Fig. 2

Energetic scenarios for the origin of eukaryotes. Filled arrows indicate FECA and the acquisition of mitochondria, empty arrows stand for LECA. Black lines roughly indicate averages in prokaryotes and eukaryotes. Prokaryotes cannot have genomes much larger than ~10 Mb (or ~10 K genes); smallest unicellular eukaryotes overlap with prokaryotes at this complexity. According to Lane and Martin [13, 50], there is an energetic barrier that prevents prokaryotes to maintain larger genomes (energy per cell values are from [13]). They claim that the early acquisition of mitochondria permit the transition of this barrier by temporarily increasing the gene count (blue curve; though the multiplier factor is only guessed by Lane, hence the dashed curves) to be able to experiment with new gene families. They maintain that amitochondriate eukaryotes cannot evolve directly from prokaryotes, only by losing the endosymbiont. Another possible scenario is to increase the area of internal respiratory membranes which provides extra energy with no additional genes (orange curve). This might just have been enough to power primitive phagocytosis. Mitochondria had to be acquired at a point where respiratory membranes could not be further exploited. Early mitochondria might induce gradual genome increase that progressively made inventions possible (green curve), though if this happened at low energetic levels, the archezoan niche (dashed oval) again could only be reached reductively. Theoretically, any trajectory between the orange and green curves is possible, either with early or late mitochondria. Ultimately, all scenarios lead to the same LECA, though starting from different FECAs. Present amitochondriate eukaryotes are secondarily derived (purple arrow), but some scenarios allow (orange and dark green) the existence of primarily amitochondriate “archezoan” eukaryotes