Question

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.

Explanation:

By observing the graph, we can see that oxygen levels sharply declined about 400 million years ago. The graph shows a significant drop - off in oxygen percentage around the 400 million years before present mark.

Answer:

Oxygen levels sharply declined about 400 million years ago.