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Complexity of life 3.5 billion years ago?

aroger at ac.dal.ca aroger at ac.dal.ca
Fri Jul 7 06:56:50 EST 1995

In article <3tf5e6$p10 at senator-bedfellow.MIT.EDU>, lluis at aaRS (Lluis Ribas) writes:
>       Hi,
>     I would like to hear educated opinions on the following question :
>     What may have been the complexity of life on earth at the moment of
>     appearance of the eukaryota/archibacteria superkingdoms ?.
>     Basicly should we assume that a large number of distinc unicellular 
>     (supposedly) organisms existed ?. 

I don't think that we should assume anything at this point.  It depends
strongly on when the splitting of archaebacteria/eukaryotes took
place.  If it was more than 3.5 billion years ago as Woese and colleagues 
argued, then it is difficult to say what kind of eubacterial diversity
was present.  If it was more like 2.0 billion years ago (just prior
to the first fossil evidence of the existence of eukaryotes) then
there must have been quite a lot of eubacterial diversity 
(provided that the 3.5 billion year old stromatolites and filamentous
fossils are of cyanobacterial nature). 

Thus it depends upon two things:

1) the timing of the radiation of eubacteria relative to the
timing of the splitting of archaebacteria/eukaryotes

2) where the root of the tree of life falls; between the
eukaryote/archaebacteria clade and the eubacteria, or
within the eubacteria (these are the two options which
appear to be the most promising right now- to me at least). 

>that would imply that we should be able >   
>     to link eukaryote/archibacteria evolution to a subpopulation of the
>      eubacteria.
This comment concerns number 2 above.  There is no evidence from
reciprocal rooting of duplicated gene families that the eubacteria
are paraphyletic-- ie- one subgroup of them gave rise to the 
euk/archae lineage.  But until these trees contain all known
eubacterial groups the jury will still be out.

For references look at the following authors in the last
5 years:

W.Ford Doolittle (especially in PNAS)
Peter Gogarten
James R. Brown
Patrick Forterre
Pierro Cammarano
N. Iwabe
W. Zillig
C. Woese

Hope this helps
Andrew Roger
aroger at ac.dal.ca

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