text: Though the KaiABC gene cluster has been found to exist only in cyanobacteria, evolutionarily KaiC contains homologs that occur in Archaea and Pseudomonadota. It is the oldest circadian gene that has been discovered in prokaryotes. KaiC has a double-domain structure and sequence that classifies it as part of the RecA gene family of ATP-dependent recombinases. Based on a number of single-domain homologous genes in other species, KaiC is hypothesized to have horizontally transferred from Bacteria to Archaea, eventually forming the double-domain KaiC through duplication and fusion. KaiC<nowiki/>s key role in circadian control and homology to RecA suggest its individual evolution before its presence in the KaiABC' gene cluster.