Yao-Hua Lau in Discover Magazine:
“Think about the period when early humans became bipedal,” says de l’Etoile. “That coincided with the pelvis narrowing, to allow walking upright, which limited the size of the infant at time of birth — all humans are born in a certain state of prematurity. We’re not like, say, horses, which are up and walking after a couple minutes.” Our inherent vulnerability as infants means human babies need an extended period of hands-on care, explains de l’Etoile, who studies infant-directed song but was not involved in Krasnow and Mehr’s research. She adds: “At the same time, the baby is growing at an exponential rate. There comes a time when it’s too big to carry all the time but still needs care. But the mom also needed to move around, to get water, prepare food.
Singing allowed the mother, the traditional caregiver, to put the infant down while still reassuring the child. “If the infant’s making a fuss, it could attract a predator,” says de l’Etoile, “A mother effective at using her voice to calm her infant would be more likely to survive — and the infant would be more likely to survive, too. Infant-directed song could be evidence of the very first music.” While not contradicting this take on the origins of lullabies, Krasnow and Mehr propose a darker element to the evolution. “The parent-infant relationship is not all cupcakes and sunshine,” says Mehr. “There is a lot of conflict.” Krasnow and Mehr believe the tug of war between an infant seeking as much attention as possible and the caregiver dividing attention among other offspring and tasks crucial for survival may have set the stage for an evolutionary arms race.