The evolution of life on Earth has resulted in most organisms relying on using oxygen. Plentiful oxygen in the atmosphere was not always present and large amounts appeared about 2.4 bya (Billion years ago). Scientists think they know why: http://www.sciencedaily.com/releases/2009/04/090408145336.htm Life can appear only in solar systems with planets with molten cores, iron/nickel in our solar system. The convection currents in those cores also produce the protective magnetic fields around Earth, keeping all those nasty particles in space at bay. - When scientists wax philosophical about life on Earth we get an interesting perspective. Are we the only life in the universe? There should be lots more, but is there? http://www.newscientist.com/article/dn16920-existential-vertigo-over-human-origins.html... Thoughts, anybody? dhw, your questions of several days ago are discussed here.