Fossils (AQA GCSE Biology): Revision Notes
Fossils
Fossils are amazing remains of living things from millions of years ago. They give us important evidence for how life on Earth has changed over time through evolution by natural selection.
What are fossils?
Fossils are the preserved remains of organisms found in rocks. These organisms lived millions of years ago, and their remains help scientists understand how life has evolved.
The fossil record provides crucial evidence for the theory of evolution by natural selection. It shows us how different species have changed and developed over vast periods of time.
How do fossils form?
Fossils can form in three main ways:
1. Traces are preserved
- Footprints, burrows, and other marks left by organisms get preserved in rock
Worked Example: Trace Fossils
Dinosaur footprints found in Arizona were made in soft mud around 200 million years ago. The mud hardened into rock, preserving the exact shape and size of the dinosaur's feet, allowing scientists to determine the animal's size, walking speed, and behaviour.
2. Hard parts are replaced by minerals
- Bones, teeth, and shells slowly get replaced by minerals as the organism decays
- This process is called mineralisation
3. Soft tissues don't decay
- Sometimes muscles, leaves, and other soft parts are preserved
- This happens when decay cannot occur due to missing conditions
- Decay needs oxygen and water, so fossils form better without these or at low temperatures
Why is the fossil record incomplete?
Scientists cannot be completely certain about early life on Earth because the fossil record has gaps.
Why the Fossil Record Has Gaps:
The fossil record is incomplete because many organisms never had the chance to become fossils. This creates significant gaps in our understanding of early life on Earth.
This happens because:
- Many early organisms had soft bodies that didn't leave traces behind
- Geological activity has destroyed many fossil traces over millions of years
- Dead organisms often decay or get eaten before fossilisation can begin
- Smaller, more delicate organisms are less likely to survive long enough to be discovered
Extinction
Extinction occurs when there are no living individuals of a species left anywhere on Earth. Once a species is extinct, it's gone forever.
Causes of extinction
Several factors can cause species to become extinct:
Environmental changes over geological time
- Organisms cannot adapt quickly enough to major environmental changes
- These changes happen over millions of years
Single catastrophic events
- Asteroid collisions with Earth
- Massive volcanic eruptions
- These cause sudden, dramatic changes that species cannot survive
New threats
- New diseases that kill all individuals
- New predators that eat all individuals
- New competitors that take all the food or territory
Why vertebrates have better fossil records
Animals with bones (vertebrates) have much better fossil records than soft-bodied animals because their hard structures are more likely to be preserved over millions of years.
This is because:
- Bones and teeth are hard and more likely to be preserved
- Soft tissues decay quickly and rarely form fossils
- Hard parts can be more easily replaced by minerals during fossilisation
Key Points to Remember:
- Fossils are remains of organisms from millions of years ago found in rocks
- Fossils form in three ways: traces preserved, hard parts replaced by minerals, or soft tissues don't decay
- The fossil record gives us evidence for evolution by natural selection
- Extinction happens when no individuals of a species are left alive
- Extinction can be caused by environmental changes, catastrophic events, or new threats like diseases, predators, or competitors