Question

What allowed the earth’s atmosphere to increase in oxygen concentration?

Answer 1

Answer:The answer is tiny organisms known as cyanobacteria, or blue-green algae.

Explanation:

These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and oxygen increase. Additionally, some sources you could use that I used to answer this question was

https://www.scientificamerican.com/article/origin-of-oxygen-in-atmosphere/