Oxygenic photosynthesis fundamentally transformed our planet by releasing molecular oxygen and altering major biogeochemical cycles, and this exceptional metabolism requires manganese to function. Not only is manganese essential for producing oxygen, but manganese oxides are also incredibly diagnostic for the presence of environmental oxygen. Thus the history of manganese oxidation recorded in the geologic record provides a valuable perspective on our planet’s environmental past, the ancient availability of oxygen, and the evolution of oxygenic photosynthesis. I will present analyses on manganese minerals in the rock record using microscale techniques to extract primary geochemical signals and process-based laboratory experiments measuring microbial mineral products to interpret ancient geobiological signals. This talk will demonstrate how the evolution of oxygenic photosynthesis and the activity of metal oxide-reducing metabolisms are linked to and recorded in the rich complexity of the manganese mineralogical record.