We hypothesize that auditory and motor entrainment evolved in early hominin fetuses in direct response to their mothers' bipedal footsteps and, later, contributed to the evolution of music and language via two related processes. First, selection for bipedalism transformed feet from grasping into weight-bearing organs, which negatively affected infants' ability to cling to their mothers, provoking the emergence of novel affective vocal exchanges between mothers and infants that became building blocks for the emergence of motherese. Second, the derived ability to entrain movements to sound was incorporated during the prehistoric emergence of wide-ranging rhythmic behaviors such as synchronized chanting of nonlexical vocables and coordinated rhythmic clapping and stomping, which became instrumental during the more recent evolution of music. Like the derived ability to keep beat with rhythmic sounds, nascent motherese entailed entrainment of motor behavior (the physical production of pitch, timing, and vocalization rate) with external sources of sound (conversational utterances). If motherese was a precursor for language evolution, as many believe, music and language share phylogenetically derived substrates for auditory and motor entrainment that stemmed directly from bipedalism. If so, bipedalism was more important for serendipitously sculpting advanced cognition in our prehistoric ancestors than previously believed.