MER GIANT- Immense insects such as gan6ra,音dragonfly w代h the wingspon of a hawk ed 300 million years ago,when the atmospheric con tion of oxygen was much higher than It is today 1. Earth's atmosphere had little if any oxygen until 2.5 billion years and the origin of chlorophyll producing plants 2. Oxygen levels have fluctuated throughout the history of the earth 3. High oxygen levels permitted great biological experimentation and greater diversity 4. Low oxygen levels caused mass extinction of those organisms requiring high levels of oxygen Information taken from Science News,Dec.17,2005,vol.168,pp.395-6 Title=Changes in the air-variations in atmospheric oxygen have affected evolution in big ways