15.6 Fossils and Extinction

A free GCSE Biology lesson for AQA (8461) from OBA School: why the fossil record has gaps, and how other organisms can cause extinction. 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

• Explain why the fossil record is incomplete
• Explain why scientists cannot be certain about how life began on Earth
• Describe what fossils can show about how organisms have changed
• Describe how new predators, new diseases and new competitors can cause extinction
• Interpret population data linked to extinction

Key words

• Extinction: When there are no individuals of a species still alive anywhere.
• Fossil record: All the fossils that have been found, and what they show about the history of life.
• Predator: An animal that kills and eats other animals (its prey).
• Competition: When organisms need the same limited resource, such as food, space or nesting sites.
• Pathogen: A microorganism that causes disease.
• Introduced species: A species brought by humans (deliberately or by accident) to an area where it did not live before.

Gaps in the fossil record

The fossil record is very useful, but it is incomplete. Scientists have fossils for only a tiny fraction of all the organisms that ever lived.

• Many early forms of life were soft-bodied, so they left few traces behind. Soft tissue decays before it can fossilise.
• Fossils form only in rare conditions, so most organisms decayed completely.
• Many fossils that did form have been destroyed by geological activity, such as rock being melted, crushed or worn away.
• Many fossils have simply not been found yet.

Key fact: Because the earliest life was soft-bodied and old rocks have been destroyed, scientists cannot be certain how life began on Earth.

What the fossils can show

Where there are enough fossils, scientists can line them up from oldest to youngest and see how a group changed. Some groups have changed a great deal, while others, such as some sharks and horseshoe crabs, have changed very little for hundreds of millions of years. Their environment stayed fairly stable, so there was little pressure for change.

Worked example: spotting a trend

Fossil teeth of an extinct mammal are measured from three rock layers.
Oldest layer: mean tooth length 12 mm. Middle: 17 mm. Youngest: 23 mm.
Increase from oldest to youngest = 23 − 12 = 11 mm, which is 11 ÷ 12 × 100 = 92% (2 s.f.).

Answer: tooth length increased steadily, perhaps as the animals ate tougher plants

Extinction caused by other organisms

A species becomes extinct when every individual has died. Changes caused by other living things include:

• New predators; How it causes extinction The prey has no adaptations to escape or defend itself, so it is eaten faster than it can breed.; Example The flightless dodo on Mauritius was hunted by people, and introduced rats and pigs ate its eggs. It was extinct by the late 1600s.
• New diseases; How it causes extinction A new pathogen kills many individuals, because none have immunity or resistance.; Example A chytrid fungus disease has wiped out dozens of frog species worldwide.
• New, more successful competitors; How it causes extinction Another species is better at getting food, space or shelter, so the native species cannot get enough to survive and breed.; Example Grey squirrels, introduced to Britain from North America, out-compete red squirrels and spread a squirrel pox virus.

Worked example: percentage decrease

A ground-nesting seabird on an island had 4800 breeding pairs in 1990. Rats arrived in 1994. In 2002 there were 900 pairs.
Decrease = 4800 − 900 = 3900. Percentage decrease = 3900 ÷ 4800 × 100 = 81.25%.

Answer: about 81% decrease in 12 years

Common mistakes

• Saying there are gaps because “scientists have not looked hard enough”. The main reasons are soft bodies, rare fossilisation and destruction of rocks.
• Confusing a species becoming rare (endangered) with extinction. Extinct means no individuals are left anywhere.
• Forgetting that introduced species often succeed because native species have no adaptations to them.
• Working out a percentage decrease by dividing by the new value. Always divide by the original value.

Revision questions

Try these, then check your answers below each one.

Question 1. Why can scientists not be certain how life began on Earth? (Choose from: Early life forms were soft-bodied and left few traces, Scientists have never looked for very old fossils, All early organisms had hard shells that dissolved, Life began too recently to leave fossils.)

Answer: Early life forms were soft-bodied and left few traces

Working: Early life was soft-bodied, so left few traces, and many old rocks have been destroyed by geological activity.

Question 2. True or false? A species is extinct when only a few individuals are left.

Answer: False

Working: False. Extinct means no individuals are left anywhere. With only a few left it is endangered.

Question 3. Which of these is not a reason for gaps in the fossil record? (Choose from: All organisms that ever lived had hard skeletons, Many organisms were soft-bodied, Many fossils were destroyed by geological activity, Many fossils have not yet been found.)

Answer: All organisms that ever lived had hard skeletons

Working: Many organisms were soft-bodied; that is a reason for gaps, not against them.

Related lessons

• Previous lesson: 15.5 Evidence of Evolution
• Next lesson: 15.7 More About 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 15.6 (AGCBH1506) 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.