Question

How did Earth's atmosphere become oxygen-rich?

Answer 1

Answer:

The answer is tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen. ... "What it looks like is that oxygen was first produced somewhere around 2.7 billion to 2.8 billon years ago.