Composite model of trpB evolution. Upon duplication and integration of an ancient trpB2 gene into the trp operon, the last universal common ancestor (LUCA) of bacteria and archaea was of species-type i2_o2. In a bacterial ancestor, the evolution of a linkage group trpB1A1 occurred. Via horizontal gene transfer (HGT), an euryarchaeal ancestor acquired this linkage group, which gave rise to a predecessor of type i1_o2. Thermoplasmata acquired trpA2 and trpB2 genes in an ancient event of HGT. For all taxonomical orders, species-types of current species are given. S2 species possess exactly one, non operon-based trpB2 gene, s2: ditto, the gene is located inside the trp operon. trpB1 was treated analogously. i2_o2 are species possessing a trpB2 gene inside and a second trpB2 outside the operon, i1_o2 are species with an operon-based trpB1 and a non operon-based trpB2, and i1_o1 are species possessing an operon-based and at least one non operon-based trpB1.