Divergence of bacilli and the purple bacteria

Sean Eddy sre at al.cam.ac.uk
Tue Feb 22 02:47:29 EST 1994

In article <1994Feb19.165753.20988 at dal1> aroger at ac.dal.ca writes:
  >IF the cyanobacterial fossils from 3.5bya are really cyanobacterial and,
  >IF this state is derived in bacteria (that is, if the ancestral state
  >of eubacteria is not cyanobacterial-like in growth properties and
  >photosynthetic properties), then it appears (PROVIDING the 16S rRNA
  >trees are accurate) that most eubacterial phyla diverged almost
  >simultaneously prior to 3.5bya.  That would place the divergence time
  >between a gram positive (bacillus) and a gamma proteobacterium (E.coli)
  >prior to 3.5bya.

What sort of probabilities would you attach to the "if"'s?

The 3.5 Bya (or even 3.8 Bya) cyanobacterial fossils seem to be widely
accepted, but maybe I'm hearing seminars from only one camp. Are the
fossils themselves in question, or is the problem that the fossils
might represent real photosynthetic one-celled organisms of some sort,
but that the modern cyanobacteria could have arisen later and they
happen to make similar structures (stromatolites, for instance)?

What odds would you give that cyanobacteria were the ancestral
bacterial state? Are there any modern eubacteria that are
chemoautotrophic (so you could imagine them living in a world without
photosynthesis)? If the whole eubacterial branch depends on
photosynthesis directly or indirectly, I might lean towards thinking
that photosynthetic eubacteria were ancestral. How do the two
alternatives fit onto the rRNA tree, parsimony-wise? 

- Sean Eddy
- MRC Laboratory of Molecular Biology, Cambridge, England
- sre at mrc-lmb.cam.ac.uk

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