Figure 1. Eukaryogenesis scenarios with respect to the origin and nature of the first eukaryotic common ancestor (FECA).

Scenarios summarized in (a) were based on the assumption that eukaryotic groups existed that lack mitochondria and which originated independently of endosymbiosis. Such models are no longer supported and we understand that the last eukaryotic common ancestor (LECA) was a mitochondrion-bearing lineage. Models in (b) speculate on a (semi-) autogenous origin of complexity. By their definition, the FECA was free of vertically transmittable endosymbionts. Genomic and cellular complexity of FECA grew through horizontally donated genes from diverse prokaryotes part of its habitat; to a degree FECA was eukaryote-like prior to endosymbiosis. Models falling under (c) differ from (b) in that both partners at the onset of eukaryogenesis were prokaryotic in nature and that complexity only began to evolve after endosymbiosis of the mitochondrial ancestor. By this definition, the FECA is characterized by the endosymbiotic event and describes the lineage that transitioned from prokaryotic to eukaryotic evolution. Note that the prokaryotic cells on the left and the LECA on the right are shown to scale on the basis of average cell diameters.