Life had to bide its time as Earth differentiated itself into a dense core of iron surrounded by a mantle of lower density during the Hadean or rockless" Eon. Cooling eventually led to the for mation of a thin and highly mobile crust.Then following the meteorite bombardment of the Arch- ean Eon, life first appeared in the early oceans. It is thought that Earth's early atmospheric gases such as ammonia, hydrogen sulfide, and hydrogen- combined with carbon dioxide to form the first simple organic compounds. Some scientists speculate that life may have been brought to Earth by the incoming meteorites.