The 9 indispensable amino acids are those which, because they cannot be synthesized in humans to any significant extent, must be supplied by food. In contrast, dispensable amino acids are those which are produced in the body from different precursors including notably other amino acids. Interestingly, these indispensable amino acids are the same among the different studied mammals, as well as among the studied insect, fish, and bird species, thus suggesting common requirements from the environment for all these animals. In mammalian cells, specific amino acids are used for macromolecule synthesis (primarily proteins but also DNA and RNAs), for ATP production, and for the synthesis of glucose and phospholipids. Amino acids are in addition precursors for the synthesis of various compounds including neurotransmitters, hormones, metabolic regulators, and heme of hemoglobin. The amino acid-derived compounds have been shown to intervene in human cell physiology by modulating proliferation, gene expression, energy metabolism and redox state, resulting in the modulation of various central physiological functions. The compounds produced from amino acids are also involved in the modification of the structure (and functions) of biomolecules such as proteins and bile acids. By comparing amino acid utilization in mammalian cells with such utilization in archaea and bacteria, striking similarities but also, as expected, major differences can be underlined. In mammalian cells, as in archaeal and bacterial cells, amino acid-derived bacterial metabolites such as S-adenosylmethionine, polyamines, and nitric oxide are involved in physiological functions related to methyl donor functions, cellular growth, and cell respiration, respectively. In contrast, the amino acid-derived compounds noradrenaline, serotonin, and dopamine, which act as neurotransmitters in mammals, act as regulators of growth and virulence in bacteria, thus indicating recycling of ancestral molecules for new functions.