A view of early cellular evolution.

Some recent puzzling data on mitochondria put in question their place on the phylogenetic tree. A hypothesis, the archigenetic hypothesis, is presented, which generally agrees with Woese-Fox's concept of the common origin of eubacteria, archaebacteria, and eukaryotic hosts. However, for the first time, a case is made for the evolution of mitochondria from the ancient predecessors of pro- and eukaryotes (protobionts), not from eubacteria. Animal, fungal, and plant mitochondria are considered to be endosymbionts derived from independent free-living cells (mitobionts), which, having arisen at different developmental stages of protobionts, retained some of their ancient primitive features of the genetic code and the transcription-translation systems. The molecular-biological, bioenergetic, and paleontological aspects of this new concept of cellular evolution are discussed.