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B14.2 Fossils and Extinction

GCSE Combined Science · AQA · Chapter B14: Genetics and Evolution

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: what the fossil record shows, why it 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 how life began on Earth
  • Describe how fossils show how much, or how little, organisms have changed
  • Describe how new predators, new diseases and new competitors can cause extinction
  • Interpret population data linked to extinction

Key words

  • Fossil record: All the fossils that have been found, and what they show about the history of life.
  • Extinct: When there are no individuals of a species still alive anywhere.
  • Endangered: A species with so few individuals left that it is at risk of becoming extinct.
  • Predator: An animal that kills and eats other animals (its prey).
  • Pathogen: A microorganism that causes disease.
  • Invasive species: A species brought into an area where it did not live before, which harms the species already there.

Gaps in the fossil record

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

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

Key fact: Because the first organisms were soft-bodied and the oldest rocks have mostly been destroyed, scientists cannot be certain how life began on Earth.

What the fossils can show

Where there are enough fossils, they can be put in order from oldest to youngest to show how a group has changed. Fossils of horse ancestors show that over about 55 million years the feet changed from several toes to a single hoof, as forests and marshes gave way to drier, open grassland where running fast on hard ground was an advantage.

Other organisms have hardly changed. Horseshoe crabs and some sharks look almost the same as fossils hundreds of millions of years old. Their environment stayed fairly stable, so there was little pressure for change.

Worked example: spotting a trend

Fossil shells of a sea snail are measured in three rock layers. Oldest layer: mean length 18 mm. Middle: 24 mm. Youngest: 30 mm.
Increase = 30 − 18 = 12 mm. Percentage increase = 12 ÷ 18 × 100 = 66.7% (3 s.f.).

Answer: shell length increased steadily over time, by about 67%

Extinction caused by other organisms

A species becomes extinct when every individual has died. Scientists estimate that over 99% of all the species that have ever lived are now extinct. Most extinct species are known only from fossils. Other living organisms can cause extinction:

  • New predators; How it causes extinction The prey has no adaptations to escape, so it is eaten faster than it can breed.; Example Brown tree snakes, carried to the island of Guam on ships, wiped out most of the island’s forest birds.
  • New diseases; How it causes extinction A new pathogen kills many individuals, because none have immunity or resistance.; Example A chytrid fungus has caused dozens of frog species to die out worldwide.
  • New, more successful competitors; How it causes extinction Another species is better at getting food, space or shelter, so the native species cannot survive and breed.; Example American signal crayfish take the food and shelter of native white-clawed crayfish in UK rivers, and also spread a disease to them.

Worked example: percentage decrease

The graph shows 3550 birds in 1984 and 700 birds in 1996.
Decrease = 3550 − 700 = 2850. Percentage decrease = 2850 ÷ 3550 × 100 = 80.3% (3 s.f.).

Answer: about an 80% decrease in 12 years

Exam tip: Introduced species often succeed because the native species have no adaptations to them: no way to escape, no immunity, or no way to compete.

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 endangered (very few left) with extinct (none left anywhere).
  • Saying fossils prove every step of evolution. The record is incomplete, so scientists must interpret the evidence.
  • Dividing by the new value in a percentage decrease. 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: The first organisms were soft-bodied and left few fossils, and many early rocks have been destroyed, No fossils older than 1 million years have ever been found, The first organisms all had shells that dissolved in sea water, Scientists have only searched for fossils in Europe.)

Answer: The first organisms were soft-bodied and left few fossils, and many early rocks have been destroyed

Working: Early life forms were soft-bodied, so they rarely fossilised, and any traces have mostly been destroyed by geological activity. So there is very little evidence about how life began.

Question 2. True or false? The fossil record shows that some organisms, such as horseshoe crabs, have changed very little over hundreds of millions of years.

Answer: True

Working: True. Their environment has stayed fairly stable and they were already well adapted, so there has been little change.

Question 3. The diagram shows fossil feet of four horse ancestors, P to S, from rocks of different ages. What do these fossils suggest? (Choose from: Over millions of years, the number of toes gradually decreased until there was a single hoof, Horses with one hoof lived before horses with four toes, Horse feet have not changed for 55 million years, Each horse grew extra toes during its lifetime.)

Answer: Over millions of years, the number of toes gradually decreased until there was a single hoof

Working: The oldest foot (P, 55 million years) has four toes; later ones have three, then side toes become smaller, and the youngest (S) has a single hoof. This gradual change is evidence for evolution.

Related lessons

Continue in the full course

This lesson is section B14.2 (AGCSC1402) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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