This blog has been looking at a long article by John L. Locke and Barry Bogin, accompanied by open peer commentary, that appeared last summer in Behavioral and Brain Sciences. Yesterday’s post (here) reported a theory that because of their immaturity and dependence on being carried, the infants of bipeds became a unique form of persistent burden to their parents.It is this persistent-burden factor that Locke/Bogin and their peer commentators Oller-and-Griebel see as something new in primate history, demanding a new answer. Young birds are a burden to their parents as well, but not a burden that goes on for years. Elephants have as long a youth as humans, but elephant infants are on their feet and walking with the herd from the day they are born.
Locke and Bogin see “parental selection” behind the rise of infant vocalizations. Parental selection is an unusual form of evolutionary selection, meaning parents pick the children they will bother to raise. Locke and Bogin write:
As care requirements grew, conflict and competition between the infant and its parents would also have escalated. Responsibility for one solution … would have been borne largely, but not exclusively, by the infant—a behavioral change that included more effective use of care-elicitation signals, and possibly an improvement in the signals themselves. … Infants who issued more effective care-elicitation signals … were better positioned to receive care than infants who issued stress vocalizations noxiously or inconsolably—behaviors that invite neglect and abuse in primates generally… The hypothesis also envisions that infants who cooed and babbled at appropriate intervals were more likely to engage with adults, to be liked by them, to receive more sophisticated forms of care as infancy progressed (pp. 265-266)
Oller and Griebel offer a different explanation, although they agree that the unusually long and dependent period is behind the problem.
Parents appear to distill fitness information from infant vocalization … presumably because it benefits parents to make wise decisions about investment of energies in infants facing a long immaturity. … Vocalization in the human infant is then a fitness indicator, providing evidence to caregiving kin about infant well-being. Because the premium on long-term investment by caregivers is higher for the human infant than for any other primate, the selection pressure on infant fitness indicators is also higher than for any other primate. (p. 294)
I confess to finding this notion of a fitness indicator very unpersuasive. The notion of a sage Homo habilis making a wise decision about what to do with the kid requires a bit more selling before I think about buying. As noted by Chris Knight and Camilla Power in their contribution to the open peer review , “Words are not costly displays.” (They were actually responding to a different point, but it is apt here too. Abstract here) Vocalizations are not peacock tails, clear evidence of strength. If our bipedal ancestors really were asking themselves whether or not to devote the next many years to raising this newborn, something as easy to produce as a goo-goo sound probably wouldn’t have provided much rational information.
On the other hand, the issue of negligence and abuse raised by Locke & Bogin sounds more credible. As they say, we already see this as a factor among both humans and other primates, and emotional solutions make sense when addressing emotional problems. Furthermore, selection for bonding behavior rather than a specific verbal precursor explains something seen to this day: deaf children and autistic children babble but do not go on to speak (without therapeutic interventions).
The persistent-burden issue solves a great many puzzles:
- Uniqueness: Only the human line developed the vocal flexibility needed to support language because only hairless bipeds were presented with the persistent burden of raising a dependent child who must be carried about.
- Babbling: Children babble before they speak because of the emotional bonding it creates, whether or not it leads to meaningful speech.
- First words: Almost inevitably, some babbling sounds are going to be associated with something or other, and the most likely target is the person who is always to hand, mama. (Discussed in a post last week, here.) It is very likely that in the earliest days nobody knew who a child’s father was, just as chimpanzees today do not know their fathers, so papa probably came much later, but the mother’s brother was known so a word like kaka for maternal uncle may also be very old. The critical point here is that, with the rise of these basic words, even if many many generations passed before meaningful speech (complete with attention triplet) appeared, the vocal connection linking emotional communication and meaningful communication was established from the beginning.
The persistent-burden issue also suggests that there are two distinct communication systems, emotional and meaningful, that have been subject to different selective pressures and, despite their entanglement in vocalization, they are probably supported different biological systems.
Lastly, the notion of a persistent burden suggests an image. Although parents do sometimes carry their children in their hands, they are more likely to use a carriage, backpack, or sling of some sort. I’m not imagining a Homo habilis textile operation providing cloth for carrying babies, but they could have (indeed, probably did) use vegetation slings. When I taught in Africa I was quite impressed by my students’ ability to create strong ropes from vines. So I can picture mama habilis with a baby slung over her shoulder, bound with vines and, say, banana leaf.
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