2nd proofs From gesture to language

From gesture to language Ontogenetic and phylogenetic perspectives on gestural communication and its cerebral lateralization Adrien Meguerditchian, Hélène Cochet and Jacques Vauclair Department of Psychology, Research Center in the Psychology of Cognition, Language and Emotion, Aix-Marseille University, Aix-en-Provence, France

The vast majority of studies of nonhuman primate communication focus on their vocal displays, and virtually all treatises with titles such as “Primate Communication and human Language” focus on the vocal channel, often without even mentioning gestures... In my view, this is a huge mistake” (Tomasello, 2008, p. 53)

Introduction Because nonhuman primates are phylogenetically close to humans, research on our cousins is likely to provide essential clues for reconstructing the features of our ancestral communicative systems. Thus, a prime question for primatologists is to investigate whether evolutionary precursors of language may be found in the communicative behaviours of nonhuman primates. Most of the studies have focused naturally on the vocal modality and many researchers have suggested that language resulted from the evolution of the vocal system in our ancestors (e.g., Seyfarth, 1987; Ghazanfar & Hauser, 1999; Snowdon, 2001; Zuberbühler, 2005). This theory is opposed to a “gestural origins” view of how language might have evolved (e.g., Arbib, Liebal, & Pika, 2008; Corballis, 2002, 2003; Kendon, 1991; Kimura, 1993; Vauclair, 2004). The hypothesis that gestural communication may be the first phylogenetic precursor of human language is supported by several evidence of shared properties between the human language and the gestural communicative system in nonhuman primates. A second aspect of the question concerns human ontogeny and the development of communicative systems. The research on gestural communication in humans provides additional supports to the gestural origins theory of language in underlining the tight links between language and gestures. It is well known that infants and children use

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From gesture to language

Adrien Meguerditchian, Hélène Cochet and Jacques Vauclair

number of different meanings expressed through gestures) was correlated to later verbal vocabulary size while gesture-speech combinations (in which gesture and speech convey two different ideas) were a strong predictor of later two-word combinations. The role of gestures in the development of children’s communicative skills can be explained by different reasons that do not mutually exclude each other. Firstly, it was shown that infants’ pointing gestures elicit verbal responses from adult caregivers (e.g., Kishimoto, Shizawa, Yasuda, Hinobayashi, & Minami, 2007). Such a production would more or less implicitly teach the child how to express a specific idea in language. It turns out that by supplying the child with the appropriate words and grammatical constructions following his/her pointing gesture, adults facilitate language learning. Interestingly, it was shown that pointing gestures provoke adults’ commentaries irrespective of whether or not the gestures are accompanied by vocalizations (Kishimoto et al., 2007). Children’s gestures are thus regarded by adults as an integral part of communication and not simply as a secondary communicative behaviour. Secondly, linguistic abilities may be promoted by gestures on their own as they allow children to express meanings of increasing complexity that cannot yet be expressed verbally. For example, they can practice sentence-like constructions through gesture-speech combinations. Finally, children’s early gesture use predicts language ability because speech and gestures share common cognitive processes, in particular the ability to represent and influence another person’s attentional state. For that reason, pointing gestures have received increasing and well-deserved attention. Regarded as the “royal road to language” (Butterworth, 2003), they first emerge at the end of the first year (Butterworth & Morissette, 1996) and enable children to direct the adult’s attention toward external objects or events. Pointing gestures invite the recipient to attend to a referent in two different contexts (Bates, Camaioni, & Volterra, 1975). In imperative pointing, children seek to obtain an attractive object by using others as “causal agents” (Camaioni, 1997). In contrast, the declarative function involves the use of objects as a means of getting adult’s attention. Declarative pointing is produced when the child is eager for an event or an object and wants to share this interest with the adult, or when he/she seeks to provide the adult with needed information about a referent (Tomasello, Carpenter, & Liszkowski, 2007). Features of human language, namely social cognition and cooperation, are thus already reflected in toddlers’ declarative pointing gestures (Liszkowski, Carpenter, Henning, Striano, & Tomasello, 2004; Liszkowski, Carpenter, Striano, & Tomasello, 2006; Liszkowski, Carpenter, & Tomasello, 2008). Declarative pointing appears to be more complex than imperative pointing in terms of cognitive processes, consistent with results of a study by Camaioni, Perucchini, Bellagamba, and Colonnesi (2004) showing that the understanding of adults’ intentions by toddlers was linked to the production of declarative, but not imperative pointing. The two types of gestures may then develop independently, with declarative pointing emerging later than imperative pointing (e.g., Camaioni et al., 2004). A study of Cochet and Vauclair (2010) can further illustrate this distinction between imperative and declarative function, considering the relationship between the form and

