The amino acid proline plays critical roles in polypeptide structure and function. However, the influence of proline residues on proto-peptide evolution is unknown. In order to begin exploring this question, we formed mixtures of model prebiotic peptides by wet–dry cycling and compared proline incorporation to that of glycine, alanine, and valine. Proline uptake was more efficient than the other amino acids into linear oligomers. However, proline was also found in small, cyclic species. In a more complex mixture of monomers and oligomers, proline displayed similar behaviorefficient uptake into linear sequences and yet also into small cyclic oligomers.