Evolution can be nigh-impossible to understand sometimes, but this one seems straightforward- as you mentioned, it was originally two discs, which gained bumps-

Evolution can be nigh-impossible to understand sometimes, but this one seems straightforward- as you mentioned, it was originally two discs, which gained bumps- bugs that had better bumps were more likely to survive, and thus the bumps probably became more defined and streamlined, forming ridges, and later "gears"- and if there's one thing the body likes to do, it's symmetry.