Figure 1: The Aristotle scenarios. (a) The traditional ladder-like evolutionary scenario, in which organisms increased in complexity from the origin of life to prokaryotes and eukaryotes, with archaea being intermediate organisms on the way to become eukaryotes; (b) the classical universal tree of life of Woese et al. [53] in red, combined with the fusion hypothesis (blue line). The LECA is the last eukaryotic common ancestor and FME the first eukaryote harbouring mitochondria; the dotted line refers to the hypothesis in which eukaryotes originated by the association of an archaeon with the mitochondrial bacterial ancestor [49]. Thick arrows indicate the emergence of eukaryotic specific features (ESFs).