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