Question

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

Answer 1

Photosynthesis

Explanation:

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).