Earths Primordial EnvironmentIn the beginning, Earth had a hydrogen based atmosphere. Examples of molecules which were present include methane, CH4, hydrogen, H2, and ammonia, NH3. In time, the early atmosphere of Earth changed from a hydrogen dominated one to one which contained oxygen-rich molecules. Examples of those molecules include carbon dioxide, CO2, water vapor, H2O, and sulphur dioxide, SO2. In its early state, however, Earth's atmosphere resembled other primitive atmospheres. Jupiter's atmosphere is an example of what such primitive atmospheres must have been like. This is because, unlike smaller planets, Jupiter has such enormous gravity that it retains every molecule, and elements of the atmosphere cannot drift away as they do on other planets. Therefore scientists think that Jupiter's atmosphere today is representative of the ancient atmospheres of the smaller planets.
Certain chemical reactions require energy to make them go. That energy comes from ultraviolet (UV) light or lightning. Ultraviolet light and lightning can link small molecules together to make larger ones. The Miller-Urey experiment showed that ultraviolet light plus lightning in a hydrogen-based atmosphere can produce interesting chemicals which form the foundation of living cells.
Earth's early environment was friendly to this process because the early atmosphere did not provide protection from ultraviolet light. On Earth today, the ozone layer, O3 absorbs almost all but the longest wavelengths of UV. On the early Earth, there was little free oxygen, so UV from the sun fell directly onto the surface of the Earth.