AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
How oxygen increased: Algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere. Over the next billion years plants evolved and the percentage of oxygen gradually increased to a level that enabled animals to evolve.
About 2.7 billion years ago, algae began producing oxygen through photosynthesis.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
About 2.7 billion years ago, algae began producing oxygen through photosynthesis. This oxygen initially dissolved in the oceans and then entered the atmosphere. Over the following billion years, plants evolved and photosynthesis continued on a massive scale, gradually raising the atmospheric oxygen concentration. Eventually, oxygen levels became high enough to support the evolution of animals that rely on aerobic respiration. This sequence of events is a key part of the history of Earth's atmosphere. In exams, you may be asked to describe the timeline, explain why oxygen increased slowly, or discuss the consequences for animal life. Remember that the increase was gradual and took billions of years, not a sudden change.
What earns the marks
- State that algae first produced oxygen about 2.7 billion years ago.
- Describe how oxygen appeared in the atmosphere soon after algae began photosynthesis.
- Explain that over the next billion years, plants evolved and continued to produce oxygen.
- Describe how the percentage of oxygen gradually increased over time.
- Explain that this increase in oxygen eventually enabled animals to evolve, as they require oxygen for aerobic respiration.
Common mistakes and how to put them right
Stating that oxygen appeared suddenly or that animals evolved at the same time as algae.
Put it right: Oxygen increased gradually over billions of years, and animals evolved much later once oxygen levels were sufficient.
Confusing the order of events, such as saying plants evolved before algae.
Put it right: Algae first produced oxygen about 2.7 billion years ago, and plants evolved later.
Assuming that oxygen levels rose to the current percentage immediately.
Put it right: The increase was gradual and took about a billion years or more to reach a level that could support animals.
Exam tips
- Use the specific time frame 'about 2.7 billion years ago' when referring to the first oxygen production by algae.
- When describing the increase in oxygen, emphasise the gradual nature and the role of both algae and plants.
- Link the rise in oxygen to the evolution of animals, explaining that animals need oxygen for respiration.
Try the lesson
The start of the How oxygen increased: Algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere. Over the next billion years plants evolved and the percentage of oxygen gradually increased to a level that enabled animals to evolve. lesson, made from checked teaching notes for this specification point.
Have a go first
Which organisms are thought to have first produced oxygen on Earth approximately 2.7 billion years ago?
Listen
About 2.7 billion years ago, algae began producing oxygen through photosynthesis. This oxygen initially dissolved in the oceans and then entered the atmosphere. Over the following billion years, plants evolved and photosynthesis continued on a massive scale, gradually raising the atmospheric oxygen concentration. Eventually, oxygen levels became high enough to support the evolution of animals that rely on aerobic respiration. This sequence of events is a key part of the history of Earth's atmosphere. In exams, you may be asked to describe the timeline, explain why oxygen increased slowly, or discuss the consequences for animal life. Remember that the increase was gradual and took billions of years, not a sudden change. A strong answer earns marks for these points. State that algae first produced oxygen about 2.7 billion years ago. Describe how oxygen appeared in the atmosphere soon after algae began photosynthesis. Explain that over the next billion years, plants evolved and continued to produce oxygen. Describe how the percentage of oxygen gradually increased over time. A common slip: Stating that oxygen appeared suddenly or that animals evolved at the same time as algae. Instead, oxygen increased gradually over billions of years, and animals evolved much later once oxygen levels were sufficient. Exam tip: Use the specific time frame 'about 2.7 billion years ago' when referring to the first oxygen production by algae.
Key words
Say what each word means before you turn the card.
Algae
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Plants
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Animals
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Gradual increase
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See every definition
- Algae
- Organisms that first produced oxygen about 2.7 billion years ago, with oxygen appearing in the atmosphere soon after.
- Plants
- Organisms that evolved over the next billion years and continued to produce oxygen, gradually increasing its percentage.
- Animals
- Organisms that evolved once the percentage of oxygen gradually increased to a level that enabled them to evolve.
- Gradual increase
- The slow rise in the percentage of oxygen over billions of years, not a sudden change.
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