PMC

Schematic summary of general types of hypotheses on the phylogenetic origin of eukaryotes in the tree of life before and after the discovery of Lokiarchaeota and related deep-branching archaea. A, archaea; B, bacteria; E, eukaryotes. The schematic organelle drawing represents the mitochondrial acquisition.

Three hypothetical mechanisms to explain the evolution of the eukaryotic cell. Focus is made on key eukaryotic cellular structures (nucleus, mitochondria, membrane system). (A) Evolution of a proto-eukaryotic lineage before the mitochondrial acquisition event; it is compatible with the classical model proposing a third primary proto-eukaryotic lineage () and with models proposing that the eukaryotic host derived from within archaea (). (B) Endosymbiosis of mitochondrial ancestor within one archaeon as the triggering event for eukaryogenesis (). (C) Endosymbiotic origin of nucleus and secondary symbiosis with ancestor of mitochondria ().