Over the past 220 million years, the earth has suffered some catastrophes of which I recall two; the end of the Permian and Jurassic Eras. After each, the oxygen level was markedly lower. Back in the Permian days, it was around 36% and the carbon dioxide level was a lot higher, too. Animals and the plants they ate grew huge. The end of the Permian Era was a mega-disaster that wiped out about 99% of the species. The crocodilians and a few others survived; talk about tough! Something happened that caused a line of volcanoes to erupt across what is now Siberia. The end of the Jurassic was milder by comparison and there was another drop in oxygen level after that event. Since then, there has been a lot of competition for that 28% remaining oxygen and if it wasn't for volcanoes and plants, we'd be in a lot of trouble. The environment tends to sequester carbon dioxide as carbonates and lock them up in rocks and shells taking the oxygen along for the ride. But then, the occasional volcano belches a bunch of CO2 back in the air that the plants then turn into oxygen and carbohydrates. Conclusion: We need some more volcanoes.