Study the graph about oxygen content of earth’s atmosphere. oxygen content of earth apostrophe s atmosphere line graph. x axis is millions of years before present. y axis is oxygen in volume percent. the line starts at 1000 millions of years before present at 4 percent oxygen, remains level until 625 millions of years before present, increases quickly to 13 percent oxygen, then increases slowly to 18 percent oxygen. the line jumps to 35 percent oxygen near 300 millions of years before present, drops quickly to 15 percent oxygen, then increases to 31 percent oxygen near 100 millions of years before present, before dropping down to 20 percent oxygen. which describes oxygen content as earth evolved over time? oxygen levels sharply declined about 400 million years ago. oxygen levels stay above 15 percent starting 200 million years ago oxygen levels remain below 5 percent starting 1000 million years ago. oxygen levels raise significantly between 1000 and 700 million years ago.