[ClearCase_guy]

Seems like an interesting issue for evolution: "I don't have any use for oxygen, but my progeny will die without it."

I'm having a hard time seeing how this would evolve via incremental steps.

[James C. Bennett]

Simultaneously having multiple energy cycles, would be evolutionarily beneficial. It could have started out that way.

[AdmSmith]

Try this:
Several unicellular organisms (prokaryotes and protozoa) can live under permanently anoxic conditions.

http://www.biomedcentral.com/1741-7007/8/30/abstract