Subclasses B1 and B2 are theorized to have evolved about one billion years ago and subclass B3s is theorized to have evolved before the divergence of the Gram-positive and Gram-negative eubacteria about two billion years ago.
As subclases B1 e B2 teorízase que evolucionaron hai uns mil millóns de anos e a subclase B3 crese que evolucionou antes da diverxencia das eubacterias grampositivas e gramnegativas hai uns dous mil millóns de anos.