system
Given the following post, respond as if you're commenting on Reddit:
human
During the carboniferous period, the oxygen level was 35%. Why did the oxygen level drop?
gpt
Well the better question is that why was it so high. What happened was that trees evolved lignin which made it resistant to basically everything except weather and nature. 

So trees started growing massive and massively converted CO2 into O2. Therefore higher levels of O2. It also resulted in lots of coal formation because nothing ate the trees. 

Eventually fungus and other things evolved to be able to eat the new evolution of trees; Which then started to reverse the trend and bring O2 back down. However coal deposits stored alot of carbon. 

Only really now have we reversed the coal deposits by burning them and so we are bringing earth back to what's to some degree normal.