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In the face of general bafflement as to the purpose of peroxisomes,
an ingenious theory has been proposed, according to which peroxisomes
and catalase arose as an early defense mechanism of primitive anaerobic
organisms against atmospheric oxygen. Many essential reactions in
a cell lead to reduced flavoproteins, which then are reoxidized
by anaerobic means. Traces of oxygen in the surroundings of an anaerobe
could upset things by reoxidizing the flavoproteins directly, and
producing hydrogen peroxide:
 
Peroxides of all kinds are reactive and dangerous oxidants, and
must be removed for the safety of the cell. Catalase may have evolved
to meet this need by using some expendable organic molecules (H R)
as a reducing donor:
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