Big bang theory explains the origin of the universe as a singular huge explosion in physical terms. The primitive Earth had no proper atmosphere but consisted of ammonia, methane, hydrogen, and water vapour. The climate of the Earth was extremely high. UV rays from the Sun split up water molecules into hydrogen and oxygen. Gradually the temperature cooled and the water vapour condensed to form rain. Rainwater filled all the depressions to form water bodies. Ammonia and methane in the atmosphere combined with oxygen to form carbon dioxide and other gases.