In early year the earth was devoid of free oxygen- oxygen that is not mixed with carbon and nitrogen that is needed for anearobic respiration, only the cyanobacteria releases free oxygen leading to it low concentration on earth. Because of the low concentration earth could not support life. Over the years a small portion of the ocean started releasing oxygen into the earth increasing the amount of oxygen in the planets earth. This is needed in anearobic respiration for the release of ATP,alcohol( Ethanol) and CO2
This oxygen makes the earth more suitable to accommodate complex life leading to diversification and evolution of new organisms.
Biologically induced molecular oxygen (dioxygen, O2) started to accumulate in Earth's atmosphere and changed Earth's atmosphere from a weakly reducing atmosphere to an oxidizing atmosphere.
