There is a theory that when the big bang happened, rocks and gases flew away from the blast. As we know the rocks hit each other and made planets when they became one. They then made a gravitational pull that gases came to the surface, and heavier gases flew to other planets with stronger gravity. A good example is jupiter which had a gravitational force so strong, the entire planet is mostly gas with a rock in the middle that u can bike around in one day. Then early life forms came into play. Today we take in air and the waste is carbon dioxide. But for some reason the early life forms took in carbon dioxide and the waste was oxygen! Then these microscopic creatures made the atmosphere and became plants and animals and adapted to oxygen. This theory is very solid and i cant think of a different way that life began.