<< But it appears that Shapiro is not so ready to concede the purines
and pyrimidines.  He has written a series of articles critiquing
the evidence for prebiotic synthesis of the bases.  Here is a
link to the one for Cytodine.  Note that he has nice things to 
say about both Cairns-Smith and Wachtershauser. ;-)  But he also
mentions Kauffman. ;-(

Robert Shapiro
Prebiotic cytosine synthesis: 
 A critical analysis and implications for the origin of life
PNAS 1999; 96: 4396-4401.
http://www.pnas.org/cgi/reprint/96/8/4396

Excerpt from Conclusion:
The evidence that is available at the present time does not support 
the idea that RNA, or an alternative replicator that uses the current 
set of RNA bases, was present at the start of life. This conclusion 
could be reversed if a prebiotic simulation were devised that produced 
all of the bases in good yield under a single set of conditions, by 
using a plausible combination of water, atmospheric components, and 
minerals. In the absence of such a demonstration, more attention 
should be given to origin-of-life theories that do not require the 
four RNA bases: (i) The first living system used a replicator 
constructed of more accessible and stable components. A number of 
possibilities may exist, with the clay system of A.G. Cairns-Smith (*) 
perhaps the best known. (ii) Life began with cycles of autocatalytic 
reactions. Storage and transfer of information at the polymer level 
came later. A number of writers have discussed this possibility, 
including F. Dyson (*) and S. Kauffman (*). One possible system has 
been described in detail by G. Wächtershäuser (*). 

TH
I have suggested that a purine world came first.
A world of folded RNA strands with mostly A and G - 
the two easiest to make in prebiotic experiments.
Then U and finally C.

Remember RNA folds on itself and there may have
been any number of base varieties before the final 4
(think other Beatles before the final 4 = J,P,G,and R)
Early on you could probably get away with other than
WC base pairing. That would be refinement that would come later.

Tom



References:
Cairns-Smith, A. G. (1982) 
 Genetic Takeover and the Mineral Origins of Life 
 (Cambridge Univ. Press, New York).  
Dyson, F. (1985) 
 Origins of Life 
 (Cambridge Univ. Press, Cambridge).  
Kauffman, S. (1993) 
 The Origins of Order. Self-Organization and Selection in Evolution 
 (Oxford Univ. Press, New York).  
Wächtershäuser, G. (1992) 
 Prog. Biophys. Mol. Biol. 58, 85-201

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