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

Question

Quizplus · Accepted Answer

The majority of the O2 in our modern atmosphere came from photosynthesis, which is the process by which green plants and algae convert carbon dioxide and water into organic matter (like glucose) and oxygen. This process began to increase approximately 2.4 billion years ago and has continued to be a major source of O2 in the atmosphere ever since.

If the Early Earth's Atmosphere Contained Little or No O2