Fig. 17. Amino acid sequence hypothesis on the origin of proteins. For a protein composed of 100 amino acid residues, there are 20100 = ca. 10130 different sequences. In the hypothesis, it is assumed that extant proteins (dots) have been produced through selection from the enormously large number of sequences. But, it is generally considered that it would be impossible to select one unique sequence from the sequence space (open ellipsoid).