Before they begin to speak, infants have already forged a link between language & core cognitive capacities; humans establish reference, convey information declaratively & pass down communicative devices via cultural transmission

Becoming human: human infants link language and cognition, but what about the other great apes? Miriam A. Novack and Sandra Waxman. Philosophical Transactions of the Royal Society B: Biological Sciences, November 18 2019. https://doi.org/10.1098/rstb.2018.0408

Abstract: Human language has no parallel elsewhere in the animal kingdom. It is unique not only for its structural complexity but also for its inextricable interface with core cognitive capacities such as object representation, object categorization and abstract rule learning. Here, we (i) review recent evidence documenting how (and how early) language interacts with these core cognitive capacities in the mind of the human infant, and (ii) consider whether this link exists in non-human great apes—our closest genealogical cousins. Research with human infants demonstrates that well before they begin to speak, infants have already forged a link between language and core cognitive capacities. Evident by just three months of age, this language–cognition link unfolds in a rich developmental cascade, with each advance providing the foundation for subsequent, more precise and more powerful links. This link supports our species' capacity to represent and convey abstract concepts and to communicate beyond the immediate here and now. By contrast, although the communication systems of great apes are sophisticated in their own right, there is no conclusive evidence that apes establish reference, convey information declaratively or pass down communicative devices via cultural transmission. Thus, the evidence currently available reinforces the uniqueness of human language and the power of its interface to cognition.

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3 Beyond humans: gestural communication among non-human great apes

Our goal in this last section is to look beyond human infants andconsider the communicative abilities of the other great apes.

Like humans, apes use a range of communicative signals—including vocalizations, facial expressions and gestures—toconvey information and to influence the behaviour of others[75]. There has been substantial research on primate vocaliza-tions,  much  of  it  focusing  specifically  on  vocalizationsproduced in the context of evolutionarily urgent survival func-tions such as responding to predators and communicatingsources of food [76,77]. Certainly, these vocalizations areimpressive and some have argued that they reflect a varietyof sophisticated cognitive and social functions [78–81]. But others have argued that ape gestures—more than their vocalizations—provide the most compelling comparisons tohuman language [82–85]. Therefore, we focus primarily onape gesture and its communicative status in comparison tohuman language and human gesture.



(a) The gestural repertoires of apes

Gestures are part of the communicative repertoires of allspecies of great apes [75]. Typically emerging at around ninemonths of age [86,87], these gestures are produced deliberatelyand voluntarily in social interactions [88–90]. Ape gestureshave been defined as‘discrete, mechanically ineffective phys-ical movements of the body observed during periods ofcommunication’[88, p. 749] and include tactile gestures (invol-ving bodily contact with another individual, e.g. hittinganother), auditory gestures (incorporating non-vocal sounds,e.g. stomping) and visible gestures (those can be seen from adistance, e.g. arm raising) [75,88,90]. Note that within the apeand human literatures, the definition of what constitutes a gesture often differs; definitions of human gesture tend to focusprimarily on silent, empty-handed movements that make nophysical contact with objects or other people (see [91] for a dis-cussion of differences in definition and coding). Nevertheless,important cross-species comparisons can be made.

Most researchers agree that apes’gestures share two key features with those of humans’gestures: flexibility and inten-tionality [13,88,90,92–94]. Regarding flexibility, apes producea variety of gesture types (e.g. arm raise; poke; object shake)flexibly across different situations (e.g. affiliation, grooming,resting, social play). They can use a single gesture typeacross multiple contexts, as well as multiple gesture typeswithin a single context [7]. Regarding intentionality, apes’ges-tures are often responsive to the attentional states of theirwould-be communicative partners: for instance, when theintended partner is looking elsewhere, chimps tend to initiateby making physical contact (touching the other) and auditorygestures (banging on the ground) [88,95].

(b) How do ape gestures differ from human gestures?

Like humans, the gestures of apes reveal an intention tocommunicate and insight into the attentional states of theirconspecifics. But apes’gestures also differ considerablyfrom those of humans. Most striking are species differencesin the presence versus absence of (i) pointing and the estab-lishment of reference, (ii) gesturing for declarative purposes,(iii) communicating without contextual support, and (iv) evi-dence for learning processes and developmental cascades.

Together, these differences raise intriguing questions aboutwhether the communicative systems of apes link to their core cognitive capacities, as is the case for humans.First, apes in the wild do not use pointing gestures to com-municate with conspecifics [96–100]. In one comparative study,researchers used the same criteria to characterize spontaneousreferential gestures produced by 1- to 2-year-old human children and chimpanzees in natural settings. Among children,nearly 25% of the gestures produced were classified as potentially referential (e.g. directed to an external location or third party). Among chimpanzees, fewer than 0.1% of their gestures met this criterion [91]. Human-reared or human-captive apes can be taught to use pointing gestures; however, these points typically occur only in communication with humans and only in contexts where the goal is to convey imperative information, typically a request for food [101–106].

