How Oxygen Increased

This lesson focuses on how oxygen levels in the atmosphere increased from virtually zero to the current level of approximately 21%, as required by AQA GCSE Chemistry specification (5.9.1). The appearance and increase of oxygen is one of the most important events in the history of life on Earth, and it was driven primarily by photosynthesis. You need to understand the organisms involved, the chemical reaction of photosynthesis, and the consequences of rising oxygen levels.

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The Absence of Oxygen in the Early Atmosphere

For the first 1–2 billion years of Earth's history, there was essentially no free oxygen in the atmosphere. The early atmosphere was dominated by carbon dioxide and water vapour, with no organisms capable of producing oxygen. Any small amounts of oxygen produced by chemical reactions (such as the breakdown of water vapour by ultraviolet radiation — a process called photolysis) were immediately consumed by reactions with reactive metals and other elements on the surface.

Exam Tip: Free oxygen means oxygen gas (O₂) that is not combined with other elements. The early atmosphere contained oxygen atoms bonded in molecules like CO₂ and H₂O, but there was no free O₂ gas.

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