There's a great book out now called, "Singing and Wellbeing - Ancient Wisdom, Modern Proof" by Kay Norton. If you are really interested in the evolution of singing in nature and culture or in the current research on how singing helps to repair the loss of brain function, I encourage you to pick a copy up for yourself.
I've highlighted some key points from just one chapter which jives with all the other credible research on the topic of "motherese". "Womb to Tomb" (borrowed from Stephen Sondheim's West Side Story libretto): Singing, Science, and the Mother's Voice reminds us that the average human fetus can hear during the last trimester of pregnancy and the mother's voice is the most intense acoustical signal in the amniotic environment. Scientists noted both fetus and newborn show heart rate decelerations in response to speech sounds (remember their heart beat is really fast!). Most of early rhythmic development is about getting the child to move to something other than their own heartbeat. The parents role is critical in this process.
As babies have heard their mother's voice for quite some time before delivery it is not surprisingly then, babies prefer the mother's voice to that of a similar-sounding female within a week after birth and when given the choice between a lullaby that had been sung twice a day by the mother during her last five weeks of pregnancy and a new story (Spence and DeCasper 1982), three-day-old newborns prefer the familiar story. Remembering to sing the same lullaby before and then after birth could help make the transition outside the womb a bit easier.
The "motherese," or "parentese", or "infant-directed speech" (IDS), evolved to ensure a close mother-child bond. We know this as a higher overall pitch, a wider range of pitch, longer "hyper-articulated" vowels and pauses, shorter phrases and greater repetition than are found in speech directed to older children and adults. Mothers who sing regularly to infants, doing so in a distinctive manner marked by high pitch, slow tempo and emotional expressiveness will have more responsive infants.
Furthermore, studies show a happy voice quality (joyful, melodious contours) was the principal contributor to soothe, interest and focus the baby's attention. It is also known by testing newborns and infants that motherese and IDS have the capacity to organize brain function.
How does singing fit in the evolutionary process? Infants do not identify the mother on the basis of vision alone until they are three months old (Burnham 1993). It appears we evolved as voice-driven beings and that we receive intrinsic rewards (care, joy, attention) when we seek out and find a meaningful, inflected voice.
As our ancestors became Bipedal and the birth canal narrowed, births became more premature and resulted in more dependent infants so longer and better maternal care was required. Singing became a fitness advantage. The voice became a third hand in child rearing when the female parent was manually engaged in the other demands of bipedal life ("I'm here little one, Mama is close by while I'm gathering fruit and nuts and tending the fire...").
Biochemistry also supplies another reason that highly musical, infant-directed speech has persisted in human evolution. For millennnia, oxytocin has infused mammalian births; it is implicated in cervical dilation, uterine contractions, lactation letdown reflex, contractions during the first few weeks of breastfeeding and bonding. Interwoven in these actions are inflected vocalizations, not just with mother but with father. Again in OT release studies, maternal vocalizations were nearly as important as touch in the regulation of stress. Both mother and fetus produce oxytocin at birth and OT crosses the placenta; maternal oxytocin thus affects the fetal brain. In cesarean births where the mother's oxytocin upsurge does not naturally occur, it is important to expose the infant immediately to the parents' speech. Skin-to-skin contact after cesarean will support the early onset of the first vocal communication (Velandia et al. (2010, abstract) and help bring up OT levels.
However the power of inflected vocal interactions surely must not wane after the birth experience! The voice can be used to soothe, direct and organize the emotional, physical and linguistic development of the child through adolescence. It becomes especially important as the child enters the "mirror stage" between six and eighteen months. It is a developmental stage where the child can recognize herself in the mirror and recognize the difference between self and image. It marks their entry into the world of symbols and language and the formation of the ego. The realization that "I'm a person too" mixes with depression over the truth that "mother is separate and I do not control her." It is this time that the child recognizes the father as part of the social order. It is important to encourage and guide the child through this period of separateness with much vocal positivity. The parent voice can model and encourage while the child discovers they can experience great power in the production of their newly found voice. There too are intrinsic rewards in self actualization!