Explain the origin of the Earth and the sequential changes that occurred on it.

(N/A) Initially,there was no atmosphere on Earth. Water vapor,methane,carbon dioxide,and ammonia were released from molten masses and covered the surface. $UV$ rays from the sun broke down water into hydrogen and oxygen,and the lighter $H_2$ gas escaped. Oxygen combined with ammonia and methane to form water,carbon dioxide,and other compounds. The ozone layer was formed. As the Earth cooled,water vapor fell as rain,filling the depressions on the Earth's surface to form oceans. Life appeared on Earth approximately $500$ million years after the formation of the Earth,i.e.,about $4$ billion years ago.