4.6 bya, constant bombardment of earth with meteorites. Meteorites might have provided the complex organic building blocks necessary to initiate life. It still begs the question as to how life evolved from organic molecules. Synthesis of organic molecules Early earth may have possessed the necessary conditions for organic synthesis. UV light readily penetrated the earth's atmosphere, thereby providing another source of energy.
Life processes involve oxidation-reduction reactions.
Oxidation and reduction involve giving up and obtaining electrons. This is how bonds are made and broken. In general, reduced compounds do 'work' when they give up their electrons, such as chemical catalysis. An early reducing atmosphere provided a rich source of reducing agents. In addition, the earth's crust contained large amounts H 2 S which provided a reducing source.