<p>Photosynthetic oxygen evolution underpins Earth’s biosphere, yet its origins remain contested. We synthesize evidence across paleontology, stratigraphy, geochemistry, biochemistry, and molecular evolutionary biology to re-evaluate the earliest sources of O₂. We hypothesize that some of the earliest detectable O₂ on Earth was produced abiotically, specifically via photo(electro)chemical-driven decomposition of bicarbonate at aqueous mineral–water interfaces, including illuminated Fe/Mn (oxyhydr)oxide-coated rock/soil surfaces and seawater–polymetallic nodule interfaces, where interfacial charge separation and electrochemical gradients can promote HCO₃^– activation, O₂ release, and CO₂ production; chemical footprints of this pathway may be retained in contemporary oxygenic photosynthesis. In this view, water-splitting photolysis emerged later as an evolutionary response to the declining availability of dissolved inorganic carbon (CO₂/HCO₃^–) in surface environments. We outline geologic and biochemical constraints consistent with this sequence and propose testable predictions to discriminate between bicarbonate-derived and water-derived O₂ signatures in the rock record and modern analogs. By reframing early Earth redox budgets and the trajectory of oxygenic metabolism, this perspective also suggests design principles for artificial photosynthetic reactors that exploit carbonate photochemistry under carbon-rich conditions and transition to water oxidation when carbon becomes limiting.</p>