Explain why the early atmosphere is thought to have contained little or no oxygen.

The early atmosphere is thought to have contained little or no oxygen because it was formed primarily from volcanic gases, which released carbon dioxide and water vapour, but not oxygen. Photosynthetic organisms had not yet evolved to produce oxygen.

A strong answer should directly address the approved learning objective to describe the Earth's early atmosphere as probably containing mainly carbon dioxide with little or no oxygen. This question belongs to The Earth's early atmosphere 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 early atmosphere, and keeps the wording specific to AQA GCSE revision.