In an evolutionary story akin to the fairytale Goldilocks and the Three Bears, it seems that the timing of the endo-symbiotic event that led to the development of mitochondria was ‘just right’. However, traces remain of archaeal-bacterial interactions that pre-date this crucial event in eukaryotic evolutionary history.

New research published in Nature this week is challenging our understanding of eukaryotic cell evolution. Mitochondria, the all-important ‘power-house’ organelles that generate energy for eukaryotic cells, originate from a single endosymbiotic event involving Alphaproteobacteria and an archaeal host. However, when this event occurred is not quite so clear.

Taking on the role of Goldilocks (who explored the Three Bears house), Pittis & Gabaldón investigated the phylogeny of proteins present in the ‘last eukaryotic common ancestor’. They confirmed the eukaryotic archaeal origin, and identified mitochondrial proteins which likely originated from the alphaproteobacterial ancestor of mitochondria. The specifics of their findings suggest that the archaeal cells already had some degree of intracellular complexity before the mitochondrial endosymbiosis, supporting an intermediate-timing for mitochondrial origin, in terms of archaeal cell evolution. Interestingly though, they also found a third group of bacteria-originated proteins that were acquired before the mitochondrial proteins. The authors speculate that they may indicate the occurrence of earlier, pre-mitochondrial, symbiosis events or horizontal gene transfer between bacteria and the host, but the answers remain elusive for now.