The Burton lab described the evolution of RNA synthesis systems (transcription systems).
RNA polymerase, the enzyme that synthesizes RNA from a DNA template, evolved around two double-psi-beta-barrels, the bridge helix and trigger loop.
General transcription factors evolved around TATA-binding protein (TBP) and Transcription Factor B (TFB). A TFB precursor gave rise to sigma factors in bacteria and TFB in archaea. Sigma factors and TFB are sufficient to describe divergence of bacteria and archaea in ancient evolution.
Fusion of multiple bacteria and multiple archaea gave rise to eukaryotes. Eukaryote complexity can largely be described through the genetic fusion and its effects on evolution of eukaryotic transcription systems. The carboxy terminal domain of eukaryotic RNA polymerase is a defining feature of eukaryote complexity.