Recent work in bioacoustics has emphasized the importance of cues to individual- and kinship-identity in ``signature'' calls produced by many animal species, including nonhuman primates. Applying the source-filter approach to sound production in monkeys and apes, it is proposed that low-pitched, tonal calls may be significantly better-suited to providing these indexical cues than are noisy or high-pitched vocalizations. As the supralaryngeal vocal tract is relatively inflexible in many primates, harmonically rich calls produced by different individuals should exhibit stable, subtly distinctive spectral characteristics due to intraspecies variation in vocal tract size, shape, and tissue properties. Based on field studies of calls from baboons and rhesus monkeys, it is suggested that protohominids routinely uttered vowel-like sounds long before the development of speech. Laboratory tests of pure-tone and formant frequency discrimination in monkeys and humans further indicates that detailed formant-related characteristics in these sounds were likely both functionally important and perceptually salient. Due to changes in facial morphology (probably reflecting dietary factors), shortening of the protohominid vocal tract created selection pressure for lower laryngeal position to maintain acoustic-signature cues. Laryngeal descent therefore set the stage for development of flexible vocal tract positioning, but was not itself an adaptation for speech.