Describe how oxygen levels increased gradually as photosynthetic organisms became more widespread.

Oxygen levels in the Earth's atmosphere increased gradually due to the process of photosynthesis carried out by algae and plants. As these organisms became more widespread, they produced more oxygen, which contributed to the gradual rise in atmospheric oxygen levels over millions of years.

A strong answer should directly address the approved learning objective to describe how oxygen levels increased gradually as photosynthetic organisms became more widespread. This question belongs to How oxygen increased within The composition and evolution of the Earth's atmosphere, so the response should use that exact curriculum context rather than a generic statement. The answer is correct when it names the key idea, explains the link to oxygen, and keeps the wording specific to AQA GCSE revision.