Fossils locked away for 1.75 billion years hold clues about key moment in Earth's history

The GEO is not a fact. It is a theory based on the idea that an ocean saturated in dissolved ferrous iron captured cyanobacterial oxygen to keep the atmosphere anoxic. Banded iron formation oxidized iron are offered as proof. Thus, oxygenic photosynthesis must have been fully operational going back into the early Archean. Otherwise, the GEO would not be necessary. There's abundant geochemical and fossil evidence to support the early evolution of photosystem I and II.