gestures for communication before their first spoken words and that manual actions play an important role in the development of speech, from the babbling stage onwards. In addition, the richness and diversity of the gestures predict vocabulary development. In short, gestures can be considered as fundamental building blocks of communication as they “pave the way for language development” (Iverson & Goldin-Meadow, 2005). A related point that needs to be mentioned is that adult speakers almost systematically accompany all their speech with expressive manual gestures (co-speech gestures, see McNeill, 2005) and that several reports indicate that speech and gesture might share the same integrated communication system (e.g., Bernardis & Gentilucci, 2006). Moreover, it has been well documented that human signed languages are full-blown languages and share the same “phonological”, morphological and syntactical properties as well as some similar cerebral areas with speech (for reviews: Bellugi, 1991; Emmorey, 2002; see also Emmorey, Mehta, & Grabowski, 2007 and see below). A last issue is helpful for supporting the gestural hypothesis and it relates to the now recognized tight interconnection in the brain between the control of action (gestures) and language processing (e.g., Willems, Özyürek, & Hagoort, 2007; Gentilucci & Dalla Volta, 2008). Borrowing from the field of human developmental and comparative psychology as well as from primatology and cognitive neuroscience, the goal of this chapter is to spell out some arguments which are in favour of the gestural hypothesis of language origin. The chapter is organized in four main parts. In the first part, a synthesis will present the most significant advances pertaining to the development of gestural communication in human infants and children. As we are interested in identifying lateralized patterns of manual gestures, Part 2 will be devoted to the description of manual asymmetries and their development during the first 2 years. Part 3 is concerned with the presentation of the properties of gestural communication in nonhuman primates. The fourth part deals with the question of the relationships between gestural communication, its lateralization and brain functioning. Finally, we will conclude by summarizing the main arguments that are relevant for advocating the crucial role played by gestures in the shaping of human communication and language in comparison to the vocal communicative system.

I. Gestural communication in human children The onset of intentional communication in human infants appears through the gestural modality and it is presently widely admitted that gestures predict and facilitate language learning. Numerous researchers have indeed found early communicative gestures to be correlated to later linguistic skills (e.g., Brooks & Meltzoff, 2008; Iverson & Goldin-Meadow, 2005; Rowe, Özçaliskan, & Goldin-Meadow, 2008; Volterra, Caselli, Capirci, & Pizzuto, 2005). For example, Rowe & Goldin-Meadow (2009) showed that toddlers’ gestures used at 18 months of age selectively predict vocabulary size and sentence complexity at 42 months of age. Children’s gestural vocabulary (defined as the

