Language and life history: A new perspective on the development ...

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Commentary/Locke & Bogin: <strong>Language</strong> <strong>and</strong> <strong>life</strong> <strong>history</strong>to have evolved a specific tendency to use elaborate vocalization as ameans of soliciting long-term investment from caregivers. Thedevelopment of such vocal capacity provides necessary infrastructurefor language development across human <strong>life</strong> <strong>history</strong>.By highlighting the unique human <strong>life</strong> <strong>history</strong>, Locke & Bogin(L&B) bring into focus the need to address development in modelingthe evolution of language. Although the emphasis in thetarget article is on stages spanning birth through adolescence,it also bears emphasizing that even in the first six months ofhuman <strong>life</strong>, vocal development is remarkable, <strong>and</strong> appears tobring the infant to a point of functional <strong>and</strong> contextual flexibilityin production <strong>and</strong> use of vocalization that is never achieved in anynonhuman primate (Oller & Griebel 2005). The foundations laidin the first six months appear to be critical to all subsequentdevelopment toward vocal language, because it would be impossibleto learn even the simplest group-specific words or even toproduce systematic imitation of novel syllable patterns in theabsence of substantial functional <strong>and</strong> contextual flexibility ofvocalization. Similarly, it is sensible to conclude that ourhominid ancestors, as they broke away from the primate background,prior to possessing language must have developedcomm<strong>and</strong> of vocal flexibility, because, without it, all furtherevolution in the direction of vocal language may well have beenimpossible.This evolutionary process must have been the product ofspecific selection forces, just as there appear to have beenspecial selection forces that have produced vocal variability <strong>and</strong>flexibility in other species with complex communicationsystems, especially some cetaceans <strong>and</strong> birds (Griebel & Oller,in press). Interestingly, there are more than 200 extant speciesof primates (Martin 1990) but only the human shows vast vocalflexibility, suggesting that primate <strong>life</strong> circumstances have overwhelminglyfavored vocal systems serving specific, immediatefunctional needs such as aggression, distress, or alarm signaling,<strong>and</strong> to serving those needs with well-defined, unambiguous calls(Griebel & Oller, in press; Hauser 1996). One of the most fundamentalquestions of language evolution then is: What was uniqueabout hominid <strong>life</strong> circumstances that yielded selection forceswhere hominids <strong>and</strong> only hominids came to possess vocalcapacities that vastly exceeded the requirements of immediatefunctional needs?The pattern of modern human development in the first sixmonths of <strong>life</strong> suggests an answer. The human infant is morealtricial <strong>and</strong> faces a longer immaturity than any other primate,as is emphasized in the target article. Consequently, the humaninfant has a need for long-term caregiver investment that dramaticallyexceeds that of any other primate. The unique topography<strong>and</strong> richness of human parent-infant face-to-face vocalinteractions strongly suggests bonding is at stake (see, e.g.,Stern 1974; Trevarthen 1979; Tronick 1982), <strong>and</strong> that elaborate,contextually flexible infant vocalization solicits parental investment(Oller 2000). Parents appear to distill fitness informationfrom infant vocalization, <strong>and</strong> in many instances to elicit it inface-to-face interaction, presumably because it benefits parentsto make wise decisions about investment of energies in infantsfacing a long immaturity (Locke, in press c; Oller 2004).Human caregivers also attend to a variety of infant vocalizationsoutside bonding interactions.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 ishigher for the human infant than for any other primate, the selectionpressure on infant fitness indicators is also higher than forany other primate. And since altriciality had already increasedin hominids by the point of bipedalism, intense selectionpressure on infant solicitation of parental investment must havebeen in place very early in hominid evolution. The pattern ofvocal development in the human infant suggests, then, that thedriving engine of evolution for hominid vocal capacity mayhave been the unique constellation of <strong>life</strong> <strong>history</strong> circumstances(including altriciality <strong>and</strong> long immaturity) that encouragedimmature hominids to seek caregiver investment, <strong>and</strong> thehominid parent (<strong>and</strong> other kin) to seek fitness indications fromthe infant in vocalization, in order to make better decisionsregarding investment of caregiving energies across a long immaturity.This special constellation of forces encouraged parentalselection (Locke, in press c) by vocalization assessment, apattern that may have increasingly resembled sexual selectionin its effects on evolution, as the priority on long-term parentalinvestment became increasingly high. Runaway selection(rapidly producing salient <strong>new</strong> traits) has long been wellrecognizedas a product of sexual selection (Darwin 1871; see areview in Miller 2000), <strong>and</strong> it may well be that the elaboratehuman vocal capacity was spurred on by parental selection inearly <strong>life</strong> of hominid infants. As noted in the target article, thisvocal capacity, fostered in infancy through parental selection,could have been amplified in adolescence through sexualselection.Of course, human parents use (<strong>and</strong> ancient hominid parents nodoubt used) a variety of fitness indicators in making decisionsabout investment in their young. But vocalization has uniquestatus as a communicative device