The majority of quorum sensing systems that fall under the "two-gene" (an autoinducer synthase coupled with a receptor molecule) paradigm as defined by the "Vibrio fischeri" system occur in the Gram-negative Proteobacteria. A comparison between the Proteobacteria phylogeny as generated by 16S ribosomal RNA sequences and phylogenies of LuxI-, LuxR-, or LuxS-homologs shows a notably high level of global similarity. Overall, the quorum sensing genes seem to have diverged along with the Proteobacteria phylum as a whole. This indicates that these quorum sensing systems are quite ancient, and arose very early in the Proteobacteria lineage.