AQA GCSE Biology Foundation (8461), Foundation tier · Inheritance, variation and evolution › Understanding of genetics and evolution
Practise Fossils. 6 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
What fossils are, the three ways they form, why the fossil record is incomplete, and what fossils tell us about how life developed. Questions include 'describe how fossils form', 'explain why scientists cannot be certain how life began' and interpreting evolutionary trees.
Key facts
Fossil = the remains of an organism from millions of years ago, found in rock
Formed by: no decay (conditions for decay absent); replacement by minerals; preserved traces
Traces: footprints, burrows, rootlet traces
Decay needs warmth, moisture and oxygen; remove one and remains may be preserved
Record incomplete: early life was soft-bodied; geological activity destroyed traces
So scientists cannot be certain how life began
Fossils show how much or how little organisms have changed
Notes
How fossils form
Fossils are the 'remains' of organisms from millions of years ago, which are found in rocks.
They may form from parts of organisms that have not decayed, because one or more of the conditions needed for decay are absent, e.g. too cold, too little oxygen or too little water. Examples: animals frozen in ice, or insects trapped in amber.
They may form when parts of the organism are replaced by minerals as they decay. Hard parts such as bones, teeth and shells are often preserved this way.
They may be preserved traces of organisms, such as footprints, burrows and rootlet traces.
Why the fossil record is incomplete
Many early forms of life were soft-bodied, so they have left few traces behind.
What traces there were have been mainly destroyed by geological activity (for example rocks being worn away, folded or melted).
This is why scientists cannot be certain how life began on Earth.
Also, most dead organisms decay completely, so only a tiny fraction ever become fossils.
What fossils tell us
diagram
We can learn from fossils how much or how little different organisms have changed as life developed on Earth.
Fossil data for extinct organisms is used with data from living organisms to build evolutionary trees.
On an evolutionary tree, each branch point is a common ancestor. Organisms that share a more recent common ancestor are more closely related.
Evolutionary tree: the more recent the common ancestor, the more closely related the species.
How to answer each type of question
Describe how fossils form
2 to 3 marksGrade 4
Give the three ways: no decay, replacement by minerals, preserved traces.
For 'no decay', say which condition was missing (e.g. oxygen).
Example. Describe three ways in which fossils can be formed. [3 marks]
Show the model answerHide the model answer
Parts of an organism do not decay because one or more of the conditions needed for decay are absent, e.g. no oxygen (1). Parts of the organism are replaced by minerals as they decay (1). Traces of the organism, such as footprints or burrows, are preserved (1).
Explain why a specimen did not decay
2 marksGrade 5
Name the missing condition: warmth, moisture or oxygen.
Link it to the decomposers: microorganisms could not grow or break down the tissue.
Example. A woolly mammoth was found frozen in ice. Its skin and hair were preserved. Explain why the mammoth had not decayed. [2 marks]
Show the model answerHide the model answer
The temperature was too low (1), so decomposers / microorganisms could not grow and break down the tissue (1).
Don’t lose marks
Saying fossils are only bones. Footprints, burrows and rootlet traces are fossils too.
Saying the conditions for decay were 'bad'. Name the missing condition (oxygen, warmth or moisture).
Saying early life left few fossils because it was 'too small'. The reason is that it was soft-bodied, and traces were destroyed by geological activity.
More tips
Memory tricks
Three ways to make a fossil: decay stopped, tissue swapped for minerals, trace stamped into rock (footprints).
Decay needs warmth, moisture and oxygen; take one away and remains can last.
Most recent common ancestor = most closely related.
Exam technique
In 'describe how fossils form', give three different methods, not three examples of the same method.
For evolutionary trees, read branch points against the time axis and use the numbers in your answer.
Link fossils to evidence for evolution: they show how organisms have changed over time.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Define a fossilThe remains of organisms from millions of years ago, which are found in rocks.
Grade 4
Describe three ways fossils formParts that have not decayed, parts replaced by minerals, and preserved traces such as footprints.
Grade 5
Explain why some remains do not decayOne or more of the conditions needed for decay (such as oxygen, warmth or moisture) are absent.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Some fossils are preserved traces of organisms rather than parts of the organisms themselves. Give one example of a preserved trace of an organism.
Footprints, burrows or rootlet traces
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Fossils are the ‘remains’ of organisms from millions of years ago, which are found in rocks.
(a) Which two of the following are ways in which fossils can form? Tick (✓) two boxes.[2]
An organism is completely broken down by decomposers
Parts of an organism are replaced by minerals as they decay
Parts of an organism do not decay because one or more of the conditions needed for decay are absent
The organism’s DNA is copied into the rock
(b) Some fossils are preserved traces of organisms rather than parts of the organisms themselves. Give one example of a preserved trace of an organism.[1]
(c) Scientists cannot be certain about how life began on Earth. Give two reasons why.[2]
Show the answer and mark scheme
(a)Answer: Parts of an organism are replaced by minerals as they decay; Parts of an organism do not decay because one or more of the conditions needed for decay are absent
(b)Answer: Footprints, burrows or rootlet traces
footprints / burrows / rootlet traces
(c)
many early forms of life were soft-bodied, so they left few traces / fossils
(most of) the traces that did form have been destroyed by geological activity
Question 2Medium7 marks
Scientists found three fossils at different sites:
fossil A: an insect preserved in amber (hardened tree resin)
fossil B: the bones of a mammal that have turned to rock
fossil C: the footprints of a dinosaur in mudstone.
(a) The insect in fossil A did not decay. Explain why.[2]
(b) Describe how fossil B was formed.[2]
(c) Fossil C is a trace fossil. Suggest one thing scientists could find out about the dinosaur from its footprints.[1]
(d) Fossils can show how much or how little organisms have changed as life developed on Earth. Explain how.[2]
Show the answer and mark scheme
(a)
the amber prevented oxygen / air / water reaching the insect
(so) microorganisms / decomposers could not survive / decay the insect / the conditions needed for decay were absent
(b)
the bones were buried (in sediment) and slowly decayed
(and were) replaced by minerals (which formed rock)
(c)
its size / mass
whether it walked on two legs or four legs
how fast it moved (from the length of its stride)
whether it lived in groups (from several sets of tracks)
(d)
compare fossils from rocks of different ages
(with each other or with) living species, to see how similar / different their structures are