In this study, we have elucidated the molecular origin of shear banding in the entangled polymeric melts. Specifically, it is shown that the inflection point corresponding to the stress-overshoot indicates the possibility of inhomogeneity and this combined with slow orientation relaxation in step-strain start-up experiments will lead to formation of local inhomogeneity along the velocity gradient direction. Once the aforementioned inhomogeneities are created, a localized jump in entanglement density and a commensurate jump in normal stress and viscosity will lead to formation of the incipient shear banded flow structure. To this end, number of step-strain and start-up simulations with different deformation rate ramp times in the planar Couette flow with entanglement densities (Z(k)) >= 17 were performed to demonstrate the effect of deformation rate ramp time on the formation of local inhomogeneities and occurrence of shear banding. It has been demonstrated that if the time scale for the deformation rate to reach its steady value is larger or on the order of the orientation relaxation time of the chain, local inhomogeneities in the velocity gradient direction will not form and the linear velocity profile will prevail. Overall, the molecular mechanism of incipient shear banding as well as its evolution to steady shear banding or to a linear velocity profile (transient shear banding) is described for the first time. Moreover, our findings are in agreement with a host of prior step-strain and start-up experiments with synthetic and natural Deoxyribonucleic acid (DNA) entangled polymeric fluids. (C) 2016 The Society of Rheology.