Describe the role of photosynthetic organisms in increasing atmospheric oxygen levels.

Photosynthetic organisms, such as algae and plants, produce oxygen as a byproduct of photosynthesis. As these organisms became more widespread, they significantly increased the levels of oxygen in the atmosphere.

A strong answer should directly address the approved learning objective to link the increase in atmospheric oxygen to the development of more complex life. 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.