Figure 3: A scenario based on divergent evolutionary trends for “prokaryotes” (archaea and bacteria) and eukaryotes. This scheme is based on the assumption that the universal tree is rooted in the bacterial branch [53]. Complexity increased from the origin of life to LUCA to eukaryotes, via the last archaeal-eukaryal common ancestor (LAECA). Reductive evolution occurred from LAECA to LACA and modern archaea, possibly triggered by thermoreduction [77] indicated by large red triangles and/or as a way to escape protoeukaryotic predators [50]. Bacteria experienced independently a similar evolutionary path. The blue arrow indicates the mitochondrial endosymbiosis.