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Its amazing. 2.3 billion years ago cyanobacteria (spelling?) started to exude oxygen as a byproduct of their metabolism. That created a major die off on the planet. The pre-cursor to Mitochondria at the the time was able to make use of oxygen. The rest is evolution. Always its about moving electrons - packets of light energy

Mitochondria is an organelle or one of many functional parts within a cell. They retain their own DNA, I am not sure if that classifies them as seperate organisms still or not. For sure they originated as bacteria, but have probably dropped some of the functionality of an independent organism, now that they are so well integrated.