Geological evidence indicates that four billion years ago, the early earth had very little oxygen. The most primitive living photosynthetic organisms are intolerant of oxygen. They conduct cyclic photophosphorylation, a form of photosynthesis that does not generate oxygen.

Geological evidence also indicates the presence of large quantities of free oxygen beginning about 2.5 billion years ago. This oxygen was generated by photosynthetic organisms that split water to obtain electrons and hydrogen atoms during noncyclic photophosphorylation, the photosynthetic processes used by most plants today.