The prebiotic geochemistry of formaldehyde

Formaldehyde (HCHO), the simplest aldehyde, is an intermediate oxidation state one carbon molecule that exists transiently but prominently in the abiological carbon cycle, and is ubiquitous in the cosmos. Its potential prebiotic importance is suggested by the fact that it readily undergoes a variety of addition and redox reactions to give products of biological significance including sugars and amino acids. It is especially important with respect to the origin of an RNA or pre-RNA world, since HCHO may be a precursor to ribose and other sugars. HCHO is introduced to the environment by a number of processes including atmospheric and aqueous phase synthesis as well as extraterrestrial delivery, balanced by various destructive processes such as photolysis and redox equilibration in hydrothermal environments. While the Strecker synthesis of amino acids can occur at very low dilution, even best case scenarios for HCHO steady-state concentrations in the primitive oceans are too low for the formation of sugars to occur. Concentration mechanisms would thus be necessary. As HCHO is volatile, direct evaporation is not possible, but other geochemical mechanisms such as eutectic freezing and conversion to non-volatile derivatives by reaction with other species present in the primitive environment, followed by evaporation, could have concentrated HCHO sufficiently to allow for sugar synthesis.