What evidence shows that Earth’s atmosphere was reducing before 2.5 billion years ago?

Evidence for a reducing (anoxygenic) atmosphere on Earth prior to 2.5 billion years ago is primarily derived from the geological and mineralogical record, which indicates an environment lacking free oxygen.

Key lines of evidence include:

• Detrital Minerals: The presence of well-preserved, unoxidized grains of uraninite and pyrite in sedimentary rocks older than 2.2 billion years indicates that these minerals were transported and deposited without undergoing the chemical weathering that occurs in an oxygen-rich atmosphere.123
• Iron Formations: The widespread deposition of banded iron formations during the Archean Eon demonstrates that iron was transported in the oceans in its soluble ferrous ($F{e}^{2+}$) state. In an oxygenated environment, iron would have oxidized to insoluble ferric ($F{e}^{3+}$) iron and precipitated much earlier, preventing the formation of these massive deposits.1243
• Paleosols: Chemical analysis of ancient fossil soils (paleosols) reveals patterns of iron depletion that are consistent with soil development in an oxygen-poor environment.13

These findings collectively support the conclusion that the early Earth's atmosphere was reducing, with free oxygen levels significantly lower than those of the present day.123