among birds <strong>and</strong> mammals.Vocalization in many species communicates immediate survivalneeds <strong>and</strong> provides a primary fitness indicator in sexual selection(Hausberger 1997; Hauser 1996; Kroodsma 1999). In humans,vocalization appears to play a significant role in sexual selection(Miller 2000), but also a critical role in survival during the mostaltricial period of infancy. The importance of this early altricialperiod in fostering the creation of vocal capabilities that have afoundational character for language has been vastly underplayedin most prior portrayals of language evolution, where the focushas so often been on how syntax in language came about (see,e.g., Bickerton 1981; Pinker & Bloom 1990). Syntax, <strong>and</strong> manyprecursors to it, could never have occurred without fundamentalcommunicative flexibility; <strong>and</strong> syntax is a feature of language thatappears in the human infant only after many months of infrastructuraldevelopment in flexibility of vocal communication.The target article is on the mark in highlighting the <strong>life</strong> <strong>history</strong>issue. The necessary refocusing on developmental issues inspeculations about language evolution will inevitably bring usto the beginning of hominid evolution <strong>and</strong> thus to the beginningof vocal development in the first months of human <strong>life</strong>.ACKNOWLEDGMENTSThis work was supported by the National Institutes of Deafness <strong>and</strong>Other Communication Disorders (R01DC006099-01, D. K. Oller PI<strong>and</strong> Eugene Buder Co-PI), by the Konrad Lorenz Institute forEvolution <strong>and</strong> Cognition Research, <strong>and</strong> by the Plough Foundation.Comparative, continuity, <strong>and</strong> computationalevidence in evolutionary theory: Predictiveevidence versus productive evidenceDavid M. W. PowersSchool of Informatics <strong>and</strong> Engineering, Flinders University of South Australia,Adelaide, SA 5001, Australia.David.Powers@flinders.edu.auhttp://www.infoeng.flinders.edu.au/people/pages/powers_davidAbstract: Of three types of evidence available to evolutiontheorists – comparative, continuity, <strong>and</strong> computational – the first islargely productive rather than predictive. Although comparisonbetween extant species or languages is possible <strong>and</strong> can be suggestiveof evolutionary processes, leading to theory development, comparisonwith extinct species <strong>and</strong> languages seems necessary for validation.Continuity <strong>and</strong> computational evidence provide the best opportunitiesfor supporting predictions.294 BEHAVIORAL AND BRAIN SCIENCES (2006) 29:3
Commentary/Locke & Bogin: <strong>Language</strong> <strong>and</strong> <strong>life</strong> <strong>history</strong>Locke & Bogin (L&B) tell two kinds of story, the first seeking todemonstrate that Homo sapiens is unique in having a childhood<strong>and</strong> that this is critical to the nature of human language, thesecond seeking to explain how evolutionary pressures mayhave given rise to these phenomena. These stories are intimatelyintertwined in the target article, with many evolutionaryhypotheses being stated baldly as facts. Nonetheless, L&B arecorrect to emphasize the role of ontogeny <strong>and</strong> provide auseful review of developmental evidence. Computationalmodels of self-organization show that significant visual, auditory,<strong>and</strong> linguistic features can self-organize with minimalinput, whereas detailed ontogenesis without self-organizationwould seem to exceed the capacity of the genome, suggestingthat a composite model is required (Willshaw & von derMalsburg 1979).The perinatal <strong>and</strong> infancy period is certainly critical but L&Bneglect the former. Since the 1980s we have known that infants,even in utero, can discriminate auditory information, can distinguishone language from another, can distinguish onespeaker of a language from another, or even distinguish smallphonemic changes in a single syllable (Mehler et al. 1988).Indeed, neonates as much as 10-weeks premature can distinguishlow frequency tones <strong>and</strong> show auditory cortical event relatedpotentials (Kurtzberg et al. 1984; 1986; 1988; Kushnerenko2003).L&B highlight an important phenomenon under the guise of“trickle-up phonetics.” There has been an implicit assumptionin linguistics <strong>and</strong> psycholinguistics that children learn thoseaspects of their target language that are not innate. Althoughthis seems to be self-evident, it is not true! The child’s idiolectis creatively constructed <strong>and</strong> negotiated in a complex socialenvironment <strong>and</strong> is not identical to that of either parent – parental,sibling, <strong>and</strong> peer language is in general modified by the interactionwith the child, <strong>and</strong> linguistic, cognitive, <strong>and</strong> social learningis part of a complex that is not conveniently modular. It is thussimplistic to assume there is a target language <strong>and</strong> that thechild’s aim is identification in the limit (Gold 1967; Powers &Turk 1989). As L&B note, imitation of the infant is morecommon than imitation by the infant, <strong>and</strong> such parental imitationis associated with increased attention. The vocabulary of a familyis changed by an infant, as child protowords are adopted, <strong>and</strong> thegrammar <strong>and</strong> subject matter <strong>and</strong> complexity of child-directedspeech are accommodated to the infant. This is most clearlyevident in the process of creolization (Bickerton 1984; 1990;1995).An underst<strong>and</strong>ing of the language acquisition process thatescapes from the naïve idea of a baby replicating his mother’sgrammar is fundamental to a theory of language evolution <strong>and</strong>predicts that considerable language evolution is possiblewithin the species. This puts the onus on a theory of macroevolutionto make truly contrastive predictions