If the early Earth's atmosphere contained little or no free O₂, then where did most of the 20 percent O₂in our modern atmosphere come from?

Question

Quizplus · Accepted Answer

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

If the Early Earth's Atmosphere Contained Little or No Free