Eukaryotes have long been thought to have arisen by evolving a nucleus, endomembrane, and cytoskeleton. In contrast, it was recently proposed that the first complex cells, which were actually proto–eukaryotes, arose simultaneously with the acquisition of mitochondria. This so–called symbiotic association hypothesis states that eukaryotes emerged when some ancient anaerobic archaebacteria (hosts) engulfed respiring αproteobacteria (symbionts), which evolved into the first energy–producing organelles. Therefore, the intracellular compartmentalization of the energy-converting metabolism that was bound originally to the plasma membrane appears to be the key innovation towards eukaryotic genome and cellular organization. The novel energy metabolism made it possible for the nucleotide synthetic apparatus of cells to be no longer limited by subsaturation with substrates and catalytic components. As a consequence, a considerable increase has occurred in the size and complexity of eukaryotic genomes, providing the genetic basis for most of the further evolutionary changes in cellular complexity. On the other hand, the active uptake of exogenous DNA, which is general in bacteria, was no longer essential in the genome organization of eukaryotes. The mitochondrion–driven scenario for the first eukaryotes explains the chimera–like composition of eukaryotic genomes as well as the metabolic and cellular organization of eukaryotes.