 When use of the amino acid appeared confined to the "Methanosarcinaceae", the system was described as a "late archaeal invention" by which a 21st amino acid was added to the genetic code.Afterward it was concluded that "PylRS was already present in the last
universal common ancestor" some 3 billion years ago, but it only persisted in organisms using methylamines as energy sources.Another possibility is that evolution of the system involved a horizontal gene transfer between unrelated microorganisms.