A Complementary Perspective on the Evolutionary Emergence of Essential and Non-Essential Amino Acids

Abstract

All animals, including humans, lack the ability to synthesize nearly half of the 20 amino acids required for protein synthesis. It has long been hypothesized that the biosynthetic pathways for these essential amino acids (EAAs) were lost due to their higher biosynthetic cost and that the remaining non-essential amino acids (NEAAs) retained their lower-cost pathways. However, the specific mechanism by which amino acid synthesis cost determines the maintenance or loss of these pathways remains unclear.
In this paper, I propose the Differential Recycling Efficiency (DRE) Framework as a new perspective on the emergence of EAAs and NEAAs. Specifically, (1) as organisms evolved, their bodies became increasingly compartmentalized, (2) this compartmentalization led to varying levels of resource-recycling efficiency across different compartments, (3) and, driven by selective pressures favoring metabolic optimization, proteins rich in low-cost amino acids came to be preferentially allocated to compartments where resource recovery is more difficult, (4) this preference, in turn, raised overall demand for low-cost amino acids, and (5) ultimately led to the retention of their biosynthetic pathways and their classification as NEAAs. These five propositions, taken together, form the basis of the DRE Framework. Under this framework, the commonly observed amino acid requirements among various phagotrophic organism lineages—exemplified by the essential amino acids in humans—can be considered as predetermined outcomes of resource optimization throughout evolution.