A structural element that could have existed independently in the prebiotic era was identified at the active site of the contemporary ribosome. It is suggested to have functioned as a proto-ribosome catalyzing peptide bond formation and non-coded elongation in the same manner that contemporary ribosomes exert positional catalysis, namely by accommodating the reactants in stereochemistry favourable for inline nucleophilic attack. This simple apparatus is a dimer of self-folding RNA units that could have assembled spontaneously into a symmetrical pocket-like structure, sufficiently efficient to be preserved throughout evolution as the active site of modern ribosomes, thus presenting a conceivable starting point for translation.Here we discuss the proto-ribosome emergence hypothesis and show that the tendency for dimerization, a prerequisite for obtaining the catalytic centre, is linked to the fold of its two components, indicating functional selection at the molecular level in the prebiotic era and supporting the existence of dimeric proto-ribosome
