Figure 4 Further implications of the rRNA data (Woese and Fox, 1977). A third group, the archaebacteria, seemed as distant evolutionarily from eubacteria and eukaryotes as these were from each other.

extreme thermophiles, and still completely uncharacterized and unseen meso- or psychrophiles, which are related to the extreme thermophiles and known only from PCR products amplified from the open ocean (DeLong, 1992). Uniting them are a number of basic characters unrelated to rRNA sequence and more than adequate to support their taxonomic and phylogenetic unity in spite of this diversity. These include unique isopranyl ether lipids (and the absence of acyl ester lipids found in eubacteria and eukaryotes); characteristic genetic organization, sequence, and function of RNA polymerase subunits; structural and functional characteristics of ribosomes and modification patterns of tRNAs; varied but unique cell-envelope polymers; and distinctive antibiotic sensitivities and insensitivities (Zillig et al., 1993).

Rooting the Universal Tree

Woese felt that the differences between archaebacteria and either eubacteria or eukaryotes were of a sufficiently fundamental nature to indicate that all three primary kingdoms must have begun to diverge during the period of progressive evolution from a progenote. But there was no way to decide the order of branching—whether the first divergence in the universal tree separated (i) eubacteria from a line that was to produce archaebacteria and eukaryotes, or (ii) a proto-eukaryotic



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