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Schneider, 1980; Kimura, 1973; Saucier & Elias, 2001) and for signing in deaf people (Vaid, Bellugi, & Poizner, 1989; see also Grossi, Semenza, Corazza, & Volterra, 1996), while several studies have reported a right bias for communicative gestures in infants and toddlers as well (Bates, O’Connell, Vaid, Sledge, & Oakes, 1986; Blake, O’Rourke, & Borzellino, 1994; Vauclair & Imbault, 2009; Young, Lock, & Service, 1985). One of the first signs of the coupling between speech and gestures in infants is observed with the emergence of babbling at around 7 months of age, as it is accompanied by an increase in repetitive right-handed activity (Locke, Bekken, McMinn-Larson, & Wein, 1995). In addition, the degree of right-hand bias for pre-pointing gestures in infant tends to increase between 8 and 12 months of age (Blake et al., 1994). These findings were interpreted as reflecting the maturation of the control mechanisms in the left cerebral hemisphere. Regarding referential and intentional gestures, an observational study conducted at a day care centre investigated spontaneous pointing gestures produced by toddlers (Cochet & Vauclair, 2010). Results confirmed the previous findings reported in experimental studies as pointing gestures were found to be predominantly right-handed. This study also revealed that the vast majority of pointing gestures (almost 90%) were accompanied by vocalizations, consisting of either words or other speech sounds. This widespread use of vocalizations contrasts with results from nonhuman primate studies (e.g., Hopkins & Cantero, 2003) and may be regarded as an evidence of the uniqueness of human communication. The strong involvement of the left cerebral hemisphere for communicative gestures might be, of course, related to the left lateralization for language. As concerns sign language in deaf people specifically, functional brain imaging studies (Positron emission tomography, PET) revealed that the production of signs activated Broca’s area in the left hemisphere (e.g., Corina, San Jose-Robertson, Guillemin, High, & Braun, 2003; Emmorey, et al., 2007). But no similar data are available so far for communicative gestures other than signs in adults and infants. Traditionally, the relationship between hand preference and speech focused on handedness for object manipulation: right-handedness in humans has been historically linked to left-hemispheric specialization for language. However, 70% of left-handed humans appear to show also a dominance of the left hemisphere for speech (Knecht et al., 2000), indicating that the direction of handedness for manipulation is a poor marker of hemispheric lateralization for language. Whether manual asymmetries for gestural communication constitutes a better landmark is still unclear. Thus, as communicative gestures are recognized as forming a crucial step in the development of infants’ communication, researchers started to examine the link between language and communicative gestures in evaluating whether handedness differs between manipulation activities and communicative gestures. The relationship between laterality for communicative gestures and for object manipulation has been studied in human infants and toddlers (e.g., Bates et al., 1986; Vauclair & Imbault, 2009) and in nonhuman primates (see part IV of this chapter, Hopkins et al., 2005; Meguerditchian & Vauclair, 2006, 2009; Meguerditchian, Vauclair,

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Figure 1. Average progression of the proportion of index finger extensions (solid line) and declarative pointing gestures (dashed line) as a function of age (adapted from Cochet & Vauclair, 2010)

function of pointing gestures. Index extensions were more frequently used with a declarative function, whereas whole-hand gestures were more frequently produced with an imperative function. Moreover, as they grew older, children tended to produce more and more pointing gestures in declarative contexts and with the index finger extended (see Figure 1). These results have led the authors to hypothesize distinct origins of imperative and declarative pointing. Imperative pointing would substitute for reaching actions by a process of ontogenetic ritualization (Tomasello & Call, 1997), whereas the development of index-finger extensions would be socially transmitted to the infant, involving an imitation process rather than a process of ontogenetic ritualization. Indeed, it is unlikely that declarative pointing gestures originate from reaching actions, as almost any of these gestures are produced with the whole hand, contrary to imperative pointing (Cochet & Vauclair, 2010). Moreover, observations of cultural differences in the form of pointing gestures (e.g., Wilkins, 2003; Kendon & Versante, 2003), indicating that index-finger pointing is not a universal form of reference, support the role of imitation in the development of declarative pointing.

