The Nature of the Last Common Ancestor

Abstract

Until the late 1970s cellular evolution was assumed to be a continuous, unbroken chain of progressive transformations that begun with the emergence of life itself and continued until the endosymbiotic origin of eukaryotes marked the major biological discontinu-ity. This scheme was challenged when the comparison of small subunit ribosomal RNA (16/ 18S rRNA) sequences led to the construction of a trifurcated, unrooted tree in which all known organisms can be grouped in one of three major monophyletic cell lineages, i.e., the domains Bacteria (eubacteria), Archaea (archaeabacteria), and Eucarya (eukaryotes). Information from one single molecular marker does not necessarily yield a precise reconstruction of evolutionary processes, but as shown by numerous phylogenies constructed from other genes such as those encoding polymerases, elongation factors, F-type ATPase subunits, heat-shock and ribosomal proteins, the identification of the three major lineages is not an artifact based solely upon the reductionist extrapolation of information derived from the rRNA tree, but a true reflection of an ancient trifurcation.