MetalPants said:
One fan theory that I've enjoyed about the ridges...
That doesn't explain the Mintakans of TNG, though, who have ridges and are described as "proto Vulcans".
The Pocket novel trilogy "Vulcan's Soul" extrapolated that some Vulcans still had ridges despite centuries of evolution but, around the time of Surak, all of the ridged Vulcans and some without, left Vulcan and settled Romulus. Some were kidnapped enroute by Triskelions, explaining why Vulcan weapons were being used there in TOS times.
During an incident on the journey to Romulan space, the ships were catapulted further than their drives permitted under normal circumstances, IIRC, making a return unlikely - and some of these Romulans were badly injured by the radiation on the journey, eventually becoming the exiled Remans.