<P> Oceanic cyanobacteria , which evolved into coordinated ( but not multicellular or even colonial ) macroscopic forms more than 2.3 billion years ago ( approximately 200 million years before the GOE ) , are believed to have become the first microbes to produce oxygen by photosynthesis . Before the GOE , any free oxygen they produced was chemically captured by dissolved iron or organic matter . The GOE started when these oxygen sinks became saturated , at which point oxygen produced by the cyanobacteria was free to escape into the atmosphere . </P>
the accumulation of free oxygen in earth 's atmosphere began with the origin of
Oceanic cyanobacteria