For the last 200 million years or so, the proportions of gases in dry air have stayed about the same.

percentage of a gas = volume of the gas ÷ total volume × 100

You are expected to know the rough figures (four fifths nitrogen, one fifth oxygen) and to work percentages out yourself.

1. A 400 cm³ sample of breath contains 66 cm³ of oxygen
2. percentage of oxygen = 66 ÷ 400 × 100 = 16.5%
3. Estimate for 400 cm³ of dry air: oxygen ≈ 1/5 × 400 = 80 cm³

Divide by the total volume of the sample, not by the volume of the other gases.

Evidence is limited because the Earth formed about 4.6 billion years ago, so these are theories, not certainties.

• In the first billion years, intense volcanic activity released gases: mainly carbon dioxide, with water vapour, nitrogen and perhaps small amounts of methane and ammonia. There was little or no oxygen, rather like Mars and Venus today.
• As the Earth cooled, water vapour condensed to form the oceans.
• Carbon dioxide dissolved in the oceans, and carbonates precipitated to form sediments.
• Nitrogen is unreactive, so it slowly built up.

About 2.7 billion years ago algae began to photosynthesise; plants followed. Over the next billion years oxygen rose until animals could evolve.

carbon dioxide + water → glucose + oxygen
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Carbon dioxide also fell because carbon was locked into sedimentary rocks (limestone from the calcium carbonate shells of sea creatures) and fossil fuels: coal from plant remains, crude oil and natural gas from plankton buried in mud.

Volcanoes added carbon dioxide. Dissolving in oceans, photosynthesis and the formation of rocks and fossil fuels took it out.