Abstract
The characteristics of the last eukaryotic common ancestor (LECA) population and the root of the eukaryotic tree have been coming into focus lately. However, the trajectory taking the host, related to present-day Asgard archaea and the endosymbiont, related to present-day alphaproteobacteria, toward such fully integrated and complex organisms is still unclear. Here I marshal recent evidence supporting the early arrival of the "mitochondrion-to-be", setting up the evolutionary dynamic for a series of mutual adaptations leading to eukaryotes. Upon critical analysis of some presuppositions in phylogenomic reconstructions of eukaryogenesis, I again propose that pre-symbiosis, efficient ATP generation, internal reactive oxygen species (ROS) formation and enhanced retention of genes supplied by horizontal gene transfer (HGT) interdependently allowed this unique transformation to occur.