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The air we breathe today — roughly four-fifths nitrogen and one-fifth oxygen — has not always been this way. For most of the Earth's 4.6-billion-year history the atmosphere was very different, and it has changed enormously over time, driven by volcanoes, by the oceans, and above all by living organisms. This lesson, part of Topic C6 of OCR Gateway Combined Science A, tells the story of how the atmosphere evolved: the carbon-dioxide-rich early atmosphere, how carbon dioxide fell and oxygen rose, and the composition of the atmosphere today. It also stresses why our knowledge of the early atmosphere is uncertain.
By the end of this lesson you should be able to describe the early atmosphere and where it came from, explain how the proportions of carbon dioxide and oxygen changed as the atmosphere evolved, state the approximate composition of today's atmosphere, and explain why evidence about the early atmosphere is limited.
This lesson is largely AO1 (recalling how the atmosphere evolved and its composition today), with AO3 when you evaluate why evidence about the distant past is limited and conclusions remain uncertain.
For roughly the first billion years of the Earth's existence, the surface was extremely hot and there was intense volcanic activity. Volcanoes released the gases that formed the early atmosphere, which was very different from today's:
This is thought to be similar to the atmospheres of Mars and Venus today, which are rich in carbon dioxide. As the intense volcanic activity continued, the water vapour built up in the atmosphere.
Exam Tip: The headline facts about the early atmosphere are: mainly carbon dioxide (from volcanoes), lots of water vapour, and little or no oxygen. Comparing it to Mars or Venus today is a useful supporting point.
As the Earth gradually cooled over many millions of years, the water vapour in the atmosphere condensed to form liquid water, which fell as rain and collected to form the oceans. The formation of the oceans was a turning point, because it set off the processes that removed carbon dioxide from the air.
The amount of carbon dioxide then fell dramatically over time, for several reasons that all locked carbon away:
Together these processes transferred huge quantities of carbon out of the atmosphere and into the oceans, rocks, fossil fuels and living things.
Exam Tip: Learn at least three reasons carbon dioxide fell: it dissolved in the oceans, was locked into sedimentary rocks (carbonates) and fossil fuels, and was used in photosynthesis. Examiners reward more than one named process.
There was almost no oxygen in the early atmosphere. Oxygen built up because of photosynthesis by living organisms:
carbon dioxide+water→glucose+oxygen
The first organisms to photosynthesise were algae, which evolved in the oceans about 2.7 billion years ago. As algae — and later plants on land — spread and photosynthesised, they steadily released oxygen into the atmosphere. Over a very long time, the oxygen level rose until it was high enough for animals to evolve and survive.
So the rise of oxygen and the fall of carbon dioxide were largely the work of the same process — photosynthesis — carried out first by algae and then by plants.
flowchart TD
A["Volcanoes release CO2 and water vapour<br/>(early atmosphere, little/no oxygen)"] --> B["Earth cools: water vapour condenses, oceans form"]
B --> C["CO2 dissolves in oceans and is locked into<br/>sedimentary rocks and fossil fuels"]
C --> D["Algae evolve (~2.7 billion years ago)<br/>and begin photosynthesis"]
D --> E["Plants spread: photosynthesis removes CO2<br/>and releases O2"]
E --> F["Oxygen rises high enough for animals;<br/>atmosphere becomes ~78% N2, 21% O2"]
Exam Tip: The single most important cause of rising oxygen is photosynthesis, first by algae (about 2.7 billion years ago) and then by plants. A common misconception is to credit plants with making the nitrogen — they did not.
After billions of years of change, the atmosphere reached its present, fairly stable composition. The approximate composition of dry air today is:
| Gas | Approximate proportion |
|---|---|
| Nitrogen (N2) | about 78% |
| Oxygen (O2) | about 21% |
| Argon (Ar) | about 0.9% |
| Carbon dioxide (CO2) | about 0.04% |
There is also a variable amount of water vapour, depending on the weather and location. The figure below shows the composition of dry air.
The proportion of nitrogen rose to its present level partly because it is fairly unreactive, so it built up and was not removed as carbon dioxide was; it was also released by some bacteria and volcanic processes. Importantly, nitrogen was not produced by plants.
Nitrogen makes up about 78% of dry air and oxygen about 21%. In a sealed 500 cm³ sample of dry air, roughly what volume is nitrogen, and what volume is oxygen?
Step 1 — find the nitrogen volume: 78% of 500 cm3=10078×500=390 cm3.
Step 2 — find the oxygen volume: 21% of 500 cm3=10021×500=105 cm3.
Step 3 — check it is sensible: 390+105=495 cm3, leaving about 5 cm3 for argon, carbon dioxide and other gases — which matches the remaining ∼1%.
Answer: about 390 cm3 nitrogen and about 105 cm3 oxygen.
Exam Tip: Learn today's composition: ~78% nitrogen, ~21% oxygen, ~0.9% argon, ~0.04% carbon dioxide, plus variable water vapour. These figures are commonly asked for directly, sometimes as a percentage-of-a-volume calculation.
Our picture of the early atmosphere is a theory, not a certainty. The early atmosphere existed billions of years ago, and there were no instruments — or people — to measure it. Scientists build their ideas from indirect evidence, such as the composition of ancient rocks and comparisons with other planets, but this evidence is limited and incomplete. As a result, scientists cannot be sure exactly what the early atmosphere was like, and ideas may change as new evidence is found. This is a good example of how science works with the best available evidence while acknowledging genuine uncertainty.
Exam Tip: If asked why we are unsure about the early atmosphere, say it was billions of years ago, so the evidence is limited/indirect and no direct measurements exist. This honesty about uncertainty is itself a mark-worthy point.
It is worth looking more closely at where the carbon that was once in the atmosphere ended up, because this links directly to the fossil fuels we burn today. When the oceans formed, carbon dioxide dissolved into the seawater; some of it reacted to form carbonate compounds, which were deposited and, over millions of years, compressed into sedimentary rocks such as limestone. Many sea creatures also used carbonate to build their shells and skeletons; when they died, their hard parts sank, accumulated and were compressed into carbonate rock too. In this way, enormous quantities of carbon were transferred from the air, through the oceans and living things, and locked away in rock, where most of it still remains.
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