What type of evidence would scientists look at to infer that atmospheric oxygen levels were lower in the past compared with today?

They would examine the fossil record for the presence of organisms that can only survive in low‑oxygen environments, such as certain early marine invertebrates, and they would analyse the isotopic composition of ancient sedimentary rocks for signatures of low oxygen, such as the abundance of iron‑bearing minerals that form only when oxygen is scarce.

A strong answer should directly address the approved learning objective to interpret simple evidence for changes in atmospheric oxygen over time. 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.