Probing the isomerization of a retinal chromophore model we have found the first ab initio realization of the so-called "bicycle-pedal mechanism". In an ensemble of 47 starting geometries generated by ground-state zero-point-energy sampling one single trajectory followed the aforementioned reaction mode which was proposed by Warshel in 1976. Furthermore restrained optimization of synchronous rotation mode shows that two-double-bond isomerization is barrierless for the conrotatory and disrotatory pathway.