Figure 3 Implications of the rRNA data of Woese and Fox (1977). Prokaryotic (eubacterial) lineages from which the eukaryotic nuclear lineage was thought to have evolved were entirely separate from that lineage. Distinct properties of rRNAs suggested that the ribosome of the last common ancestor was a primitive ribosome and that the last common ancestral cell was a primitive "progenote," still experiencing progressive Darwinian evolution.

molecular differences among the three major groups would be in refinements of functions that occurred separately in the primary lines of descent, after they diverged from the universal ancestor.

In other words, the cenancestor was a progenote—one of the series of ancestral forms in which the phenotype-genotype coupling was actively evolving, and we might learn about progressive Darwinian evolution by comparing prokaryotic and eukaryotic (nuclear) molecular biology. Woese went on:

… it is hard to avoid concluding that the universal ancestor was a very different entity from its descendants. If it were a more rudimentary sort of organism, then the tempo of its evolution would have been higher and the mode of its evolution highly varied, greatly expanded (Woese, 1987).

This view came to play a dominant role in the molecular biology and evolutionary microbiology of the 1980s and early 1990s. The prokaryote/eukaryote dichotomy remained, but as a vertical split, separating living things into two camps from the very beginning rather than marking a more recent but crucial transition in the grade of cellular



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