Figure 5 Rooting method of Iwabe and coworkers (1989). Two unrooted trees are constructed for eubacteria (B), archaebacteria (A), and eukaryotes (E), one with sequences of A genes and one with sequences of A' genes. A and A' genes are products of a gene duplication that must have predated the time of the last common ancestor because both are found in B, A, and E. The A tree can be rooted with A' sequences and vice versa, so that the cenacestor (the universal root) is in the position shown by the star for either tree.

biologists in Munich in 1981, one could sense a general feeling that archaebacteria were somehow "missing links" between eubacteria and eukaryotes. Zillig in particular stressed the (still-supported) eukaryote-like structural and functional characteristics of archaebacterial RNA polymerases (Zillig et al., 1982). In the subsequent 7 or 8 years, further gene sequences for proteins of the information-transfer system (ribosomal proteins, DNA polymerase) that looked strongly eukaryote-like had appeared. Although not rootable, these data too seemed to support a specific archaebacterial/eukaryotic affinity (Ramirez et al., 1993).

In 1990, Woese, Kandler and Wheelis incorporated the Iwabe rooting in a new and broader exegesis on the significance of the tripartite division of the living world (Woese et al., 1990). This treatment elevated the rank of the three primary kingdoms to "domains" (since kingdom



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