Because of the constantly changing nature of our planet, the atmosphere was never the same composition all the time. Before there was terrestrial plant life, the Earth had an atmosphere with a lot more carbon dioxide, which made it impossible for modern terrestrial life forms to breathe. On the other hand, the atmosphere of the Carboniferous Period had twice as much oxygen as today. Because of the high oxygen levels, insects and vegetation were allowed to grow to massive size. Unfortunately, the high oxygen levels increased the chances of forest fires, which is why so many scorched fossils were found from that era. In addition to giant insects and colossal plants, the Carboniferous Period was ruled by amphibians the size of crocodiles and saw the evolution of the earliest reptiles.
