The order Chlamydiales has been set apart from the other great groups of Bacteria since ancient times. Throughout most of its evolutionary history, the Chlamydiales lineage has been an obligate intracellular parasite. It may have had longer to adapt to intracellular life than any other group of bacteria. The basic properties of the chlamydial developmental cycle were already established when the Chlamydiales families diverged. Extant representatives of the four families show a number of phenotypic differences. Chlamydiaceae is only one of the several end products of Chlamydiales evolution.
[JWM]