II. Asymmetries of vocal and gestural communicative behaviours in humans The existence of speech-gesture links can be further illustrated when focusing on laterality for communicative gestures. In adult speakers, a right-sided asymmetry has been observed for gestures accompanying speech (e.g., Dalby, Gibson, Grossi, &

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III. Properties of gestural communication in nonhuman primates

& Hopkins, 2010). Lateralization in communicative behaviours was found to show a stronger degree of asymmetry than lateralization for object manipulation. For example, in a study of children born from deaf parents using sign language, their right-hand preference for signing was much stronger than for nonsign actions (Bonvillian, Richards, & Dooley, 1997). Vauclair and Imbault (2009) assessed hand preferences for pointing gestures and for object-manipulation in 123 infants and toddlers aged 10–40 months. In this study, the number of right-handed participants was significantly higher in pointing than in object manipulation. Moreover, a large proportion of the lefthanded and ambidextrous children regarding object manipulations pointed with their right hand, whereas very few right-handers shifted to the left hand for pointing. The degree of right-asymmetry thus appears to be more pronounced for communicative gestures than for non-communicative actions. More surprisingly, it was also shown that hand preferences for object manipulation and pointing gesture were significantly correlated, though lightly, between the ages of 18 and 20 months and between 29 and 32 months, whereas this correlation became non-significant in the interim periods (Vauclair & Imbault, 2009). These two age ranges were considered to encompass two key periods in the development of linguistic abilities, namely the lexical spurt and the acquisition of grammatical morphology, respectively. The authors explained this result by a “hypermobilization” of the left hemisphere for language functions. Greater demand would be placed on left hemisphere resources during these key periods. Finally, the two types of laterality were only partially linked, and at specific moments in language development. This difference in laterality patterns supports the view for the existence of a bimodal communication system (gestural and vocal) in the left cerebral hemisphere that differs from the system controlling the purely motor functions of manipulation (e.g., Gentilucci & Dalla Volta, 2008). This theory is consistent with the results reported in a study investigating the relationship between laterality of communicative gestures and language development (Vauclair & Cochet, submitted). In this study, language level was assessed using the “language” subtest of the revised Brunet-Lézine scale (Josse, 1997), which leads to the calculation of a developmental quotient for language. Pointing gestures were found to be more right-handed in children with either a low or a high developmental quotient for language, that is in children who encounter relative difficulties in language acquisition, and conversely in those who seem to have a higher learning ability. In both cases, a specific cognitive load would be generated in the left cerebral hemisphere, which significantly affects laterality patterns. These collective findings support the hypothesis that (1) communicative gestures play an active role in the development of children’s linguistic skills and that (2) a single integrated communication system in the left cerebral hemisphere might be in charge of both vocal and gestural communication.

It is well documented that great apes, particularly chimpanzees, and in a lower degree some monkey species, use manual gestures and body movements to communicate with conspecifics in various social contexts such as play, threat, aggression, greeting, invitation for grooming, in case of shared excitation, of reassurance-seeking after stress, and for food begging (e.g., Goodall, 1986; Pika, Liebal, Call, & Tomasello, 2005, for reviews; Pollick & De Waal, 2007). The investigation of this communicative system in our closest phylogenetic cousins provides some evidence of continuities with some key properties of human language, such as flexibility of morphology and use, intentionality and referential properties (see for reviews, Meguerditchian & Vauclair, 2008; Pika, 2008).

1. Flexibility Firstly, the gestural modality in apes turns out to be flexible. It has been frequently observed that chimpanzees raised by humans can acquire and use some human-like gestures (Tomasello & Camaioni, 1997). Moreover, the research projects aimed at teaching human language to apes, such as the chimpanzee Vicki, were quite unsuccessful concerning the vocal modality, whereas the chimpanzee Vicki was very good at imitating human gestures (Hayes, 1952) and more than a hundred signs from the American Sign Language were learned by the chimpanzees Washoe (Gardner & Gardner, 1969) and Nim (Terrace, 1979), the gorilla Koko (Patterson, 1978) as well as the orangutan Chantek (Miles, 1990). These data indicate the remarkable ability of apes to learn and to use flexibly novel manual signs to communicate with humans rather than novel vocalizations. In addition, observations of groups of apes revealed some variation of the gestural repertoire not only among individuals in the same social group but also between different populations (Pika et al., 2005 for a review). It appears that bonobos and chimpanzees communicate with gestures independently of the social behavioural contexts, whereas the facial and the vocal systems appear to be context-dependent, indicating again the singular flexibility of use of the gestural communicative system (Pollick & De Waal, 2007). In fact, among individuals, different gestures may be produced for the same goal and, conversely, similar gestural signals may be used for divergent goals (Tomasello et al., 1985; Tomasello, Gust, & Frost, 1989).

