Mitochondria = cellular energy factories
In Greek: mitos, thread; chondrion, granule. Mitochondrion have layered membranes, granulated appearance, and ATP synthases that generate energy through the Krebs cycle (every Bio 1 student's nightmare) or when things go wrong, anaerobic respiration (nightmare of anyone with a septic patient).
Before discussing mitochondria & their role in evolution, here's a disclaimer (that anyone involved in Core curriculum at UC Merced is familiar with). Evolution and creationism are not either/or. You can believe in God/Allah/____ and evolution at the same time. God/Creators are outside of time, so who's to say they did not create all matter & energy (E=mc^2 etc.), including life (through evolution) in 7 "days" (which could very well be 4.5 billion years)?
However evolution got started, mitochondria, one of the most awesome little powerhouses to ever appear on earth, are key to life as we know it. Mitochondria and the tree of life is more entertwined in its branches than some older textbooks led us to believe, however.