At the very beginning of life on Earth: the thiol-rich peptide (TRP) world hypothesis.

Life developed on Earth probably about 3.8 billion years ago, on a planet that was already largely covered by oceans and where the atmosphere was very humid. The reactions, which may have led to the formation of the first polymers, particularly to the first peptides and nucleic acids, must have been compatible with these conditions. This is the case of the reaction of nitriles with aminothiols, such as cysteine and homocysteine. Since aminonitriles are the probable precursors of amino acids, this condensation reaction has been able to rapidly yield dipeptides, tripeptides, oligomers and even true polymers, each containing thiol functions. These thiol-rich peptides (TRP's) would then have assumed the various catalytic roles that the peptides containing cysteine residues play today. They allowed a rapid bloom of life in the primitive ocean. In this scenario, RNA's are not the first polymers, but have been synthesized, like DNA's, thanks to the catalytic properties of thiols in a mostly TRP world. In this world, due to its ability to form a thiolactone, homocysteine may have played the leading role in enabling the previously formed oligomers to be stappled together, thus accelerating the formation of long peptide chains.