The Non-Biological Meaning of the Term “Prokaryote” and Its Implications

Massimo Di Giulio

The meaning of the term prokaryote is critically analyzed. The conclusion reached is that this term does not have a real biological sense, above all because we are not able to link to this term a specific biological characteristic, i.e. the hypothetical evolutionary stage of the prokaryote would seem to have been unable to result in a completed cell, which could possibly be due to the recapitulation of the fundamental characteristics that might have been common to bacteria and archaea. This would define a biological immaturity of this evolutionary stage because otherwise we would have found traits already clearly defined at this level of cellular evolution. Therefore, the lack of well-defined traits characterising the prokaryote would seem to imply an evolutionary stage still in rapid evolution, i.e. with a tempo and a mode of evolution typical of a progenote. This in turn would seem to imply that the last universal common ancestor (LUCA) has been a progenote at least when the domains of life are only two-the bacterial and archaeal domains-because, in this case, the LUCA's node should coincide with that of prokaryote on the tree of life. Instead, if the root of the tree of life would be placed in the bacterial domain or in the archaeal one, we might again, very likely, have a LUCA with a character of progenote being, under these conditions, the LUCA a prokaryote-like organism.