2. Learning We cannot exclude that, like speech acquisition, forms of social learning or observational imitation processes underlie the emergence of some of these gestures within a group of apes (for a review: Pika, 2008). To explain such a variability of the gestural repertoire, Tomasello (1996) has suggested that the main process for the emergence of

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auditory or tactical gestures when the recipient is inattentive (Tomasello et al., 1994; 1997; for similar evidence in other great apes, see also Liebal, Pika, & Tomasello, 2004, 2006; Pika, Liebal, & Tomasello, 2003, 2005). These observations clearly demonstrate that great apes are able to distinguish an attentive from an inattentive state of the audience and to adjust their gestural behaviours to the attentional state of the recipient. In short, the above examples offer solid evidence of intentional communication.

gestures is at the individual learning level, namely via “ontogenetic ritualization”, rather than at the social level. In other words, as it has been described in human infants for some pre-linguistic imperative gestures ritualized from actions across interactions with the mother (see part I of this chapter), most gestures in apes might first be initiated at an individual level from manual actions and progressively become, during dyadic interactions, ritualized into communicative signals (see Pika, 2008 for a precise description of the different steps of this ritualization process). Finally, contrary to human speech, some gestures of the repertoire in a given ape species appear to be species-typical and stereotyped (like gorillas’ chestbeat) and might thus be mainly genetically determined (see Genty, Breuer, Hobaiter, & Byrne, 2009).

4. Pointing and referential properties The production of referential pointing in nonhuman primates remains more controversial (see Gomez, 2005; Leavens, 2004; Tomasello, 2006). Referential pointing might be defined as the ability to direct the arm or a finger to draw the attention of an audience to an external object of interest, an event, a direction or a location for an imperative (i.e., for attaining a goal) or a declarative purpose (i.e., for the sake of sharing attention about an object, an event). Imperative pointing behaviours has been frequently observed in both captive apes and captive monkeys (reviewed in Leavens & Hopkins, 1999). For example, Leavens, Hopkins and Thomas (2004) demonstrated that, in the presence of a human recipient, captive chimpanzees were able to direct their arm (or even their index-finger) towards one of the two out-of-reach opaque boxes where food had previously been hidden by another experimenter. Contrary to humans, there are poor evidence of declarative pointing in apes and most of these gestures are quasi-exclusively imperative insofar as only specific goals motivate their use (e.g., usually for obtaining food). In addition, it must be noted that pointing has rarely been described in wild populations or between conspecifics but rather in captive subjects (but see Inoue-Nakamura & Matsuzawa, 1997; Veà & SabaterPi, 1998). This limitation supports the hypothesis (see part I) that the emergence of imperative pointing might especially result from an ontogenetic ritualization that, in the case of chimpanzees, is related to the captive conditions of the individuals and their frequent interactions with humans for obtaining out-of-reach food (Leavens, Hopkins, & Bard, 2005). However, Pika and Mitani (2006) have reported that wild chimpanzees were able to produce and to comprehend a surprising referential gesture. During mutual grooming, these authors showed that a signaller can indicate the precise part of his body he wants to be groomed by scratching this area in front of his social partner. Interestingly, in most cases, the recipient reacted by changing the location of his grooming behaviour to the desired area of the signaller, indicating that he is able to understand the meaning of the directed scratching gesture (see Figure 2). Such a shared understanding of the gestural orienting signal reflects the existence of a typically referential communication.

