How did the Earth's atmosphere evolve to its present composition?
The composition of today's atmosphere; how the early atmosphere formed and changed; how oxygen increased; and how carbon dioxide decreased over time.
A focused answer to AQA GCSE Chemistry 4.9.1, covering the composition of today's atmosphere, how the early atmosphere formed from volcanic activity, how oxygen increased through photosynthesis, and how carbon dioxide levels fell over billions of years.
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What this dot point is asking
AQA wants you to state the composition of today's atmosphere, describe how the early atmosphere is thought to have formed, explain how the amount of oxygen increased and how carbon dioxide decreased, and the role of plants, oceans and the formation of rocks and fuels. Because there is little direct evidence from billions of years ago, the spec stresses that these ideas are theories supported by limited evidence.
Today's atmosphere
The early atmosphere
As the Earth cooled, the water vapour condensed to form the oceans, which then played a major role in removing carbon dioxide from the atmosphere.
How oxygen increased
When algae and later plants evolved, they carried out photosynthesis, taking in carbon dioxide and releasing oxygen:
Algae appeared first, around 2.7 billion years ago, and over billions of years this process slowly increased the oxygen level until it was high enough for animals to evolve.
How carbon dioxide decreased
Carbon dioxide levels fell because it was:
- Dissolved in the oceans and used by marine organisms to make shells and skeletons.
- Used in photosynthesis by algae and plants, which locked carbon into living matter.
- Locked up in sedimentary rocks (such as carbonates formed from shells) and in fossil fuels (coal, oil and gas) formed from dead organisms over millions of years.
The carbon stored in these rocks and fuels is the same carbon that human activity is now releasing by burning fossil fuels, which connects this dot point to climate change: processes that took hundreds of millions of years to lock carbon away are being reversed in a few hundred years.
How we know
Because there were no instruments billions of years ago, scientists build the picture of the early atmosphere from indirect evidence: the gases released by volcanoes today, the composition of the atmospheres of Mars and Venus (thought to resemble the early Earth), the chemistry of the oldest rocks, and the timing of the first oxygen-producing algae recorded in rock layers. This evidence is limited and open to interpretation, so the spec stresses that the model of how the atmosphere evolved is a theory that could be revised if new evidence appears, rather than a settled fact.
Try this
Q1. State the approximate proportions of nitrogen and oxygen in today's atmosphere. [2 marks]
- Cue. About nitrogen and oxygen.
Q2. Explain how oxygen first built up in the atmosphere. [2 marks]
- Cue. Algae and plants evolved and carried out photosynthesis, releasing oxygen.
Q3. Name one way carbon was locked up out of the atmosphere. [1 mark]
- Cue. In sedimentary carbonate rocks (or in fossil fuels).
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksDescribe how the Earth's early atmosphere is thought to have formed, and explain how the amount of carbon dioxide in the atmosphere decreased over the following billions of years.Show worked answer →
A 4-mark Paper 2 question on the early atmosphere and falling carbon dioxide.
Early atmosphere (2 marks): for the first billion years intense volcanic activity released gases, giving an atmosphere thought to be mainly carbon dioxide with little or no oxygen, plus water vapour (which condensed to form the oceans) and small amounts of methane and ammonia. Carbon dioxide decreased (2 marks): carbon dioxide dissolved in the newly formed oceans; it was used in photosynthesis by algae and plants; and carbon became locked up in sedimentary (carbonate) rocks and in fossil fuels formed from dead organisms.
Markers reward the volcanic origin and at least two distinct mechanisms for the fall in carbon dioxide.
AQA 20213 marksExplain how the evolution of algae and plants changed the composition of the Earth's atmosphere, and write a word equation for the process responsible.Show worked answer →
A 3-mark question on photosynthesis and the rise of oxygen.
Change (1 mark): algae and later plants evolved and carried out photosynthesis, which took in carbon dioxide and released oxygen; over billions of years this increased the level of oxygen (and decreased carbon dioxide) until animals could evolve. Word equation (2 marks): carbon dioxide + water gives glucose + oxygen.
Markers reward the photosynthesis link (oxygen up, carbon dioxide down) and a correct word equation.
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Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)