This article explores the “putting the baby down” (PTBD) hypothesis. The hypothesis states that vocal interactions between early hominin mothers and infants resulted in a sequence of events that led, eventually, to the ancestors' earliest words and, later, to the emergence of protolanguage. The modern motherese is more melodic, slower and more repetitious, has a higher overall pitch, uses a simpler vocabulary, and includes special words, as compared to adult-directed speech. Contemporary motherese is known for its musical quality, or prosody. Prosody provides the melody or tone-of-voice in adult speech, coloring it with nuance and revealing emotions. Motherese, as defined in the PTBD hypothesis, is verbal and also covers facial expressions, body language, touching, patting, caressing, and even laughter and tickling. The clarity of motherese that normal babies are exposed to is associated with their development of speech discrimination skills, and infants who are best at perceiving speech sounds at seven months of age score higher when they are older on language tests measuring the number of words they can say and the complexity of their speech. The studies on French- and English-speaking parents and their one- to two-year-old infants show that the extent to which parents incessantly label objects and encourage repetition of names is associated with their babies' vocabulary growth, as well as their ability to manipulate and categorize objects.