15.5 Evidence of Evolution

A free GCSE Biology lesson for AQA (8461) from OBA School: what fossils are, the three ways they form, and how rock layers show their age. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Define a fossil
• Describe the three ways in which fossils can be formed
• Explain why some remains do not decay
• Use rock layers and simple calculations to estimate the age of fossils
• Explain how fossils give evidence for evolution

Key words

• Fossil: The remains of organisms from millions of years ago, which are found in rocks.
• Decay: The breakdown of dead material by microorganisms. It needs warmth, moisture and oxygen.
• Mineral fossil: A fossil formed when the parts of an organism are slowly replaced by minerals as they decay, forming a rock copy.
• Trace fossil: A preserved sign of an organism, such as a footprint, burrow or the track left by a root (rootlet trace).
• Sediment: Small particles such as sand, mud and silt that settle in layers and slowly turn into rock.
• Fossil record: All the fossils that have been found, and what they tell us about the history of life.

What is a fossil?

Fossils are the remains of organisms from millions of years ago, which are found in rocks. They show what organisms living long ago looked like and how they have changed over time.

Most dead organisms decay completely. Decay is caused by microorganisms, which need warmth, moisture and oxygen. A fossil forms only when something stops or slows down decay, or when the remains are quickly buried.

Three ways fossils form

• 1. Parts that have not decayed; What happens One or more of the conditions needed for decay is missing, so the organism is preserved.; Examples Mammoths frozen in permafrost (too cold); insects trapped in amber (no oxygen or water); bodies in acidic peat bogs
• 2. Parts replaced by minerals; What happens Hard parts such as bones, teeth and shells are buried in sediment. As they slowly decay, minerals from water replace them and harden into rock.; Examples Most dinosaur bones; ammonite shells; fossil wood
• 3. Preserved traces; What happens Marks left by an organism are covered by sediment that sets into rock.; Examples Footprints, burrows, rootlet traces, droppings

Worked example: explaining preservation

A frog fell into sticky resin on a pine tree 30 million years ago. The resin hardened into amber.
Inside the amber there was no oxygen and no water, so decomposers could not grow or respire.

Answer: the frog did not decay, so its body was preserved

Exam tip: For “parts that have not decayed”, always name the missing condition: oxygen, water / moisture, or a warm enough temperature.

Reading the rocks

Sediment builds up in layers over millions of years. In undisturbed rock, the deepest layers were laid down first, so they are usually the oldest. Fossils in deep layers are generally older than fossils in layers above them. Scientists can also date rocks using radioactive elements.

Worked example: estimating age from sediment

In a lake, mud settles at a rate of about 0.4 mm per year. A fossil leaf is found 4 m below the lake bed.
4 m = 4000 mm. Age = 4000 ÷ 0.4 = 10 000 years.

Answer: about 10 000 years old

Fossils as evidence for evolution

• Comparing fossils from layers of different ages shows how much (or how little) organisms have changed over time.
• Simple organisms appear in the oldest rocks; more complex ones appear later.
• Fossils show species that are now extinct, and in-between forms linking groups of organisms.
• Fossils of the same group from different ages can be put in order to show gradual change in a feature, such as body size or the number of toes.

Common mistakes

• Saying fossils are only bones. Traces such as footprints and burrows, and whole bodies in ice or amber, are fossils too.
• Saying decay stops because “there are no bacteria”. Decay stops because the conditions microorganisms need (oxygen, water, warmth) are missing.
• Thinking a mineral fossil contains the original bone. In mineralised fossils the original material has been replaced by rock.
• Forgetting units when calculating ages from sediment rates – convert metres to millimetres first.

Revision questions

Try these, then check your answers below each one.

Question 1. What is a fossil? (Choose from: The remains of an organism from millions of years ago, found in rocks, Any dead animal found in soil, A rock that looks like a living thing, The skeleton of an animal that died recently.)

Answer: The remains of an organism from millions of years ago, found in rocks

Working: Fossils are the remains (or traces) of organisms from millions of years ago, found in rocks.

Question 2. True or false? In undisturbed rock, the deeper layers are usually older than the layers above them.

Answer: True

Working: True. Sediment builds up from the bottom, so lower layers were laid down first.

Question 3. A mammoth was found frozen in permafrost with its skin still present. Why had it not decayed? (Choose from: It was too cold for decomposers to work, Mammoths contain no microorganisms, The ice replaced its body with minerals, There was too much oxygen in the ice.)

Answer: It was too cold for decomposers to work

Working: Freezing stops decomposers growing and their enzymes working, so decay does not happen.

Related lessons

• Previous lesson: 15.4 Evolution and Speciation
• Next lesson: 15.6 Fossils and Extinction
• In this chapter: 15.1 The History of Genetics
• In this chapter: 15.2 Theories of Evolution
• In this chapter: 15.3 Accepting Darwin's Ideas

Continue in the full course

This lesson is section 15.5 (AGCBH1505) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.