3. Intentionality Intentional signalling is one of the key properties of human language and is usually characterised in the literature (e.g., Leavens, 2004) by 3 main criteria: (1) the behaviour of the signaler is produced and directed towards a recipient; (2) the visual orienting behaviours alternate between the recipient and the distal event or object of interest (gaze alternation); (3) when the social partner is not attending or responding, the signaller repeats or adjusts its behaviour to the attentional state of the recipient (persistence). Firstly, it is well documented that the production of gestures in apes fulfils these criteria and is then under an intentional control (e.g., Bard, 1992, Call & Tomasello, 1994; Leavens & Hopkins, 1998; Leavens, Hopkins, & Bard, 1996; Tomasello et al., 1994). In effect, the production of gestures always involves a social partner (a conspecific or a human), within a dyadic interaction (reviewed in Leavens, 2004 and Pika et al., 2005, see also Genty et al., 2009). Secondly, it has been frequently described that captive chimpanzees alternate their gazes between the food and the human recipient when they produce pointing gestures in the presence of a human and a desired out-of-reach food (Krause & Fouts, 1997; Leavens & Hopkins, 1998, 1999). Finally, captive chimpanzees and orangutans repeat and modulate their gestures when the human recipient is not responding and does not deliver the desired out-of-reach food (Leavens, Russell, & Hopkins, 2005; Cartmill & Byrne, 2007). Moreover, attention-getting gestures before initiating an interaction have been frequently observed both in social groups (e.g., Goodall, 1986) and during interactions with humans (e.g., Krause & Fouts, 1997). This happens particularly when the recipient is not responding or is not attending (Tomasello, 2003), such as for example in ground hand slapping when a young gorilla wants to invite a social partner to play (Pika, Liebal, Tomasello, 2003) or in clapping and cage banging in captive chimpanzees when begging for out-of-reach food (Hostetter, Cantero, & Hopkins, 2001; Leavens, Hostetter, Wesley, & Hopkins, 2004). Interestingly, great apes can change their position to face the recipients before producing gestures towards them (e.g., Liebal, Pika, Call, & Tomasello, 2004). Moreover, in play invitations, young chimpanzees produce significantly more visual signals when the social partner is attentive but more

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฀ Adrien Meguerditchian, Hélène Cochet and Jacques Vauclair


for reviews). Such measures have led to non significant or contradictory results, supporting the historical view that right handedness and hemispheric specialization are unique to human evolution and exclusively associated to the emergence of human speech (e.g., Crow, 2004; Ettlinger, 1988; Warren, 1980). The latter view has been challenged by a large body of evidence in a host of vertebrates that have demonstrated behavioural and brain asymmetries at a population level (Rogers & Andrew, 2002; Vallortigara & Rogers, 2005), including nonhuman primates (reviewed in Hopkins, 2007; Hopkins & Vauclair, in press). Interestingly, a significant predominance of right handedness has been frequently reported, particularly in captive chimpanzees and in captive monkeys (Hopkins, 2007). As compared to the rest of the literature, the singularity of these reports might be related to the use of both large samples of subjects and more complex motor tasks than simple unimanual reaching (e.g., bimanual feeding, a bimanual coordinated tube task, throwing, tool use, etc.), indicating that the conflict in the literature may be easily reconciled on the basis of both task complexity and sample-size rather than the contrast usually drawn between wild versus captive subjects (Fagot, & Vauclair, 1991; Hopkins, 2006a,b).

Figure 2. The referential directed-scratch in wild chimpanzees. During mutual grooming, a chimpanzee scratches a part of his body in order to indicate the precise area that he wants to be groomed by his social partner. Such behaviours show a shared understanding of the gestural directed-signal that is typical of referential communication (Pika & Mitani, 2006). Drawing: © Adrien Meguerditchian

