Homochiral use of l-amino acids has been regarded as a signature of life. Although ribosomal protein synthesis requires homochiral use of l-amino acids, chiral conversion from l- to d-configuration is also detected both in free amino acids and amino acid residues of proteins/peptides in non-ribosomal physiology. This paper reviews how homochirality of l-amino acids is sustained in life, with major focus on mammals, and the biological significance of use of d-amino acids.

  All living organisms on present Earth have evolved from the last universal common ancestor. Little is known about the ancestor or its close relatives. Characterization of deep-branching lineages of modern organisms will provide insights into the ancestral characteristics. In this review paper, the physiology, ecology, and evolution of Nanobdellati archaea, which is one of the deepest-branching lineages among all organisms, are summarized.