Mitochondria were originally free-living bacteria with their own division machinery, which took up residence within another cell two billion years ago. The host cell subsequently tamed mitochondrial division using dynamin, a membrane-pinching protein. We have found that a single ancient dynamin capable of pinching both mitochondria and vesicles duplicated independently in plants and animals into specialized mitochondrial and vesicle variants. A "living fossil" of this ancient bifunctional dynamin still survives in scattered eukaryotic lineages, along with the original bacterial FtsZ division protein. The mitochondria of these organisms, preserved as if in amber, might teach us how the fateful partnership between host and endosymbiont was first established.