The biochemical capability to use H2O as the source for electrons in photosynthesis evolved once, in a common ancestor of extant cyanobacteria. This transforming event occured early in Earth’s history, at least 2450-2320 million years ago. Evidence from geobiological studies of Archean sedimentary rocks indicates that life existed 3500 Ma, but the question of when oxygenic photosynthesis evolved is still unanswered.

A clear paleontological window on cyanobacterial evolution opened about 2000 Ma, indicating an already-diverse biota of blue-greens. Cyanobacteria remained the main producers throughout the Proterozoic Eon (2500-543 Ma), because the redox structure of the oceans favored photoautotrophs capable of nitrogen fixation.

Green algae joined blue-greens as major primary producers on continental shelves near the end of the Proterozoic, but only with the Mesozoic (251-65 Ma) dinoflagellates, coccolithophorids, and diatoms did primary production in marine shelf waters take modern form.

Cyanobacteria play an essential role in marine ecosystems as primary producers in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as the plastids of marine algae.