I like to try to imagine previous geological periods and try to relate them to present life. Now, it really is impossible to imagine anything that far back, but it's fun to try. Currently I'm studying the Carboniferous period, a vast time from 299 to 359 million years ago, the coal-producing period.
In the carboniferous period, the oxygen percentage in the atmosphere was higher than any other time in history, about 35% oxygen. It's currently about 21%.
It was that high because of the abundance of land plants and phytoplankton in the ocean. The atmosphere favored plants and phytoplankton since it was high in carbon dioxide because of high volcanic venting activity and, consequently, warm. The two massive continents were centered on the equator providing extensive ranges for land plants.
Trees were abundant and were very large supported mainly by a very thick bark-like material and little wood. When the trees fell, the bark was broken down into a peat-like substance, but it remained in that form because it's main constituent, lignin, couldn't be broken down. The fungi and bacteria necessary to break down lignin hadn't evolved yet, so this peat accumulated for thousands or millions of years and eventually transformed into coal. At the end of the Carboniferous period, lignin-digesting fungi and bacteria did evolve and that was the end of the coal-producing period.
It's nice to think how wonderful it would be to have an atmosphere with 35% oxygen. Imagine how our energy levels would increase, but there is a downside: forest fires would be constant.