Abstract
The claim by Nelson et al. (2001) that the reactionof cyanoacetaldehyde and urea provides `an efficient prebioticsynthesis' of cytosine is disputed. The authors have not dealt withthe important points presented in a criticism of this reaction(Shapiro, 1999): (1) The reactants undergo side reactions with commonnucleophiles that appear to proceed more rapidly than cytosineformation, and (2) No reactions have been described thus far thatwould produce cytosine at a rate sufficient to compensate for itsdecomposition by deamination, and permit accumulation over extendedperiods of time. Instead, Nelson et al. have conducted `drying-down' experiments, inan effort to simulate evaporations on the early Earth, but thedesign of these experiments is flawed. The initial reactantconcentrations are much higher than might be expected in a naturalsetting, and potentially interfering substances such as glycine,cyanide and thiols have been excluded. `Drying beaches and dryinglagoons' have been invoked as sites for such a reaction but noeffort has been made to describe the characteristics of such sitesor to estimate their frequency with reference to the present Earth.In the absence of contradictory data, the conclusion put forward inShapiro (1999) remains valid: `It was quite unlikely that cytosineplayed a role in the origin of life'.
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Shapiro, R. Comments on `Concentration by Evaporation and the Prebiotic Synthesis of Cytosine'. Orig Life Evol Biosph 32, 275–278 (2002). https://doi.org/10.1023/A:1016582308525
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DOI: https://doi.org/10.1023/A:1016582308525