Earth's earliest atmosphere was transformed in ways that allowed the development of complex life forms. A. Volcanoes changed Earth's earliest atmosphere in ways that allowed life to develop, and bacteria that used oxygen produced by photosynthesis further altered the atmosphere to what we find today. B. When the sun became brighter, early bacteria removed methane and carbon dioxide from the atmosphere, preventing Earth from becoming too hot for life to survive. C. Early bacteria provided the oxygen that was needed to support the metabolism of complex life forms and to form an ozone shield against deadly UV radiation. D. Volcanic carbon dioxide in Earth's early atmosphere kept Earth warm enough for life to begin during the time when the Sun was too dim to provide much warmth. E. Earth has been able to support life because its position relative to the Sun provided it with enough heat, but not too much heat for early bacteria to evolve. F. Complex life evolved on the Earth's surface, but not on Mars or other planets in the solar system because on those planets, early surface life was killed by UV radiation.