It has been argued that a PriA-like gene product could have been the common evolutionary ancestor for both HisA and TrpF. Ancestral sequence reconstruction has also been used to suggest that ancient HisA precursors were likely bifunctional, and that this bifunctionality persisted over at least a two-billion-year time span. However, as shown in Figure , HisA and PriA exploit loops 1, 5 and 6 to facilitate activity, whereas TrpF lacks an analog for loop 1, and isomerizes PRA harnessing just two catalytic loops, 3 and 6. Our results suggest that an evolutionary trajectory from a PriA-like ancestor to an extant TrpF would be surprisingly complex.