Abstract

Available evidence suggests that two prokaryotes, an archaeon and a bacterium, collaborated in the eventual formation of nucleated cells with arguably increased complexity of form and function. However, the mechanisms by which bacteria and archaea cooperated in the formation of eukaryotes, and the selective pressures that promoted this partnership, remain a mystery. Mitochondria are eukaryotic organelles thought to be derived from respiring, alpha-proteobacterial endosymbionts capable of generating ATP by oxidative phosphorylation. The earliest eukaryote likely harbored mitochondria, since all characterized eukaryotic lineages show evidence of containing, or having once contained, these organelles. Consequently, it has been argued that mitochondria, and particularly the ATP that can be generated by these compartments, allowed for evolution toward an expanded number of proteins, an increase in overt specialization achievable by eukaryotic cells, and the eventual formation of complex multicellular organisms. However, the relationship between mitochondrial ATP generation and its potency in allowing genome expansion has been a matter of debate. Moreover, how and why an endosymbiont not yet converted to an organelle might purposefully provide ATP to its host is not clear. Here, I propose that the initial driving force for integration of the proto-mitochondrial endosymbiont within the proto-eukaryotic host may not have been provision of ATP to its archaeal partner, but rather that heat generated by the endosymbiont allowed the archaeal host to endure lower temperatures at the outset of eukaryogenesis. I discuss how this arrangement may have led to the increased apparent complexity that is characteristic of eukaryotes.