Darwin (1871) noted that the human musical faculty "must be ranked amongst the most mysterious with which he is endowed". Indeed, previous research with human infants and young children has revealed that we are born with variable musical capabilities. Here, the adaptive purpose served by these differing capabilities is discussed with reference to comparative findings regarding the acoustic behavior of nonhuman primates. The findings provide evidence supporting Darwin's hypothesis of an intermediate stage of human evolutionary history characterized by a communication system that resembles music more closely than language and possibly acting as a precursor for both current language and music.