For much of early Earth, the atmosphere contained almost no free oxygen. Follow the evidence for the Great Oxidation Event, when microbial photosynthesis and a changing balance of chemical sinks allowed oxygen to persist, reshaping rocks, climate, oceans, and the possibilities for later life. Inside this episode: - Earth before oxygen accumulated in the atmosphere - How oxygenic photosynthesis changed the planetary balance - Sulfur isotopes, red beds, iron formations, and other evidence - Why oxygen sources and sinks mattered - Climate connections and the stepwise oxygenation of Earth Sources include peer-reviewed geochemistry and Earth-system research, NASA technical reviews, and published studies of early atmospheric oxygen. This episode is a researched narrative summary. Historical records may be incomplete, translated differently, or interpr ...