Treffer: How many and which types of primary cells (domains of life) emerged from LUCA, identified employing different genetic codes.

The progenote- > cell transition defines the evolutionary stage of the formation of a domain of life. Indeed, the progenote evolutionary stage cannot be or have a domain of life, because the progenote stage does not, by definition, contain cells, being an evolutionary stage with a genotype-phenotype relationship still in formation. Therefore, the progenote stage is certainly cell-free and therefore cannot be a domain of life, which is instead made up of cells. Consequently, it is at the moment in which a cell emerged from the progenote stage that the first domain of life was defined. In fact, this would be the first appearance of a cell in the history of life, since the progenote stage is cell-free. All of this would define the birth of a domain of life because it would represent the first and precise evolutionary point at which the first cell would be born, after the progenote stage. Furthermore, the progenote- > cell transition is completely reflected in the transition from evolving genetic code to frozen genetic code. That is to say, a genetic code in evolution, i.e. still originating, would precisely reflect the definition of progenote because a code that is still originating should have a genotype-phenotype relationship yet to be defined and would identify, by definition, the stage of the progenote. A frozen genetic code, a fully formed, modern genetic code, would imply the cell stage, that is, the abandonment of the progenote stage. Indeed, nothing more than a modern genetic code might indicate the achievement of the cell stage because a fully developed genetic code would imply modern proteins, which in turn would imply that all the structures of that particular evolutionary stage would certainly belong to a cell stage, precisely because they are structures to be considered, in some sense, complete and definitive. In conclusion, the simple observation of specific variants of the genetic code would provide the opportunity to easily identify the domains of life because a genetic code possessed only by a certain type of organism would witness the transition from evolving genetic code to frozen genetic code and consequently the transition from progenote to cell, and would therefore identify a domain of life. In this way, that is to say, by using some variants of the genetic code, the following domains of life have been identified: Bacteria, Methanogens, and Non-Methanogenic Archaea. Eukaryotes, having the same genetic code as non-methanogenic archaea, would belong to the same domain as the latter. The discussion focuses on these three domains of life and their relationship with the domains of life suggested by other hypotheses, and on properties of the variants of the genetic code used to identify these domains.
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Declaration of competing interest Author declares no conflict of interest.