well, you VASTLY over simplified that. the earth was a huge ball of molten material, that slowly cooled. as it cooled, chemical reactions occurred that caused water to form, and a thin atmosphere. simple life evolved in the water, and SLOWLY evolved to fit its environment. as more of these creatures grew, they started to effect their environment. as they evolved more, the plants started to produce more oxygen, which allowed larger life forms to evolve, which eventually led to pot heads wasting time arguing with each other about how it all happened.