The Great Oxygenation Event (GOE) was a time interval during the Earth’s Paleoproterozoic era (2,500 to 1,600 million years ago) when the Earth experienced a rise in the concentration of free oxygen from practically none. The GOE is inferred to have been caused by cyanobacteria, which evolved chlorophyll-based photosynthesis that releases dioxygen as a waste byproduct of water photolysis. Scientists believe the GOE may have caused an extinction of many early Earth organisms, including the retinal using (Purple Earth hypothesis) organisms that lived during that time. The adaptation of surviving archaea (leading to mitochondria) may have led to the rise of eukaryotic organisms and eventually multicellular life-forms. So Earth may have been purple before, and we’re only here thanks to cyanobacteria!