with <strong>new</strong> innatemechanisms associated with <strong>new</strong> species. Although L&B do notclearly enunciate a model of language acquisition/ontogenesis,they do provide many details <strong>and</strong> hypotheses that could beuseful in elaborating a testable theory of ontogenesis; thoughthese details are far from a testable theory in relation to theevolution of innate cognitive <strong>and</strong> linguistic capabilities <strong>and</strong>distinctives.An evolutionary theory may be tested in two main ways: (1) bymaking predictions that may be validated archeologically, genetically,or biochemically; or (2) by building computational modelsthat demonstrate that proposed mechanisms produce the predictedeffects. A developmental theory of language may betested analogously by making predictions that may be validatedneurologically, behaviourally, or linguistically; or again, byusing computational modelling.Existing fossil <strong>and</strong> comparative linguistics evidence is “productive”in inspiring theory production – “productive evidence”has its value in producing the theory but has no value, zerovalue, in establishing its validity. Validation requires making<strong>and</strong> confirming theory predictions <strong>and</strong> requires <strong>new</strong> evidencethat fits the “predictive” framework <strong>and</strong> either confirms orrefutes the theory – “predictive evidence” must not havebeen utilized in formulating the theory <strong>and</strong> is the only kindof evidence that can provide support for the theory, althoughit can never prove it. The riskier the prediction, the moreunexpected the evidence; <strong>and</strong> the greater the contrast withthe predictions of competing theories, the better the theoryis <strong>and</strong> the more support predictive evidence can bring(Popper 1963).L&B make no explicit predictions <strong>and</strong> offer no confirmatoryevidence. In particular, their Figure 3 is speculative <strong>and</strong> the proposedevolutionary sequence has been disconfirmed by genetic,biochemical, <strong>and</strong> other evidence. L&B themselves acknowledgethat “the only reliable data are associated with Pan <strong>and</strong> H.sapiens”(sect. 9.1 of the target article). These are the endpointsin the proposed chain – the missing links are still missing <strong>and</strong>saltational theory has failed to hypothesize mechanisms thatcould explain the leaps.We turn therefore to the third kind of evidence, that of computationalmodelling for both evolution of language (Steels1998) <strong>and</strong> the development/ontogenesis of language (Powers &Turk 1989). The modelling of the evolution of language naturallydivides into several aspects (Hauser et al. 2002; see target article,sect. 12): evolution of languages within the human species;evolution of communication capabilities independent ofspecies; continuous macroevolution <strong>and</strong> adaptation of organiccognitive, communicative, <strong>and</strong> linguistic capacities acrossspecies; <strong>and</strong> saltational mechanisms that might explain themissing links <strong>and</strong> missing ancestors.The use of the term “evolution” in relation to language as amedium rather than a faculty involves a metaphorical step:Darwinian, Lamarckian, <strong>and</strong> saltational evolution involve hypothesizedmechanisms for genetic mutation, inheritance, <strong>and</strong> naturalselection (ignoring the problem of the evolution of the genetic<strong>and</strong> proteomic mechanisms themselves); evolution of humanlanguage within H. sapiens depends only on social <strong>and</strong> ontogeneticprocesses. L&B espouse a theory of evolution that requiresmacroevolutionary steps; however, much of what they describeseems not to be evolutionary in the macro sense but rather concernsthe emergence of language within the species. In eithercase, computational modelling is one of the most promisingmeans for testing our theories.To deal first with the question of computational models ofmacroevolution, it is necessary to define a starting substrate –species A with specific capabilities – <strong>and</strong> a condition for recognizingenhanced communicative capability (e.g., second signallingof facts not in evidence in the here <strong>and</strong> now). Incomputational terms, the distinction between evolutionary, selforganizing,<strong>and</strong> learning systems arises in relation to the programrather than the hardware. Evolutionary development involvesdifferent individuals having different programs, <strong>and</strong> normallyincludes both genetic variants (not true evolution) <strong>and</strong> mutation(not macroevolution of itself) <strong>and</strong> falls into the domains of artificial<strong>life</strong>, genetic programming, <strong>and</strong> evolutionary robotics. It may, forexample, involve the evolution of different types of neural networks(Luerssen & Powers 2003) or it may be undertaken at a higherlevel of abstraction (Olsson & Powers 2003).A developmental theory of the emergence of language in boththe species <strong>and</strong> the individual has more of the character of selforganization<strong>and</strong> unsupervised learning (Powers & Turk 1989).Such processes involve the interaction of innately determinedstructures <strong>and</strong> capabilities, physical <strong>and</strong> environmental conditions,<strong>and</strong> boundary <strong>and</strong> interaction constraints, to producedetailed neural structure as well as define an individual’s idiolect<strong>and</strong> a society’s language. This has been demonstrated for variouskinds of visual <strong>and</strong> auditory features as well as for phonological,morphological, <strong>and</strong> phrase-level structure (Powers 1992), <strong>and</strong>some multi-organism language creation (Steels 1998) also hassomething of this unsupervised character – although in aBEHAVIORAL AND BRAIN SCIENCES (2006) 29:3 295
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