Explain why the increase in atmospheric oxygen is linked to the development of more complex life.

The increase in atmospheric oxygen allowed for the evolution of more complex life forms, as oxygen is essential for cellular respiration, which provides energy for larger and more complex organisms. Higher oxygen levels supported the development of diverse ecosystems and the evolution of animals.

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.