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Summary
There is much evidence (in particular from comparisons of sequenced genomes) to indicate that cellular organelles were originally free-living bacteria that entered into a cosy symbiotic relationship with their host cells and then decided to hang around in eukaryotic cells for good. As Bill Martin explains in a Perspective, new evidence about the nature of the ancestral mitochondrion comes from studies of a eukaryotic alga called Mallomonas, which has two forms of a cell division protein called FtsZ found in bacteria but not yeast or higher eukaryotic cells (Beech et al.). The form found in mitochondria of the alga is very similar to that found in ?-proteobacteria, bolstering the view that these bacteria gave rise to the mitochondria of eukaryotic cells.
