisms constantly take in carbon (including the radioactive isotope carbon-14). When they die, the carbon-14 starts to decay at a known rate (half-life ≈ 5730 years). By measuring how much carbon-14 is left, scientists can calculate how long ago the organism died.
- Carbon-14 cannot be used for very old fossils (e.g. dinosaurs), because after about 50,000 years the carbon-14 level is so low that it is no longer detectable.
- Other isotopes with longer half-lives (e.g. Uranium-238 → Lead-206 or Potassium-40 → Argon-40) are used to date rocks and fossils millions of years old.
Speciation
Speciation is the process by which one species splits into two or more new species, usually as a result of genetic divergence over time.
- Allopatric speciation – occurs when a population is geographically isolated, causing the separated groups to evolve independently.
- Example: Galapagos finches – evolved different beak shapes in response to different food sources on each island.
- Sympatric speciation – speciation that occurs without physical separation, usually due to changes in behaviour, timing, or ecological niche.
- Example: Howea palms on Lord Howe Island – evolved on the same island but adapted to different soil types and flower at different times.
