Fossil Hunters
Dig into deep time: the three ways fossils form, why the record has gaps we can never fully fill, and what wipes a whole species off the Earth.
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Fossil Hunters
Fossils are messages from millions of years ago. The closest thing we have to a photo album of life on Earth. This module digs into how those messages survive, why so many are missing, and what makes an entire species vanish. (All of 4.6.3 is **Biology-only**.)
Evidence in the rocks
A **fossil** is the remains or traces of an organism from millions of years ago, preserved in rock. Fossils are a major piece of **evidence for evolution**: by comparing fossils of different ages, scientists can see **how organisms have gradually changed** over enormous spans of time.
Three ways to make a fossil
Fossils form by three different routes. An exam question usually hands you an example and asks which route it took, so learn each route by its examples.
Match how it formed
- A mosquito trapped in amber
- A mammoth frozen in Siberian ice
- A tree trunk turned to stone
- A dinosaur bone that is now rock-like mineral
- A dinosaur footprint in rock
- A worm burrow preserved in mudstone
- Preserved because it did not decay
- Hard parts replaced by minerals
- A trace or impression left behind
Why amber preserves
An insect sealed in tree resin (amber) does not rot away. Why not?
- The resin seals out the conditions decay microbes need, such as oxygen and moisture
- Its body is quickly replaced by minerals
- Amber is always frozen solid
- It only leaves an impression, not the body
What decay needs
Decay microbes need warmth, moisture and oxygen. A mammoth frozen in ice does not decay because there is no _____. A body buried in deep waterlogged mud may not decay because there is no _____. Whenever decay is prevented like this, the remains can last long enough to become a _____.
The missing pages
The **fossil record has big gaps**, especially for the earliest life. Two reasons: • Many early organisms were **soft-bodied**, so they decayed before they could fossilise, leaving few traces. • Many fossils that did form have since been **destroyed** by geological activity (heat, pressure, movement of rock). Because of this, scientists **cannot be certain** exactly how life on Earth first began.
Find the bogus reason
A student lists reasons for the gaps in the fossil record. Tap the ONE that is NOT a real reason.
- The record has gaps because
- many early organisms were soft-bodied and decayed before they could fossilise
- , because
- many fossils that did form were later destroyed by heat, pressure and movement of rock
- , and because
- scientists have simply not searched thoroughly enough in the right places
- .
What fossils can tell us
Pick the TWO things the fossil record CAN show scientists.
- How organisms have gradually changed over very long spans of time
- That some species which once lived are no longer alive today
- Exactly how life on Earth first began
- The complete DNA sequence of every extinct organism
- Which species will become extinct over the next century
- A complete record of every soft-bodied organism that ever lived
When a species ends
**Extinction** is when there are **no individuals of a species left alive** anywhere. AQA names four kinds of cause, and an exam question usually describes one and asks you to name it.
Spot the cause
- Humans introduce cats to an island of ground-nesting birds
- A fungal infection sweeps through frogs with no resistance to it
- An asteroid strike blocks out sunlight for years
- Huge volcanic eruptions darken the sky worldwide
- The climate cools slowly over tens of thousands of years
- A newly arrived species eats the same food but gathers it faster
- A new predator
- A new disease
- A new competitor
- A catastrophic event
- Environmental change
Trace an extinction
A giant asteroid strikes Earth, throwing up dust that blocks sunlight for years. Follow the knock-on effects.
- With little sunlight, plants die off. What happens to the plant-eating animals?
- With far fewer plant-eaters left, what happens to the predators that hunted them?
- One species cannot adapt quickly enough, and no individuals are left alive anywhere. What is that called?
- Which of the four causes of extinction does this whole event count as?
Reading the record
Millions of years later, palaeontologists find plenty of fossils from just before that event but very few from just after it. Which explanation fits best?
- Far fewer organisms were alive to fossilise, and conditions for fossilisation were poor
- Fossilisation stopped working as a process once the asteroid had struck
- The rocks from after the event have never been examined by anyone
- Every organism alive after the event happened to be soft-bodied
Write it up
Explain why the fossil record is incomplete, and describe two different causes of extinction.
- Give BOTH reasons for the gaps: what early organisms were like, and what happened to fossils that did form.
- Say what that means for what scientists can claim about how life on Earth began.
- Name two causes of extinction from different categories, not two examples of the same one.
- For each cause, say how it actually removes the species, not just that it happened.
- Define extinction precisely: no individuals of the species left alive anywhere.