Life began about 3500 mya (million years ago). That most people agree on, but what the first life form looked like and how it evolved into the three great domains of Archaea, Bacteria, and Eukarya are matters of intense controversy. Fortunately, we can leave these uncertainties behind and move on to only 2000 mya when there occurred two somewhat less controversial events of importance to the origins of Chlamydiales (Figure 1).

Figure 1. Some events of importance in the evolution of Chlamydiales. Data from Benton (1997), Doolittle et al., (1996), Feng et al. (1997), and Kumar and Hedges (1998).

First, Bacteria began to diverge with the splitting into cyanobacteria, Gram-positives, and Gram-negatives. Second, primitive eukaryotes interiorized relatives of present-day alpha-proteobacteria and established symbiotic relations with them to become the first Eukarya with mitochondria. I suspect that the Chlamydiales ancestor diverged thereabouts and that it also became endosymbiotic with primitive eukaryotes. Sequence comparisons of ribosomal RNA genes and protein-coding genes of many eubacteria suggest an early divergence of the Chlamydiales lineage (Cousineau et al., 1992; Ludwig et al., 1993; Boorstein et al., 1994; Viale et al., 1994; Eisen, 1995; Gupta, 1995; Gruber and Bryant, 1997). Early evolution of Chlamydiales could have taken many pathways. None can be proven or disproven. I only think that the one I am going to describe is the most likely.