Evolution of the Atmosphere

This lesson covers how Earth's atmosphere changed over time from being rich in CO₂ to the nitrogen-and-oxygen atmosphere we have today, as required by the Edexcel GCSE Combined Science specification (1SC0). The key process is photosynthesis by early organisms.

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The Atmosphere Today

Before understanding how the atmosphere evolved, you need to know what it looks like now:

Gas	Approximate Proportion
Nitrogen (N₂)	78%
Oxygen (O₂)	21%
Argon (Ar)	~0.9%
Carbon dioxide (CO₂)	~0.04%
Water vapour	Variable (0–4%)

Compare this with the early atmosphere:

• CO₂: from ~95% → ~0.04% (massive decrease)
• O₂: from ~0% → 21% (massive increase)
• N₂: from small amounts → 78% (gradual accumulation)

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How Did Oxygen Appear?

The oxygen in our atmosphere was produced by photosynthesis.

The Role of Algae and Plants

1. Around 2.7 billion years ago, the first photosynthetic organisms appeared — these were cyanobacteria (blue-green algae) living in the oceans.
2. They used photosynthesis to convert CO₂ and water into glucose and oxygen:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

(carbon dioxide + water → glucose + oxygen)

3. Over hundreds of millions of years, these organisms released enormous quantities of oxygen into the atmosphere.
4. Later, green plants evolved on land and continued to add oxygen through photosynthesis.

Timeline of Atmospheric Change

graph LR
    A["4.6 billion years ago<br/>Earth forms<br/>Volcanic atmosphere<br/>~95% CO₂, no O₂"] --> B["3.5 billion years ago<br/>First cyanobacteria<br/>Photosynthesis begins<br/>O₂ starts to build up"]
    B --> C["2.4 billion years ago<br/>Great Oxidation Event<br/>O₂ rises significantly<br/>CO₂ continues to fall"]
    C --> D["400 million years ago<br/>Land plants evolve<br/>O₂ increases further<br/>Ozone layer forms"]
    D --> E["Today<br/>78% N₂, 21% O₂<br/>0.04% CO₂"]

    style A fill:#d32f2f,color:#fff
    style B fill:#e65100,color:#fff
    style C fill:#f9a825,color:#000
    style D fill:#2e7d32,color:#fff
    style E fill:#1565c0,color:#fff

Exam Tip: When explaining where oxygen came from, always state photosynthesis by algae (cyanobacteria) and later plants. This is the only significant source of atmospheric oxygen.

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How Was CO₂ Removed?

The massive decrease in atmospheric CO₂ was caused by several processes working together:

1. Photosynthesis

Photosynthetic organisms used up CO₂ from the atmosphere to make glucose. As more organisms evolved, more CO₂ was consumed.

2. Dissolving in the Oceans

CO₂ is soluble in water. As the oceans formed and expanded, huge quantities of CO₂ dissolved into them.

3. Formation of Sedimentary Rocks

• Marine organisms with shells or skeletons made of calcium carbonate (CaCO₃) died and accumulated on the ocean floor.
• Over millions of years, these remains were compressed to form limestone and chalk.
• The carbon that was once in atmospheric CO₂ became locked in these rocks.

4. Formation of Fossil Fuels

• The remains of plants and marine organisms that did not fully decompose were buried under layers of sediment.
• Over millions of years, heat and pressure converted them into coal (from plants), crude oil and natural gas (from marine organisms).
• The carbon became stored underground as fossil fuels.

Process	Effect on CO₂	Where Carbon Ends Up
Photosynthesis	CO₂ taken in by organisms	In organic molecules (glucose, biomass)
Dissolving in oceans	CO₂ dissolves in water	In ocean water as dissolved CO₂/carbonates
Sedimentary rocks	Carbonates form limestone	Locked in rocks (CaCO₃)
Fossil fuels	Carbon stored underground	In coal, oil, natural gas

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The Build-Up of Nitrogen

Nitrogen gas (N₂) was released by volcanoes, but also produced by:

• Denitrifying bacteria in the soil that convert nitrogen compounds into N₂ gas.
• Reactions between ammonia (from volcanoes) and oxygen (once available).

Because N₂ is very unreactive (it has a strong triple bond, N≡N), it was not easily removed from the atmosphere. Over billions of years, it accumulated to become the most abundant gas at 78%.

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The Ozone Layer

Once oxygen levels rose sufficiently:

1. High-energy UV radiation from the Sun split some O₂ molecules:

O₂ → 2O (oxygen atoms)

2. These oxygen atoms reacted with more O₂ to form ozone (O₃):

O + O₂ → O₃

3. The ozone layer formed in the upper atmosphere (stratosphere).
4. The ozone layer absorbs harmful UV radiation, making it possible for life to move onto land.

Exam Tip: The ozone layer could only form after oxygen levels had risen due to photosynthesis. Without photosynthesis, there would be no oxygen and no ozone layer.

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Comparing the Early and Modern Atmospheres

Feature	Early Atmosphere	Modern Atmosphere
CO₂	~95%	~0.04%
O₂	~0%	~21%
N₂	Small amounts	~78%
H₂O vapour	Very high	Variable (0–4%)
Ozone layer	None	Present (stratosphere)

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Summary

• Photosynthesis by cyanobacteria (algae) and later plants produced the oxygen in our atmosphere.
• CO₂ levels decreased due to photosynthesis, dissolving in the oceans, formation of sedimentary rocks and fossil fuels.
• Nitrogen accumulated because it is unreactive and was not removed from the atmosphere.
• The ozone layer formed once there was enough oxygen, allowing life to colonise land.
• The atmosphere changed from ~95% CO₂ (no O₂) to ~78% N₂, ~21% O₂ and ~0.04% CO₂.

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Worked Examples

Worked Example 1 — Photosynthesis word and symbol equations

Question: Write word and balanced symbol equations for photosynthesis.

Word equation: carbon dioxide + water → glucose + oxygen.

Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Check: left has 6 C, 18 O, 12 H; right has 6 C, (6 + 12) = 18 O, 12 H. Balanced.

This reaction, carried out by cyanobacteria (blue-green algae) from about 2.7 billion years ago and later by plants, is responsible for removing CO₂ and producing the O₂ in the atmosphere.

Worked Example 2 — Explaining the rise in nitrogen

Question: The amount of nitrogen in the atmosphere increased over geological time, yet very little new nitrogen was added. Explain why the percentage of N₂ rose to ~78%.

Answer: Nitrogen is a very unreactive gas because the N≡N triple bond is extremely strong. Other gases (CO₂, water vapour) were continually removed (dissolved in oceans, trapped in rocks, used by plants), but N₂ was not. The percentage of N₂ therefore rose by relative accumulation rather than by being added.

Worked Example 3 — Ozone layer formation

Question: Why could life only colonise land once an ozone (O₃) layer formed?