On Thu, 16 Dec 2004 16:23:52 +0000 (UTC), 
Paul Ciszek  wrote:
> In article ,
> Maarten D. de Jong  wrote:
>>
>>During a discussion about primitive forms of life, someone popped the ques-
>>tion why there is so little biochemical variation around. His argument ran
>>that most life on Earth has some form of DNA or RNA inside it, and that there
>>is little evidence for anything more primitive. He then proceeded to say that
>>regular biological evolution breaks down at the biochemical level, and that 
>>you would need a different theory (but not any form of creationism, he was 
>>quite emphatic about that) in order to explain all sorts of basic biological
>>structures, like for example DNA.
> 
> I am not a biologist, but Archea show far more fundamental variation in
> chemistry than Bacteria or Eukaria, both in the types of chemical reactions
> they can cause and/or exploit, and in the substances that they are made
> of.  

I don't think this is correct. Archaebacteria have a fairly limited repertoire 
of the basic biochemical pathways. The other bacteria consist of species that
have a much wider variation in pathways. This includes most of the pathways
and enzymes found in archaebacteria *plus* lots of additional enzymes.

For example, there are no archaebacteria that have a complete citric acid
cycle but among other bacteria there's a wide variety of pathways ranging
from the simple reactions seen in archaebacteria to the complete cycle seen
in eukaryotes. The same observation applies to many other carbohydrate
pathways. 

Archaebacteria don't do photosynthesis. Other bacteria do it in many different
ways including the full-blown version that was passed to eukaryoties. This
is a very powerful argument in favor of the diversity of other bacteria and
the relative lack of diversity among archaebacteria.


Larry Moran
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