The key O₂ sources are O₂-producing (oxygenic) photosynthesis accompanied by burial of photosynthetic organic matter, and the photo-dissociation of water vapor and the loss of hydrogen to space. Key O₂ sinks are biological O₂ respiration, reduced volcanic gases, and weathering of rocks containing reduced elements such as carbon, sulfur and iron.

Before oxygenic photosynthesis arose, the relatively minor O₂ source of water vapor photodissociation was overwhelmed by abiotic and biotic fluxes of reduced species. Oxygenic photosynthesis appeared after a succession of diversifications in the light-harvesting reaction centers of photosynthetic bacteria finally led to cyanobacteria. These diversifications were driven by competition both for reduced species for organic synthesis and for various wavelengths of light. Oxygenic photosynthesis provided a virtually unlimited supply of reducing power. Life could exist anywhere that light, liquid water, nutrients and a stable substrate were available.

Oxygenic photosynthesis arose long before O₂ became abundant in the atmosphere. Fossil evidence of cyanobacterial ecosystems appeared before 3.0 billion years (Ga) ago. Sulfates (at 3.4 Ga) and oxidation of organic matter (3.0 Ga) indicate that O₂ was present locally around photosynthetic ecosystems. Atmospheric O₂ levels rose substantially between 2.2 and 2.07 Ga. Oxygen-sensitive detrital minerals (FeS₂, UO₂) disappeared, iron was retained as Fe³⁺ in soils, and redbeds and O₂-requiring eukaryotes arose. Banded iron formations, Fe²⁺-rich finely-laminated deposits, disappeared by about 1.8 Ga, indicating that O₂ had finally permeated the deep oceans.

The history of O₂ increases reflects planetary and biological change. Sources of O₂ strengthened. Oxygenic photosynthesis evolved; cyanobacteria proliferated over widening continental shelves; and plankton arose. Stabilized continents enhanced the preservation of photosynthetic organic carbon. Sinks of O₂ weakened, as fluxes of reduced volcanic species (H₂, Fe²⁺, sulfides, etc.) decreased.

Earth's history offers insights for a survey of extrasolar planets. Life arose very early and depended upon reduced volcanic emanations. Even after oxygenic photosynthesis arose, perhaps 1 Ga or more passed before O₂ became a substantial atmospheric constituent. A dependable O₂ supply was essential for the development of complex (plants and animals) and intelligent (us?) life.