1. Manual asymmetries for gestures Does gestural communication in nonhuman primates involve a left-hemispheric dominance like human speech does? The study of manual asymmetries for gestural communication in apes and monkeys constitutes an indirect approach to investigate this question. A study in 227 captive chimpanzees revealed a significant predominance of right-handedness at the population level for a gesture (begging for out-of-reach food) directed towards humans (Hopkins et al., 2005; see also Hopkins & Cantero, 2003; Hopkins & Leavens, 1998; Hopkins & Wesley, 2002). In an observational study in 60 captive baboons, Meguerditchian and Vauclair (2006) also showed a significant population-level right handedness for a species-typical manual gesture, namely “hand slap” (see Figure 3), which consists in a repetitive slapping or rubbing of the hand on the ground towards a focused subject (a conspecific or a human observer) in order to threaten or intimidate it (see Kummer, 1968, for a first description of this gesture). Interestingly, baboons and chimpanzees revealed the same degree of populationlevel right-hand bias for gestures and such biases turned out to be much more pronounced that the right-hand biases – also quasi-identical in these two species – reported for a bimanual manipulative task (see Figure 4), i.e., “the tube task”, which consisted in removing food with fingers of one hand from inside a PVC tube while holding it with the opposite hand (Hopkins, 1995; Hopkins, Wesley, Izard, Hook, & Schapiro, 2004; Vauclair, Meguerditchian, & Hopkins, 2005). This may indicate a greater activation of the left hemisphere for gestural communication. In addition, in both species, there were no correlation of hand preferences between gestures and the bimanual tube task within the same individual performing both types of manual actions (Hopkins et al., 2005; Hopkins & Wesley, 2002; Meguerditchian & Vauclair, 2006).

IV. Gestural communication, lateralization and the brain In humans, most of the language functions are under the control of the left hemisphere of the brain and involve neural networks in which Broca’s and Wernicke’s areas play a key role for the production and for the comprehension of language respectively (Broca, 1865; Wernicke, 1874). Within an evolutionary framework about the origin of language, studying the lateralization and the cerebral substrate of the communicative systems in our primate cousins might shed some light on the phylogenetic precursors of language and its left lateralization from our common ancestor (Vauclair & Meguerditchian, 2007). To discuss these potential implications of the gestural modality, we review in the next section the findings on manual asymmetries related to gestural communication and on their neural correlates in nonhuman primates.

1. Handedness in nonhuman primates Handedness is a relevant functional marker of hemispheric specialization of the brain (the left hemisphere controls the right side of the body, including the right hand, whereas the right hemisphere controls the left side; then right handedness indicates a left-hemispheric dominance of the brain). Most handedness investigations in both wild and captive nonhuman primates have focused on manipulative motor behaviours in various sample-sizes, particularly for simple unimanual measures of hand use such as reaching (see McGrew & Marchant, 1997 and Papademetriou, Sheu, & Michel, 2005

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Figure 3. Manual gesture performed by a male baboon. A young baboon intimidates a human observer by quickly slapping his right-hand on the ground. A predominance of the right-hand at a group level has been measured, suggesting thus a left hemisphere dominance for gesture production (Meguerditchian & Vauclair, 2006). Time is indicated in milliseconds (ms). Picture: © Adrien Meguerditchian

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Such findings support the hypothesis of a possible neural dissociation between communicative gestures and manipulative motor functions (as suggested in human infants, see part II) as well as the existence of a specific left-lateralized communicative cerebral system involved for the production of gestures. Does the communicative nature of gestures induce this specific pattern of right-handedness? Further investigations have recorded hand preferences in other categories of communicative gestures in the same populations of captive baboons and chimpanzees. Thus, the production of ritualized “food beg” gestures directed to humans revealed a trend towards right-handedness in 33 baboons (Meguerditchian & Vauclair, 2009) while species-typical gestures such as “arm threat”, “hand slap” and “extended arm” revealed a significant population-level right-handedness in 70 chimpanzees for both conspecific-directed and human-directed species-typical gestures (Meguerditchian et al., 2010). We can note that this right-hand bias is almost identical to the one reported for food beg gestures in a previous study with chimpanzees (Hopkins et al., 2005). Interestingly, in both species, hand preferences between different categories of gestures correlated with each other within the same individuals. The contrast of hand preferences between communicative gestures and non communicative actions was confirmed in both species: (1) the degree of right-hand biases for gestures at a population level was stronger than the one observed for the bimanual tube task and (2) individual hand preferences for these gestures were not correlated with the hand preferences of any non communicative manual action within the same individuals (Meguerditchian & Vauclair, 2009; Meguerditchian et al., 2010). In these studies, and for comparative purposes, the measures of the asymmetries of “nose wipe”1, a “neutral” manual self-directed

