The Origins of Oxygenic Photosynthesis Photosynthesis, in its earliest form, likely emerged around 3.5 billion years ago as anoxygenic photosynthesis, a process that did not produce oxygen.

Requires sunlight. Same process used by plants. Provides the oxygen we breathe. 2) Anoxygenic photosynthesis: Converts carbon dioxide and sulfide into sugar and sulfate. An ancient variant of ...