In a post last week (here) I noted an article that appeared this summer in Behavioral and Brain Sciences, “Language and Life History,” by John L. Locke and Barry Bogin. The article was followed by extensive “open peer commentary.” (Links to abstracts and open peer commentary here.) Altogether the journal provides 65 pages of extensive coverage of speech, its evolution and development in individual speakers. From time to time this blog will explore one of the discussion’s themes. Last week we noted that there appear to be two distinct, equally unique threads of vocal communication, emotional and meaningful. Today I want to look at Locke and Bogin's treatment of infant babbling and the rise of the emotional thread.
The basic premise of the Locke and Bogin article as it relates to infancy is that at age 7 months infants start to babble, making ma-ma and goo-goo type syllables in order to create an emotional bond between infant and parent. In the open peer commentary D. Kimbrough Oller and Ulrike Griebel voice their strong agreement. A short response titled, “How the language capacity was naturally selected: Altriciality and long immaturity” (abstract here) goes further, saying that, “The foundations laid in the first six months appear to be critical to all subsequent development toward vocal language” because “bonding is at stake.”
Whenever anyone proposes a selective pressure that led to language, even indirectly, the explanation must include some reason why the pressure was not also in force for other species, especially for chimpanzees. Why are we the only species with this speaking skill? As Oller and Griebel note:
There are more than 200 extant species of primates but only the human shows vast vocal flexibility, suggesting that primate life circumstances have overwhelmingly favored vocal systems serving specific, immediate functional needs … with well-defined, unambiguous calls. … What was unique about hominid life circumstances that yielded selection forces where hominids and only hominids came to possess vocal capacities that vastly exceeded the requirements of immediate functional needs? (p. 294)
Locke and Bogin also addressed this question and defined the source of the difference between the human line and other primates very sharply:
An important factor in the evolution of human infancy was bipedalism, which realigned the spine and narrowed the pelvis. This change created an unfavorable ratio between the smaller maternal birth canal and the large fetal head … and this produced … an “obstetrical dilemma” … [that] was eased when some amount of skull and brain growth were adaptively deferred into the postnatal period, increasing infant dependency and the need of postnatal care. … Anthropological accounts … indicate that most hunger-gatherer mothers rarely put their babies down, and then do so for no more than a few seconds, usually remaining within a meter. … When infants cannot be carried, they are often left in the care of others. In most other primate species, infants are equipped to cling to their mother’s body hair, and are less likely to be cared for by other embers of the social group. Thus, although the duration of infancy is not longer in humans than in some of the other primates, its quality is higher because the greater handling required by the human infant produces more intense social stimulation during a period in which the brain grows at a compensatorily rapid rate. (p. 261, italics in original)
This passage forces attention to how little we know about the most basic things:
- Bipedalism: How long has this been going on? We can say that it began 5 (or maybe even 6) million years ago, and that by 1.8 million years ago was fully established in the modern form. The Laetoli Footprints date from 3.6 million years ago and have a very modern look to them, but the recently discovered Selam fossil (discussed here) is 3.3 million years old and indicate that bipedal locomotion in those days included climbing: “three lines of evidence [the shoulder, hand, and semicircular canals of the Selam fossil] suggest that the locomotion of A. afarensis was unlikely to have been restricted to walking on two feet.” (See here for full story.)
- Obstetrical dilemma: when did it become necessary for bipedal infants to be born with such an immature brain that their motor control was very underdeveloped? Were Australopithecus infants equally helpless, or could they begin walking much more quickly. Did this begin before meaningful speech had begun or after? If before, what was driving the brain enlargement?
- Hairlessness: When did humans become so hairless that their infants could not hang on for the ride? The most popular thesis is that hairlessness is an adaptation to promote heat regulation, substituting sweat glands for hair. Alternate theories include the idea that it reduces parasite infestation and that humans had an aquatic phase before taking to the savanna. No clear date has been settled upon. It strikes me that hairlessness may be more important than bipedalism because it forced mothers to carry their infants, turning them into a persistent burden.
I’ll look at the persistent-burden issue in tomorrow’s post for it underlies the great question: why did all this happen only in the bipedal line? Why don’t chimpanzees say goo-goo?