In article <1993Oct31.222749.125610 at embl-heidelberg.de>, ouzounis at embl-heidelberg.de writes:
>> Probably what Woese suggested as the progenote, would represent a
> large number of qualitatively different steps of evolution, including
> an RNA genome, later a DNA genome, probably still fragmented and perhaps
> compartmentalized in a primitive cell membrane. Before attacking a rather
> interesting and not well-explored idea, I would give it a further chance.
>> Christos Ouzounis
Sorry, I should have clarified the problem a bit better. Let me suggest a
useful distinction between the progenote CONCEPT and the progenote
HYPOTHESIS (this distinction can be seen in Woese, 1987, Microbiological
Reviews vol. 51 pp. 262-264):
PROGENOTE CONCEPT: a 'progenote' is defined as a type of cell with a loose
genotype/phenotype connection, short genes, segmented genomes, etc.
PROGENOTE HYPOTHESIS: the most recent common ancestor of known cellular life
(archaebacteria, eubacteria, eukaryotes) was a progenote, so that all three
'domains' independently evolved genotic features.
I have no wish to argue with the progenote concept-- its a "theoretical
construct" (Woese, 1987, p. 263) and its validity is not dependent on whether
or not a 'progenote' ever existed in earth's history.
By contrast, the progenote hypothesis is a falsifiable proposition, and is
deeply contradicted by modern molecular data. To believe the progenote
hypothesis is to believe no less than that eukaryotes, prokaryotes, and
archaebacteria each (starting with short RNA genes in a small segmented
genome) separately and independently lengthened and converted their short
RNA genes or otherwise evolved long DNA genes for homologous
and conserved rRNA, EF-Tu, etc, as well as for a dozen homologous and
conserved ribosomal proteins (whose operon order would also be independently
derived in archaebacteria and eubacteria). An analogous claim about, say,
lambdoid phages (that they all independently evolved DNA genomes with
homologous CI proteins, homologous capsid proteins, homologous recombination
proteins, etc) wouldn't be entertained as a serious alternative to common
ancestry. Why is this different?