In article <SRE.94Feb22074729 at al.cam.ac.uk>, sre at al.cam.ac.uk (Sean Eddy) writes:
> 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
[stuff deleted]
>> 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
It doesn't seem to me (as a non-expert) that the biogenic origin of the 3.5
BY-old Apex microfossils is in question: see Schopf's 1993 _Science_ article
(vol. 260: 640).
> 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)?
Schopf thinks many of the microfossils in this formation are "probable
cyanobacteria," based mainly on the fact that they are filamentous
prokaryotes with sizes in the range characteristic of modern
filamentous cyanobacteria (which tend to be larger than filamentous
prokaryotes that are not cyanobacteria). Other than the morphological
similarity to cyanobacteria, there is no reason to suppose that the
microfossils represent photosynthetic (as opposed to non-
photosynthetic) organisms.
These cyanobacterium-like microfossils are free or loosely
associated-- they are not actually part of the stromatolite-like
macrofossils that also occur in the Apex formation.
Archean and proterozoic stromatolites are similar in shape to some
modern cyanobacterial colonies, and I'm assuming (though I haven't
actually seen it clearly stated anywhere) that this kind of colony
morphology is not found among extant non-cyanobacteria. Thus, the
simplest inference would be that the organisms that made the stroma-
tolites (of all ages) are cyanobacteria-related. Nevertheless,
it seems that Schopf and others recognize the possibility that a
stromatolite colony shape could arise from an aquatic colonial
mucus-secreting autotrophic prokaryote, whether or not it is a
cyanobacterium or even an oxygenic phototroph (the colony would have
to have a mucilaginous surface growth layer that incorporates
particulate matter, slowly adding layer upon layer of rock to its
base).
The same kind of interpretive problem would arise if someone were to
find a 1.2 BY-old mushroom-shaped (or earthworm-shaped) fossil.
One could suggest that it is an ancient representative of modern fungi
(or annelids), or perhaps it is a completely different type of
organism, and the mushroom (or earthworm) shape appears repeatedly in
evolution because it can be generated by a simple set of developmental
rules.
> 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?
There are many diverse lifestyles among the eubacteria, many
photoautotrophs, chemoautotrophs, and heterotrophs. If we understood
these diverse trophic mechanisms in detail (so that we could tell
which are homologous to which others), and if we knew the
relationships of bacterial phyla with some certainty, then (as you
suggest) it would be possible to infer "parsimony-wise" an ancestral
state. But these conditions have not been met, yet. There is no
evidence that the 16S rRNA tree gives reliable information on
eubacterial inter-phylum relationships.
Arlin
