(A) Didermic firmicutes have a cytoplasmic membrane (shown in blue), a peptidoglycan cell wall (gray) and an outer membrane (green), whereas monodermic firmicutes have a cytoplasmic membrane and a peptidoglycan cell wall, but no outer membrane. Antunes et al. show that the ancestral didermic cell plan of the Firmicutes phylum has been lost at least five times. Most lineages lost their outer membranes to become monoderms (thick gray lines), but the Negativicutes and the Halanaerobiales retained the ancestral didermic cell plan (thick green lines). (B) Major transitions between bacterial cell plans within the Firmicutes phylum. Ancestral sporulating diderms (similar to the Negativicutes and the Halanaerobiales) convergently gave rise to classical sporulating monoderms (e.g., Bacillus and Clostridium), which lost the capacity to form endospores in some linages (e.g., Lactobacillus). Endospores are shown as cells within cells.