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

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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.

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