The most widely accepted chronology of the Great Oxygenation Event suggests that free oxygen was first produced by prokaryotic and then later eukaryotic organisms that carried out oxygenic photosynthesis , producing oxygen as a waste product .
The most widely accepted chronology of the Great Oxygenation Event suggests that oxygen was first produced by photosynthetic organisms ( prokaryotic , then eukaryotic ) that emitted oxygen as a waste product .