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.