The view that non-human primates lack significant voluntary control over their vocalizations and that their vocalizations lack propositional, referential, or symbolic qualities has been a persistent one, especially among those who have examined the evolutionary origin of human speech. This view would seem to favor proposals of human language having originated in the form of manual gesture, a modality that in non-human primates appears to be subject to greater voluntary control. Recent field and laboratory investigations of the information conveyed in the natural vocalization system of primates and comparative neurological data, however, point to important similarities between the vocalization system of non-human primates and human speech. In particular, use of innovative methodology such as the “playback” technique has shown in several New and Old World monkey species that calls convey more than the sender's emotionalmotivational state, including also information about the sender's sex, group membership, and social relationships. In some species, calls apparently also have semantic qualities, in that they encode specific information about external objects or events, and there is evidence that the decoding of calls may be governed by simple syntactical rules and that perceptual mechanisms similar to those in human speech may be employed. This structural and functional complexity of calls is also reflected at the level of governing neural mechanisms. Not only is there strong evidence for volitional components in calls and the involvement of neocortical mechanisms, but there may also be a differentiation of neural control mechanisms according to call type. Furthermore, in some species hemispheric asymmetries exist in auditory perception and perhaps also production of calls. Despite the absence of significant neurobiological data from apes, these data suggest that the vocal-auditory machinery of the earliest hominids was far more ready to take on “primordial” speech functions than has been previously supposed.