About the last common ancestor, the universal life-tree and lateral gene transfer: a reappraisal

An organismal tree rooted in the bacterial branch and derived from a hyperthermophilic last common ancestor (LCA) is still widely assumed to represent the path followed by evolution from the most primeval cells to the three domains recognized among contemporary organisms: Bacteria, Archaea and Eucarya. In the past few years, however, more and more discrepancies between this pattern and individual protein trees have been brought to light. There has been an overall tendency to attribute these incongruities to widespread lateral gene transfer. However, recent developments, a reappraisal of earlier evidence and considerations of our own lead us to a quite different view. It would appear (i) that the role of lateral gene transfer was overemphasized in recent discussions of molecular phylogenies; (ii) that the LCA was probably a non-thermophilic protoeukaryote from which both Archaea and Bacteria emerged by reductive evolution but not as sister groups, in keeping with a current evolutionary scheme for the biosynthesis of membrane lipids; and (iii) that thermophilic Archaea may have been the first branch to diverge from the ancestral line.