Explain the importance of the buildup of free oxygen in the atmosphere for the evolution of animal life.

Prior to the buildup of oxygen, organisms were mainly
anaerobic and used mechanisms where no oxygen was present or needed.  Once the buildup
of oxygen starting occurring, organisms were forced to either evolve to survive or they
would die off due to the presence of oxygen. The organisms needed to evolve so that they
could use and benefit from the abundance of oxygen. The presence of oxygen is a huge
evolutionary milestone that kick started the evolution of many new species.  Organisms
began to metabolize and perform all of its functions with oxygen
present.