There can be little doubt that mitochondria do not need much of an introduction. It is widely known that they are the “powerhouses” of the cell and that they produce all of the ATP (adenosine triphosphate) needed to sustain life. In addition, all eukaryotes (organisms with a cell nucleus) contain these important organelles—or so it was thought. Here lies an interesting paradox: Although it was generally believed that all eukaryotes did have mitochondria, it was also generally believed that the serial endosymbiosis theory was correct; the endosymbiosis theory required the existence of eukaryotes without mitochondria. This assumption was formalized with the now-disproven Archezoa hypothesis, which stated that several groups of “primitive” eukaryotes were of premitochondrial descent. This paradoxically defined group of amitochondriate eukaryotes has resulted in a spate of publications that have significantly changed the perception of the role of mitochondria in overall cellular metabolism and that have important ramifications for our understanding of the origin of eukaryotic life.