Mitochondrial evolution. Mitochondrion has played crucial role in the evolution of the eukaryotic cell. Some two billion years ago this organelle, which now produces most of the cellular ATP and mediates programmed cell death, used to be a free-living gram-negative bacterion. The early endosymbiotic relationship was followed by the transfer of bacterial genetic information into the host cell nucleus and resulted in the enslavement of the endosymbiont. Hence mitochondrial proteins are translated on free cytosolic ribosomes and transported into the organelle. To this aim eukaryotic cell installed new translocases in the mitochondrial membranes. Origin of these multimeric complexes nicelly illustrates mechanisms of molecular evolution. This review summarizes several aspects of the mitochondrial evolution and our current ideas on the origin of mitochondrial transocases.