Figure 2

A hypothetical scenario of the advent of the machinery of apoptosis and its evolution. A: Initially a prokaryote world containing archaeal cells and α-proteobacteria existed. Multiple gene transfers occurred at this time that may have resulted in incorporation of metacaspases into the archaea and addiction modules into the bacteria. B: The ancestral archaeal cell contained metacaspases and the ancestral α-proteobacteria contained an addiction module and elements such as cytochrome c and porins. C: The parasitization of the archaeal cell by this α-proteobacteria gave rise to the ancestral eukaryote cell. D: A nucleus evolved and gene transfer from the proto-mitochondrion to the host nucleus occurred, giving rise to the last common eukaryote ancestor (LCEA). E: The early branching eukaryotes emerged, including the parasitic protozoans. F: The last common eukaryote ancestor also gave rise to a crown group exhibiting multicellularity. G: The metacaspases family evolved and diversified giving rise to the animals. Caspases and paracaspases and the control elements such as the Bcl family emerged in some groups, possibly as a result of further gene transfer. In general, great diversification of the molecular machinery of apoptosis occurred. H: Secondary endosymbiosis occurred between the crown group ancestor and a cyanobacterium. I: The stabilization of the plastid gave rise to the plants. This scenario supposes this secondary endosymbiosis occurred after the crown group ancestor became multicellular. However, multicellularity may have arisen twice. There does not appear to be a role in the apoptosis machinery for addiction modules homologous to extant bacteria and these may have been lost once the mitochondrion became stabilized in the host cell.