Overwhelming molecular evidence has established that eukaryotes acquired mitochondria when an alpha-proteobacterium became an endosymbiont. Modern-day alpha-proteobacteriainclude pathogenic Rickettsias. When the two formerly independent cells established a stable, endosymbiotic relationship, the Bacterium contributed molecular machinery for ATP synthesis by oxidative phosphorylation. The host cell might have supplied organic substrates to fuel ATP synthesis. Together, they had a reliable energy supply for processes such as biosynthesis, regulation of the internal ionic environment, and cellular motility. Given that some primitive eukaryotes lack full-fledged mitochondria, the singular event that created mitochondria was believed to have occurred well after eukaryotes branched from prokaryotes.
An alternative idea is that the recipient of theα- proteobacterium was an archaean cell rather than a eukaryote. If so, this union could have created not only the mitochondrion but also the first eukaryote! This parsimonious hypothesis is consistent with some but not all of the available data, so it is currently impossible to rule out other scenarios.
The mitochondrial progenitor brought along its own genome and biosynthetic machinery, but over many years of evolution, most bacterial genes either moved to the host cell nucleus or were lost. Like their bacterial ancestors, mitochondria are enclosed by two membranes, with the inner membrane equipped for synthesis of ATP. Mitochondria maintain a few genes for mitochondrial components and the capacity to synthesize proteins. Nuclear genes encode most mitochondrial proteins, which are synthesized in the cytoplasm and imported into the organelle. The transfer of bacterial genes to the nucleus sealed the dependence of the organelle on its eukaryotic host.
Even though acquisition of mitochondria might have been the earliest event in eukaryotic evolution, some eukaryotes lack fully functional mitochondria. These lineages apparently lost most mitochondrial genes and functions through reductive evolution in certain anaerobic environments that did not favor natural selection for respiration. The most extreme example is the anaerobic protozoan Giardia, which has only a remnant of a mitochondrion and only one mitochondrial gene in the nucleus. The protist Entamoeba histolytica is a less extreme example. It lacks mitochondria but has a remnant mitosome consisting of two concentric membranes with some rudimentary mitochondrial functions.
