Oxygen levels rose and fell several times on early Earth before stabilizing in the Earth's atmosphere
The rise and fall of oxygen levels on early Earth - Earth science news

What can nitrogen isotopes and selenium in ancient rocks that were once at the bottom of the sea tell us about oxygen levels on early Earth? For one thing, they show that oxygen levels were not stable in the early life of our planet, but rather came and went before oxygen levels stabilized in the atmosphere at levels as we know them today. Nitrogen isotopes reveal the activity of certain marine microorganisms that use oxygen to form nitrate. Selenium is released from sulfur minerals on land by oxygen. Thus, by analyzing nitrogen isotopes and selenium in rocks of different age, it is possible to determine how abundant oxygen must have been at a certain time. Also, the lack of oxygen during certain periods does not necessarily mean the lack of microorganismal life, and this might be important for the search of life on other planets whose atmosphere does not contain oxygen today.

Read the full story: University of Washington
Scientific publication: Proceedings of the National Academy of Sciences of the USA