Abstract
The genes involved in amino acid metabolisms and biosyntheses tend to have deteriorated, rather than evolved, over time in relation to living and feeding habits of organisms. The essential amino acids are basically the same among diverse animals, suggesting that they were determined at the time of emergence of animals. The presence of the animal essential amino acids has resulted from deterioration of genes that became dispensable in relation to environments. A good example is provided with the nearly complete complementary sets of essential amino acids between hosts and symbionts. We discuss evolutionary characteristics of enzymes or receptor molecules for amino acids that act as neurotransmitters and taste stimuli. We also argue these characteristic features during human evolution.
