Besides nucleobases, the primeval RNA-like polymers may have contained
ribose and phosphate entities. Ribose may have been abundant as one of the products
of the autocatalytic “formose” reaction, which was discovered by Butlerov in
1861 [304] and which yields a mixture of pentoses and hexoses from formaldehyde.
Although Butlerov’s reaction remains the only known autocatalytic reaction
that does not require specific catalysts, the importance of this reaction for prebiological
syntheses has been questioned since the yield of ribose in the product mixture
is usually low. Recent studies have shown, however, that the yield of ribose
can be selectively enhanced by the presence of phosphate in the reaction medium
[305], by UV illumination [16], and by conducting the reaction in the presence
of catalytic mineral templates [306]. More recently, it has been demonstrated
that the yields of pentoses increase to 60% and those of the ribose proper rise to
20% in the presence of a zinc-proline complex as a catalyst [15]. The Zn world
settings may have favoured autocatalytic ribose formation from photosynthesized
substrates by providing mineral templates, UV irradiation, and plenty of Zn2+
ions as catalysts.