✓ Revise Right course
A fossil is the preserved remains or traces of organisms that lived millions of years ago, found in rocks. Free notes with key terms and 12 questions answered.
What are fossils and how are they formed? is part of The development of understanding of genetics and evolution in GCSE AQA Higher Biology.
4 min
Reading time
27
Lessons in this topic
GCSE AQA Higher
Pathway
Reviewed by Revise Right
Content review
A fossil is the preserved remains or traces of organisms that lived millions of years ago, found in rocks. Free notes with key terms and 12 questions answered.
AQA GCSE Biology mock draft, 26–31 Oct. Draft coverage: Biology Paper 1 (Higher tier). Reviewed for an original-practice sitting; check student availability before registering. Marking and results will follow later, after separate approval. Eligible new accounts receive a five-day Pro trial when signup email verification completes. Previous trials keep their original expiry. After expiry, new papers need Pro unless you have an explicitly confirmed free sitting offer for this event. That offer adds no Pro time or AI learning. Saved attempts and released results remain readable. No card or automatic charge.
Use this lesson to stay inside The development of understanding of genetics and evolution while connecting the detail back to the wider Inheritance, variation and evolution topic.
Read the core explanations from What are fossils and how are they formed? before testing yourself from memory.
Imagine finding a perfect dinosaur skeleton buried in rock, or discovering ancient footprints preserved in stone. These are fossils - windows into life from millions of years ago.
Fossils are incredibly rare. Think about it - when you see a dead bird or mouse, what normally happens? Bacteria break it down completely within weeks. For a fossil to form, this normal decay process must be stopped or slowed down dramatically.
Trap: Many students think fossils are just old bones. Actually, fossils can be shells, leaves, footprints, or even preserved poo (called coprolites). They're any evidence of past life preserved in rock.
There are three main ways organisms can become fossils. Each requires special conditions that prevent normal decay from destroying the remains completely.
Method 1: No Decay ConditionsSometimes organisms are preserved because one or more conditions needed for decay are missing. Bacteria need oxygen, warmth, and moisture to break down dead organisms. Remove these, and decay stops.
Examples include insects trapped in amber (no oxygen), mammoths frozen in ice (too cold), or organisms buried in acidic peat bogs (wrong pH for bacteria).
Method 2: MineralisationHere's where it gets clever. As the organism slowly decays, minerals from surrounding water seep in and replace the original material bit by bit. The shape stays the same, but it's now made of rock instead of bone or wood.
This is how we get petrified wood - the wood fibres are replaced by minerals like quartz, creating a perfect stone copy of the original tree.
Method 3: Trace FossilsSometimes the organism itself doesn't survive, but evidence of its activity does. Footprints in mud, burrows in sand, or root traces in soil can be preserved when sediment fills them and hardens into rock.
Fossil formation needs a perfect storm of conditions. This explains why fossils are so rare and why our record of past life has huge gaps.
Worked example: The Fossil Formation Process
Trace how a dead fish might become a fossil over millions of years.
Answer: A perfect fish-shaped fossil preserved in sedimentary rock
Most organisms never make it past step 1. They're eaten, decay completely, or are destroyed by geological processes. Only organisms with hard parts (shells, bones, teeth) in the right environment have a chance.
Fossils are like a library with most of the books missing. The record is biased towards organisms with hard parts, those that lived in environments where burial was likely, and those from more recent time periods.
Soft-bodied organisms like jellyfish rarely fossilise. Mountain-dwelling organisms are less likely to be buried than those living near rivers or seas. This means our fossil record gives us a skewed view of past life.
Despite these limitations, fossils provide crucial evidence for evolution. They show us how life has changed over time, reveal extinct species, and help us understand ancient environments.
Fossil questions often test your understanding of the formation process and why fossils are rare. Here's how to tackle them successfully.
Command word focus: 'Explain' questions want you to give reasons. Don't just describe what happens - explain why it happens. 'Suggest' questions about fossil rarity should mention the specific conditions needed and why they're uncommon.
Remember, fossils are our best evidence for life in the distant past. They may be incomplete and biased, but they're the only direct record we have of organisms that lived millions of years before humans existed.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
A fossil is the preserved remains or traces of organisms that lived millions of years ago, found in rocks. Fossils are not just old bones: they can be shells, leaves, footprints or even preserved poo, called coprolites.
Fossils form in three main ways: from parts of organisms that have not decayed because conditions for decay were absent, through mineralisation where minerals gradually replace the original material, and as preserved traces such as footprints, burrows and root traces that fill with sediment and harden into rock.
Petrified wood is a fossil formed by mineralisation, where the wood fibres are replaced by minerals like quartz, creating a perfect stone copy of the original tree. The shape stays the same, but it is now made of rock.
Coprolites are fossilised poo, preserved in rock. They show that fossils are not just old bones but any evidence of past life, including shells, leaves and footprints.
A trace fossil is evidence of an organism's activity, such as footprints, burrows or faeces, rather than its body remains. Trace fossils form when sediment fills marks like footprints in mud and hardens into rock.
Fossils are rare because bacteria normally break down dead organisms completely, so fossil formation needs decay to be stopped or slowed, usually by quick burial. Most organisms are eaten, decay completely or are destroyed by geological processes, and only those with hard parts in the right environment have a chance.
The fossil record is biased towards organisms with hard parts such as shells, bones and teeth, those living where burial was likely, such as near rivers or seas, and those from more recent times. Soft-bodied organisms like jellyfish rarely fossilise.
Mineralisation is a fossilisation process where minerals gradually replace original organic material. As the organism slowly decays, minerals from surrounding water seep in and replace it bit by bit, so the shape stays the same but it is now rock; petrified wood forms this way.
Organisms can be preserved when one or more conditions needed for decay are missing, because bacteria need oxygen, warmth and moisture. Examples include insects trapped in amber (no oxygen), mammoths frozen in ice (too cold) and organisms buried in acidic peat bogs (wrong pH for bacteria).
Fossils form in three ways: from parts of organisms that have not decayed because conditions for decay are absent, by mineralisation where minerals gradually replace the original material, and as trace fossils, such as footprints, burrows and root traces preserved in rock.
Bacteria need oxygen, warmth and moisture to break down dead organisms. If one of these is missing, decay stops, which is how organisms can be preserved in amber (no oxygen), ice (too cold) or acidic peat bogs.
A dead fish becomes a fossil when it sinks and is quickly buried by mud and sand, so oxygen is cut off and decay slows. More sediment builds up over thousands of years and compresses into rock, minerals replace the bones and scales, and erosion or excavation later exposes the fossil.