If early Earthʹs atmosphere contained little or no O2, then where did most of the O2 in our modern atmosphere come from?

Question

Quizplus · Accepted Answer

Photosynthesis by plants and cyanobacteria has been the primary source of oxygen in Earth's atmosphere. Through this process, these organisms use sunlight to convert carbon dioxide and water into glucose and oxygen, significantly contributing to the oxygen present in the atmosphere today.

If Early Earthʹs Atmosphere Contained Little or No O2, Then