Question

why don't oxygen in the atmosphere degree​

Answer 1

Answer:

While photosynthetic life reduced the carbon dioxide content of the atmosphere, it also started to produce oxygen. The oxygen did not build up in the atmosphere for a long time, since it was absorbed by rocks that could be easily oxidized (rusted).

Answer 2

Yet, the percentage of oxygen in the atmosphere is constant for many years and does not decrease because the green plants which are capable of photosynthesis take up CO2 from the atmosphere and produce O2 in return. It is this process which keeps the oxygen levels fairly constant