Rock-a-Bye-Baby… in a sling.

The focus of the Discover article, Rock-a-Bye-Baby’s Rocky Rootsby Yao-Hua Law, is the research of  Shannon de l’Etoile, a graduate student of the University of Miami’s Frost School of Music. L’Etoile’s theorizes that early humans invented lullabies for hands-free parenting because human babies require a lot of care and are too big to carry. Citing the obsolete Obstetric Dilemma, that the female pelvis is inadequate for birthing mature infants, as a reason for our helplessness at birth. L’Etoile explains, “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.” L’Etoile imagines a human past in which humans had tools for transporting water and tools for preparing food, but no tools for carrying infants. What is a mother to do? Sing. (Effectively.) According to l’Etoile, “If the infant is making a fuss, it could attract a predator. 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.” If this sounds familiar to you, it is because its the theory Falk (2004) used to explain the invention of Motherese.

There a few things I would like to point out:

  1. With regards to the obstetrical dilemma, humans gestate longer and give birth to larger infants than our precocious evolutionary ape cousins (Dunsworth). Like many other species (whose pelvises are considered adequate for birth), we’re altricial; our development at birth is not due to a narrow pelvis resulting in prematurely born infants.
  2. Humans, like all other primates, are biologically riders. This means, among other things, that our breastmilk composition necessitates frequent feedings, which requires the infant to always be near, or preferably on, the body of its mother (Ross). Our ancestors needed to keep their infants in close proximity for frequent feedings.
  3. Yet, our babies are too big to carry in-arms, simply in terms of energetic costs. Without a sling or infant carrier, a paleolithic hunter-gatherer mother could literally starve to death while carrying her baby around, in-arms, foraging for food. “The cost of carrying an infant in one’s arms would have been meaningful enough to reward the development of carrying tools rapidly following the advent of bipedalism,” (Wall-Scheffler).
  4. The evolutionary trend for large newborns began at least four-million-years-ago with our first fully bipedal ancestor, Australopithecus afarensis. At that time, the Infant-Mother Mass Ratio (IMMR) jumped from 2% to around 5-6%, where it has remained (conserved) into our own species (DeSilva). It was at this time that parents would have been seeking “hand-free parenting” in the form of a tool for carrying infants.
  5. Fossil evidence has shown that Australopithecus afarensis would have vocalized similar to that of a chimpanzee due to the structure of the hyoid bone (Alemseged). They would not have had spoken language and they would not have been singing. Should the similarity to chimpanzee vocalization extend to music appreciation, research demonstrates that music is irrelevant to chimpanzees (Wallace).
  6. After our ancestors had developed language and music, lullabies would not be the first strategy to soothe a crying baby– studies have shown that proprioceptive stimulation (being held during a walk) is more soothing to infants than both maternal touch and maternal vocalization (Esposito; Korner; St.James-Roberts; Yoo). Besides, singing would attract a predator just a much as a crying infant.
  7. Not all human cultures use lullabies, let alone motherese, to soothe infants. If there were selection pressures for mother-infant communication it would likely be from infant to parent, not the other way around (Rosenberg, 2004).

Our human ancestors had no need to invent lullabies for hands-free parenting because tools for hands-free parenting preexisted our species in the form of infant carriers. In fact, we wouldn’t be here without them because bipedalism would have been just another evolutionary dead-end. Infant carrying tools helped (and continue to help) parents to get their work done, socialize, walk about, flee from dangerous situations, care for older children and themselves, gather or prepare food, sooth fussy infants, feed hungry infants all while keeping their hands-free.

“It seems unlikely to us that hominin mothers 2 millions years ago routinely would have set their children down, and it seems more probable that they would have carried them in their arms, or more likely, in a sling while they foraged.”  (Rosenberg, et al. 2004)

L’Etoile’s theory of lullabies-as-hands-free-parenting-strategy was inspired by Max Krasnow’s and graduate student Samuel Mehr’s theory of lullaby origins as how parents won the “evolutionary arms race” with their offspring, published in Evolution and Human Behavior. Krasnow’s and Mehr’s view of infant-parent relations is that of conflict and scarce resources, namely parental attention. I strongly disagree with the infant-parent-conflict premise. What is good for the infant is generally good for the adult i.e. survival, passing on their genes to future generations, provided that the baby gets what it wants, which is to survive, which requires parental attention. Remember that Harlow’s monkeys chose the soft comforting surrogate over the metal one with food; an example of infants wanting physical contact more than sustenance. The infant-parent relationship may feel fairly conflicted if what the baby needs to survive is more than the parents are willing or able to give– but parental attention is not a scarce resource when using a carrier as the baby becomes an extension of the parent’s body.

