Explanation of Why Life on Earth is Made Out of Only 20 Amino Acids When Hundreds are Possible| PNAS

“One of the long-standing questions in origins-of-life research centers on how the proteinaceous side chains and the protein backbone were selected during the earliest phases of evolution. Here we have studied
oligomerization reactions of a group of positively charged amino acids, both proteinaceous and nonproteinaceous. Amino acids spontaneously oligomerized without the use of enzymes or activating agents, under mild, hydroxy acid-catalyzed, dry-down conditions. We observed that the
proteinaceous amino acids oligomerized more extensively and with greater preference for reactivity through their α-amine compared with nonproteinaceous amino acids, forming predominantly linear, protein-like backbone topologies. These findings provide a purely chemical basis for selection of the positively charged amino acids found in today’s proteins.”

Moran Frenkel-Pinter, et al. “Selective incorporation of proteinaceous over nonproteinaceous cationic amino acids in model prebiotic oligomerization reactions.” PNAS.