Ancient proteins are the ancestors of modern proteins that survive as molecular fossils. Certain structural features of functional importance, particularly relating to metabolism and reproduction, are often conserved through geologic time. Early proteins consisted of simple amino acids, with more complicated amino acids being formed at a later stage through biosynthesis. Such late-arising amino acids included molecules like: histidine, phenylalanine, cysteine, methionine, tryptophan, and tyrosine. Ancient enzymatic proteins performed basic metabolic functions and required the presence of specific co-factors. The characteristics and ages of these proteins can be traced through comparisons of multiple genomes, the distribution of specific architectures, amino acid sequences, and the signatur