Krasnow commented on l’Etoile’s research, “Attention is invisible. You need an honest signal of its quality. That’s where singing comes in. I can’t be singing to you while I’m running away from a predator, or while I’m just having a conversation with someone else. Even turning the head affects the quality of the voice. An infant can gauge where the parent’s attention is oriented. These are things that can’t be faked.” If Krasnow is correct, that even turning your head away from the baby while singing enough to reduce the quality of the attention– and therefore its effectiveness– then how exactly is singing a way for a mother to get work done, unencumbered with the baby, as l’Etoile postulates? Alternatively, from the infant’s perspective, being carried on the body of a caregiver is quality attention, even if the caregiver’s conscious attention is directed elsewhere– whether talking to another adult, tending older children, foraging, eating, or escaping predation– the infant gets the sensory input from the movement, touch, smell, and sounds of the caregiver’s body.

Lullabies are not a weapon in the “evolutionary arms race” between infant and parent as Krasnow and Mehr theorize, nor are they the answer for hands-free parenting as l’Etoile believes. Law, the author of the article, included criticism from Sandra Trehub, professor emeritus of psychology of the University of Toronto Mississauga, “Trehub doubts that the need to soothe infants pushed vocalizations to evolve into lullabies. Humans use various means to calm infants: Rocking and carrying on their own, for example, can lull an infant to sleep. ‘Songs are not a unique solution for soothing infants,’ Trehub says, which makes creating a solely evolutionary basis for them problematic.”

And what of the whereabouts of the infant whose mother chose singing lullabies as a “hands-free” parenting strategy when the group is running from a predator? I have no doubt that humans (and very likely archaic humans) used infant-directed song to soothe and entertain their infants and children. But it would have been in addition to, not in lieu of, a much older invention: that of the infant carrier.


Alemseged, Zeresenay, et. al. “A juvenile early hominin skeleton from Dikika, Ethiopia.” Nature 443, 296-301 (21 September 2006).

Bjorklund, David F. “The Role of Immaturity in Human Development.” American Psychological Association Psychological Bulletin 122.2 (1997):153-169.

DeSilva, Jeremy M. “A Shift toward Birthing Relatively Large Infants Early in Human Evolution.” Ed. C. Owen Lovejoy. Proceedings of the National Academy of Sciences of the United States of America 108.3 (2011): 1022-027. PNAS. Web. 19 Jan. 2015.

Dunsworth, H. M., A. G. Warrener, T. Deacon, P. T. Ellison, and H. Pontzer. “Metabolic Hypothesis for Human Altriciality.” Proceedings of the National Academy of Sciences 109.38 (2012): 15212-5216. Web. 19 Jan. 2015.

Esposito, Gianluca, et. al. “Infant Calming Responses during Maternal Carrying in Humans and Mice.” Current Biology 23.9 (2013): 739-45. Web. 15 July 2015.

Korner, Anneliese, and Evelyn B. Thoman. “Visual alertness in neonates as evoked by maternal care.” Journal of Experimental Child Psychology 10.1 (1970): 67-78.

Law, Yao-Hua. “Rock-a-Bye-Baby’s Roots.” Discover (June 2017). Web.

Rosas, Antonio, et. al. “The growth pattern of Neanthertals, reconstructed from a juvenile skeleton from El Sidrón (Spain).” Science 357(2017): 1282-1287

Rosenberg, Karen R., Roberta M. Golinkoff, and Jennifer M. Zosh. “Did australopithecines (or early Homo) sling?” Behavioral and Brain Sciences 27:4 (2004), 522.

Ross, Caroline. “Park or Ride? Evolution of Infant Carrying in Primates.” International Journal of Primatology 22.5 (2001): 749-71.Springer. Web. 26 Jan. 2015.

St. James-Roberts, Ian, et. al. “Supplementary Carrying Compared With Advice to Increase Responsive Parenting as Interventions to Prevent Persistent Infant Crying.” Pediatrics 95.3 (1995): 381-388.

Wallace, Emma K., et. al. “Is music enriching for group-housed captive chimpanzees (pan troglodytes)?PLoS ONE 12.3 (2017):e0172672.

Wall-Scheffler, C.m., K. Geiger, and K.l. Steudel-Numbers. “Infant Carrying: The Role of Increased Locomotory Costs in Early Tool Development.” American Journal of Physical Anthropology 133.2 (2007): 841-46. Web.

Yoo, Kyung-Hee. “The Effects of Auditory and Vestibular Stimulation of Stress Hormones in Preterm Infants.” Journal of Korean Academic Fundamental Nursing 11.2 (2004): 203-212.

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