Second, beyond the case of pointing, ape gestures appear tobe reserved exclusively for imperative purposes. They gestureto regulate face-to-face interactions in the here-and-now suchas play, grooming, fighting or tandem travel [98]. For example,most gestures between apes are produced in dyadic contexts,aimed to get the attention of a would-be social partner [107].

There is little to no evidence that apes gesture declaratively to direct another’s attention simply for the sake of sharing interest in it or commenting on it. By contrast, human infants frequently gesture for declarative purposes, sharing their intentions with their carers [48,98,102].

Third, ape gestures are considerably more dependent on thecontextual support of the present than are those of human chil-dren. Although both children and captive apes can use gesture to refer to non-present entities (i.e. they can point to an empty plate that used to contain food), these gestures are still dependent on referencing present objects (i.e. the now-empty plate)[104–106]. In wild ape populations, spontaneous gestures typically require the use of a present object. For example, to request‘play’, an ape may hit a conspecific; to request ‘being carried’ a juvenile may place their hand on their mother’sback.

Humans, too, can use contextual support to express ideas via gesture (e.g. pointing to an object that we want). Yet, in addition,we also ubiquitously gesture in the absence of any referent object, (e.g. using one’s hands to describe the shape of a missing puzzle piece; demonstrating how to cut with scissors, even when none are present). There is no clear evidence of this typeof iconic gesture production in apes [108].

Fourth, there is little evidence that ape gestural repertoiresare readily learned through imitation or through cultural trans-mission [88,93,98,109–111]. Instead, the ape gestural repertoireconsists primarily of species-typical behaviours [88–90]. Somehave interpreted the scant variability in gestural repertoire sacross groups of apes as evidence that ape gestures areinnate, acquired primarily through genetic transmission[88,93]. Others have claimed that certain types of ape gesturesare adaptations of full-fledged actions to create a more restricted gestural form that will elicit a target behaviour, a process known as ontogenetic ritualization [111]. For example, to request a climb on its mother’s back, infants first push down her rear end to gain access to climb. But over time, this behaviour is streamlined: to elicit the response, the infant need only touch the mother’s back [112]. Certainly, this process involves learning, but the learning occurs only in that particular interaction in that dyad concerning that action.

Finally, there are dramatic differences in the developmental course of gesture systems among apes and humans. The most striking difference is that in humans, early gestures are integrated spontaneously into a rapidly burgeoning linguistic system; this system is at once more comprehensive in its communicative and symbolic reach and more precise in itsexpression than the systems observed among apes. Humaninfants initially rely heavily on gestures but this reliance decreases steadily [52,56]. As their linguistic capacities advance, infants move systematically from producing gestures alone toproducing gesture + language (and then language + language) combinations. By contrast, apes’ reliance on gesture for communication does not seem to change, even among apes trained by humans to acquire new symbolic signals [113].

Certainly, there are cases in which apes, raises by humans,learn complex symbol systems including spoken words,pictograms or sign language [114–119] (see [120] for a comprehensive review and discussion of the many controversies surrounding this topic). In such cases, learning to use discrete symbols is achieved only with considerable repetition or reward-based paradigms. Evidence like this offers insightinto the capabilities of the ape mind, given a set of symbols.

Nevertheless, we are cautious in drawing strong conclusions from these examples, as they are rare and have been observed only as a result of human intervention.The fact that apes are capable of learning new symbols ystems speaks to their impressive intelligence, and to the obvious evolutionary links between the ape brain and the human brain. Additionally, there is evidence that some language-trained apes can successfully group novel exemplars into lexical categories, raising the intriguing possibility that learning human-like abstract symbols may support object categorization in non-human apes [121]. However, it typically takes apes manytrials of learning with rewards to acquire basic use of these symbols. Note that this differs from how human infants spontaneously acquire language, as well as how they can easily adapt novel symbols as category markers or object labels, given only a single session of seeing these symbols embedded in a communicative interaction [21,23]. One perhaps important counter-example bears mention: two infant bonobos, Kanzi and Makula, may have spontaneously learned symbols on which their mother had previously been trained [118,119]).

Even for an ape that has mastered productive use of a signal system with their human trainers, there are sharp boundary conditions on their use. Language-trained apes use acquired symbol systems almost exclusively for imperative purposes in interactions with humans [122,123]. Furthermore,in stark contrast to humans, there seems to be an upper limit toapes’combinatorial abilities. Even language-trained apes overwhelmingly produce symbols in isolation; the virtual absence of combinations that exceed two symbols reveals a compelling difference between children and apes [124].

Taken together, the existing evidence reveals that althoughapes in the wild show impressive usage of communicativegestures, produced intentionally and with flexibility, these ges-ture systems differ dramatically from human communication (for a more nuanced discussion, see [88]). They do not makeuse of pointing gestures, gesture for imperative purposes only, typically require present context to gesture, do not pass down their gestures through cultural transmission, and do not undergo significant developmental shifts in gesture use.

Finally, despite the fact that some apes have, with great training, learned a limited set of human-like symbol systems, their learning processes are distinct from human language learning and their use of these symbols is largely limited.