Proteins are essential biomolecules that are responsible for nearly all biological processes. Therefore, understanding the origin and evolution of proteins is necessary to solve the mystery of the origin of life. Although many modern proteins have huge and complex structures, they are thought to have originated from shorter and simpler peptides. However, how simple such ancient peptides were is still a mystery. The authors have recently demonstrated that a DPBB structure, one of the ancient protein folds conserved in the active cores of RNA polymerase and other proteins, can be constructed with a peptide consisting of 43 residues of seven amino acid types.