What was needed to evolve before there was a significant amount of #O_2# in the atmosphere?

1 Answer
Jan 23, 2018



The increase in atmospheric oxygen was due to the evolution of organisms that were capable of photosynthesis. These were Cyanobacterica, what we use to classify as "blue-green algae". After about 200 million years of photosynthesis, there was enough oxygen in the atmosphere to cause the Great Oxygenation Event which was a mass extinction of organism that were obligate anaerobes (organisms that were obligated to live in an oxygen free environment).