The origins of the cell membrane, the progenote, and the universal ancestor (LUCA)

Abstract

I analyse the consequences that the diversity of cell membranes present in bacteria and archaea would have for the stage reached by the evolution of cellularity in the Last Universal Common Ancestor (LUCA). Regardless of how the bacterial and archaeal membranes were distributed in the LUCA, the conclusion that would seem to emerge is always the same. That is to say, it is more likely that LUCA was a progenote rather than a genote. Indeed, if LUCA hosted both types of membranes then this would have been a transitional condition which would imply, in itself, that LUCA was a progenote. In other words, the two types of membranes evidently had to segregate in that of bacteria and that of archaea, and this segregation would imply enormous changes not expected for a cellular stage but the norm for that of progenote. Instead, in the case in which LUCA hosted only one type of membrane, for example that of archaea, then the evolution of the other type of membrane - presumably in the ancestor of bacteria - would imply a progenotic LUCA precisely because such a late origin would seem be an expression, at that time, of a still rapid and progressive evolution because it involves a fundamental and profound genetic trait, and should therefore reflect the progenotic stage. Finally, in the event that LUCA had been devoid of a membrane, then the late origin of the two types of membranes - in the ancestor of archaea and in that of bacteria - would seem to be a clear expression of a rapid and progressive evolution typical of progenote. Indeed, this would imply, for example, that all mechanisms and functions associated with the two cell membranes still had to evolve which would express the progenotic stage precisely because it would seem to be an evolution still in progress, and therefore typical of the stage of progenote. Therefore, LUCA was most likely a progenote also because there would be no strong arguments in favour of the hypothesis that LUCA had instead reached the evolutionary stage of cellularity.

Keywords: Bacterial and archaeal ancestors; Evolution of cellularity; Genote; LUCA –; Progenote.