The early earth was an environment not suitable for very many organisms, and indeed only microbes that could survive extreme environments could survive. One condition they had to live under was anoxic conditions, which involved using various terminal electron acceptors to perform their metabolic functions.
The early cyanobacteria evolved a mode for respiration that released Oxygen, and over millions of years enough Oxygen in the atmosphere was concentrated to allow for aerobic respiration. The element Iron was also fundamental in the increasing concentrations of Oxygen absorbed in our atmosphere.
The link below is supplemental information provided by the University of Berkely. It has great images and goes into substantial detail of the conditions of early earth.
http://www.ucmp.berkeley.edu/bacteria/cyanointro.html
