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B14.1 Evidence of Evolution

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

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: why the theory of evolution is accepted, what fossils are and how they form. 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

  • Describe what the theory of evolution by natural selection says
  • Give the main kinds of evidence that support the theory
  • Describe the three ways in which fossils can form, and explain why some remains do not decay
  • Use rock layers and sediment rates to estimate the age of fossils
  • Explain how fossils such as Archaeopteryx give evidence for evolution

Key words

  • Evolution: A change in the inherited characteristics of a population over time, which may lead to new species.
  • Natural selection: Individuals best suited to their environment are more likely to survive, breed and pass on their alleles.
  • Common ancestor: An earlier species from which two or more later species evolved.
  • Fossil: The remains or traces of an organism from millions of years ago, found in rocks.
  • Decay: Breakdown of dead material by microorganisms, which need warmth, moisture and oxygen.
  • Fossil record: All the fossils that have been found, and what they show about the history of life.

The theory of evolution

The theory of evolution says that all species of living things have evolved from simple life forms that first developed more than three billion years ago. Over a very long time, natural selection has produced the huge variety of species alive today, each adapted to its own environment.

When Charles Darwin published his ideas in 1859, many people rejected them. They clashed with the belief that each species was created separately, and nobody yet knew how characteristics were inherited. Today the theory of evolution by natural selection is widely accepted, because the evidence for it has grown.

Key fact: Natural selection is the mechanism (how it happens). Evolution is the result: a change in a species over time.

What is a fossil?

Fossils are the remains of organisms from millions of years ago, which are found in rocks. Most dead organisms decay completely, because microorganisms break them down. A fossil forms only when decay is stopped or slowed, or when the remains are buried quickly.

  • 1. Parts that have not decayed; What happens One or more conditions needed for decay is missing, so the remains are preserved.; Examples Animals frozen in ice or permafrost (too cold); insects in amber (no oxygen or water); bodies in peat bogs (acidic, no oxygen)
  • 2. Parts replaced by minerals; What happens Hard parts are buried in sediment. As they slowly decay, minerals from water replace them and harden into rock with the same shape.; Examples Dinosaur bones, fish skeletons, shells, fossil wood
  • 3. Preserved traces; What happens Marks left by an organism are covered by sediment that sets into rock.; Examples Footprints, burrows, eggs, rootlet traces

Worked example: explaining preservation

A spider was trapped in sticky resin 25 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 spider did not decay, so its body was preserved

Exam tip: For remains that have not decayed, always name the missing condition: oxygen, moisture, or a warm enough temperature (or a suitable pH).

Reading the rocks

Sediment builds up in layers over millions of years. In rock that has not been folded or disturbed, the deepest layers were laid down first, so the fossils in them are usually the oldest.

Worked example: estimating age from a sediment rate

In a lake, mud settles at about 0.3 mm per year. A fossil leaf is found 2.1 m below the lake bed.
2.1 m = 2100 mm. Age = 2100 ÷ 0.3 = 7000 years.

Answer: about 7000 years old

Fossils as evidence for evolution

  • Comparing fossils from layers of different ages shows how much – or how little – organisms have changed.
  • Simple organisms are found in the oldest rocks; more complex ones appear in younger rocks.
  • Some fossils show in-between forms. Archaeopteryx had feathers and wings like a bird, but teeth, claws and a long bony tail like a reptile. This suggests that birds evolved from reptiles (dinosaurs).
  • Series of fossils, such as those of horse ancestors, show gradual changes in a feature over millions of years.

Summary: Fossils show change over time. Resistant bacteria show natural selection happening now. Genes explain how characteristics are inherited.

Common mistakes

  • Saying fossils are only bones. Footprints, burrows and whole bodies preserved in ice, amber or peat are fossils too.
  • Saying decay stopped because “there were no bacteria”. Decay stops because conditions decomposers need (oxygen, moisture, warmth) are missing.
  • Thinking a mineral fossil still contains the original bone. The bone has been replaced by rock.
  • Forgetting to convert metres to millimetres before using a sediment rate in mm per year.

Revision questions

Try these, then check your answers below each one.

Question 1. Which statement best describes a fossil? (Choose from: The remains or traces of an organism from millions of years ago, found in rock, Any bone found buried in soil, A rock shaped like an animal by the wind, The body of an animal that died last year.)

Answer: The remains or traces of an organism from millions of years ago, found in rock

Working: Fossils are the remains of organisms from millions of years ago, found in rocks. Traces such as footprints and burrows also count.

Question 2. The diagrams A–D show the stages in the formation of a fossil fish skeleton, but they are in the wrong order. Which order is correct? (Choose from: B → C → A → D, C → B → A → D, B → A → C → D, D → A → C → B.)

Answer: B → C → A → D

Working: B: the living fish. C: it dies and sinks, and is covered by mud. A: more layers of sediment build up and minerals slowly replace the bones. D: the rock is worn away and the fossil is found.

Question 3. The diagram shows fossils W, X, Y and Z in layers of rock that have not been disturbed. Which fossil is most likely to be the oldest? (Choose from: Y, W, Z, X.)

Answer: Y

Working: In undisturbed rock the deepest layer (layer 5) was laid down first, so fossil Y is likely to be the oldest. X, in the top layer, is likely to be the youngest.

Related lessons

Continue in the full course

This lesson is section B14.1 (AGCSC1401) 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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