The early earth or aka Mordor.

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

Cyanobacteria are the perveyors of that sweet, molecule, oxygen!

As you may or may not know but the cyanobacteria microbes roughly 3 billion years ago started to produce oxygen, the molecule that allows for oxygenic photosynthesis and aerobic respiration.

Cyanobacteria are phototrophic organisms, which means they contain pigments that absorb light to be used as an energy source. This energy is used to excite electrons which is a form of energy conservation used to drive atp synthesis.

More specific to cyanobacteria they perform oxygenic photosynthesis, which O2 is expelled thus adding to the concentrations of Oxygen for aerobic respirers like us to use for our metabolic functions.

The earth several billion years ago was an anoxic environment which means there was no Oxygen to be used for respiration. Cyanobacteria were critical in the evolution of life as they were the first to produce Oxygen.