On the evolutionary descent of organisms and organelles: a global phylogeny based on a highly conserved structural core in small subunit ribosomal RNA.

To probe the earliest evolutionary events attending the origin of the five known genome types (archaebacterial, eubacterial, nuclear, mitochondrial and plastid), we have analyzed sequences corresponding to a ubiquitous, highly conserved core of secondary structure in small subunit rRNA. Our results support (i) the existence of three primary lineages (archaebacterial, eubacterial, and nuclear), (ii) a specific eubacterial ancestry for plastids and mitochondria (plant, animal, fungal), and (iii) an endosymbiotic, evolutionary origin of the two types of organelle from within distinct groups of eubacteria (blue-green algae (cyanobacteria) in the case of plastids, nonphotosynthetic aerobic bacteria in the case of mitochondria). In addition, our analysis suggests (iv) a biphyletic origin of mitochondria, with animal and fungal mitochondria branching together but separately from plant mitochondria, and (v) a monophyletic origin of plastids. The method described here provides a powerful and generally applicable molecular taxonomic approach towards a global phylogeny encompassing all organisms and organelles.