We discuss the optical and radio early afterglow emission of the reverse shock that crosses a baryonic ejecta as it interacts with the external interstellar medium (ISM). We show that the peak of the optical flash divided the reverse shock light curve into two distinctive phases. The emission after the peak depends weakly on the initial conditions of the ejecta and therefore it can be used as an identifiable signature of a reverse shock emission. On the other hand, the emission before the optical peak is highly sensitive to the initial conditions and therefore can be used to investigate the initial hydrodynamic profile of the ejecta. In particular, if the prompt γ-ray emission results from internal shocks, the early reverse shock emission should resemble a smoothed version of the prompt γ-ray light curve.