A qualitative criterion for identifying the root of the tree of life.

Highlights • I suggest a criterion for identifying the root of the tree of life. • The group of organisms with the greatest molecular variability in phylogenetic deep characters represents the root of the tree of life. • Several arguments are advanced for justifying this suggestion. • DPANN archaea and CPR bacteria are suggested as roots of the tree of life. Abstract I suggest –as a criterion for identifying the root of the tree of life – that the group of organisms with the greatest molecular variability in phylogenetic deep characters represents the root of this tree. Indeed, it is expected that in the circumstance of the origin of a given trait several biochemical pathways would have contemporary evolved, for example. The presumed very strong selective pressure acting for the first time on the appearance of the function of that phylogenetically deep trait would have caused its multiple appearance exactly because these traits were originating for the first time. As a result, several pathways would have evolved simultaneously to solve that impelling function. In addition, the evolutionary stage of the progenote would seem that favours per se the formation of these multiple traits – that is to say, performing equivalent or similar function – because it would represent the evolutionary stage in which the origins of all cellular structures occurred. Therefore, it is in this particular evolutionary circumstance that it was – by definition – in the condition that allowed the emergence, for example, of several alternative biochemical pathways to solve the same evolutionary task. Consequently, I here discussed the possibility that the superphylum of DPANN archaea and that of CPR bacteria – especially if considered as a single group of organisms – are/is considered the root of the tree of life, exactly because they would seem to exhibit relatively higher molecular variability in phylogenetically deep characters when compared with other phyla of archaea and bacteria. [ABSTRACT FROM AUTHOR]