From Amino Acids to Peptides before the Coming of Ribosomes

They may have arrived from space, or have been synthesized on our planet; either way, amino acids were present on the early Earth. In this chapter, we present the various ways in which amino acids were able to polymerize to form peptides. Today, proteins are synthesized in ribosomes. In evolved cells, amino acids are first activated into phosphoric-carboxylic anhydrides, before forming esters on tRNAs. Mixed anhydrides are also used in the synthesis of non-ribosomal peptides, but the second intermediates are then thioesters. Amide bonds are obtained from these esters and thioesters. Should we think that anhydrides, esters and/or thioesters were already used before the coming of ribosomes? Or should we imagine simpler ways, directly from acids on mineral surfaces or by making use of activating agents forgotten in today's biology? Masked by the entire genetic history and the inevitability of the ribosome, the answer remains uncertain. It is even difficult to know whether the pre-ribosomal peptides really had an influence on the origin of life as we know it today.