of ribonucleotides. Benner's group also (and much more persuasively) concluded, from the evident homology of DNA (or RNA) polymerases in the three domains, that the transition from RNA to DNA genomes had itself already been made by that last common ancestor, whatever its residual reliance on ribozymology (Benner et al., 1989).

To make such inferences about past events through use of parsimony arguments to reconstruct common ancestors from knowledge of the different paths taken by descendants, we must know that the contemporary groups compared really did begin to diverge at an appropriately ancient date. In the rest of this review, we consider developments in our thinking about the relationships between basic kinds of living things primarily as they bear on this issue, asking if there is any reason to believe that the cenancestor was a progenote.

Of the two new terms introduced here, cenancestor is Walter Fitch's for "the most recent common ancestor to all the organisms that are alive today (Fitch and Upper, 1987)". Progenote is George Fox and Carl Woese's descriptor for "a theoretical construct, an entity that, by definition, has a rudimentary, imprecise linkage between its genotype and phenotype (Woese, 1987)"—a creature still experiencing progressive Darwinian evolution, in other words.

The Basic Kinds of Living Things

For more than a century, microbiologists suspected that bacteria, because of their small size and seemingly primitive structure, might differ fundamentally from animals, plants, and even fungi. The blue-green algae (now "cyanobacteria") might be intermediate, looking like bacteria but acting like plants. Chatton in 1937 (Chatton, 1937) and Stanier and van Niel in 1941 (Stanier and van Niel, 1941) proposed that these two groups share a common cellular organization distinguishing them as prokaryotes from the rest of the living world, or eukaryotes. A clear statement of the differences, however, required further work in biochemistry, genetics, and cellular ultrastructure. By 1962, Stanier and van Niel (Stanier and van Niel, 1962) were prepared to define prokaryotes in terms of the specific features they shared as well as the eukaryotic characteristics they lacked. They wrote that:

… the principal distinguishing features of the prokaryotic cell are:

  1. absence of internal membranes which separate the resting nucleus from the cytoplasm, and isolate the enzymatic machinery of photosynthesis and respiration in specific organelles;
  2. nuclear division by fission, not by mitosis, a character possibly related to the presence of a single structure which carries all the genetic information of the cell; and

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