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Figure 4. Degrees of predominance of right-handedness at the group level for the coordinated bimanual tube task and for manual gestures in both chimpanzees (Hopkins et al., 2005) and baboons (Vauclair et al., 2005; Meguerditchian & Vauclair, 2006). N = 104 baboons for the tube task; N = 60 baboons for communicative hand slapping gestures. N = 166 chimps for both the tube task and food-begging gestures. These degrees correspond to Mean Handedness Index scores (MHI) ± SE. On the basis of total left- and right-hand responses, an individual Handedness Index (HI) was calculated for each subject and varied on a continuum from -1.0 to 1.0. The sign indicates the direction of hand preferences: positive, right-hand preference; negative, left-hand preference. The absolute values reflect the strength of hand preference. The error bar represents the standard error around the MHI score. All the MHI scores differed significantly from zero, p < 0.05

behaviour – that does not involve manipulation or communicative intention but rather a nervosity state – revealed neither manual bias at the group level in both baboons and chimpanzees, nor any correlation of hand preferences with other measures of hand use. To sum up, different communicative gestures in both baboons and chimpanzees showed a similar pattern of hand preferences with each other and may thus share partially the same cerebral system, whereas non communicative actions exhibit different patterns of handedness when compared with manual communication. These collective findings thus provide additional support to the existence of a specific left-lateralized system involved in the production of communicative gestures that may differ from the

1. “nose wipe”: a non communicative self-directed manual action which consists of a quick passage of the hand across the bridge of the nose, usually expressed when an individual is nervous (Wallis, 2000).

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1. Pointing, a step for the emergence of language?

system involved in purely motor functions. It might be then hypothesized that such a specific system in chimpanzees and baboons constitutes an ideal prerequisite of the cerebral substrate for human language.

The status of pointing behaviours in nonhuman primates is controversial among primatologists and comparative psychologists. The underlying cognitive processes of the referential gestures described particularly in apes, are still unclear and there is no demonstration of either understanding of the mental states of others or imitation. Then, it is likely that the referential behaviours such as imperative pointing to out-ofreach food in captive chimpanzees (Leavens et al., 2005) or the directed-scratch in wild chimpanzees (Pika & Mitani, 2006) are the results of a mutual ontogenetic ritualization process at the individual level without the intervention of social learning processes. Because of this absence of evidence of shared understanding of mental states of others (theory of mind) when producing or responding to pointing gestures, some authors consider that such behaviours fail to reveal referential properties and that labelling these gestures as “pointing” is then incorrect (Baron-Cohen, 1999; Povinelli, Bering, & Giambrone, 2003; Tomasello, 2003, 2008). However, as suggested by Gomez (2007), whereas the term “referential pointing” is commonly used in the literature for labelling imperative behaviours in infants, their underlying processes remain as uncertain and equivocal as the production of imperative gestures in apes. In fact, we propose that the production of imperative pointings, in both humans and apes, does not require especially the ability to infer mental states of others. Then, the absence of firm demonstration of such an ability for these behaviours should not be incompatible with its referential property, namely drawing the attention of the other on a desired goal. The probable discontinuity for referential gestures between infants and apes might rather be at another level: whereas declarative pointing has seldom been described in nonhuman primates and seems to be unique to humans, imperative pointing gestures are produced both by human and nonhuman primates (e.g., Leavens & Hopkins, 1999; Pika, 2008). Considered as ritualized productions from non communicative actions, imperative pointings involve an individualistic motive for getting others to do what one wants, most commonly to obtain a desired object. Moreover, declarative pointing is produced in order to share an attitude with someone about a common referent or to provide needed information to the recipient. Its emergence, in phylogeny as well as in ontogeny, seems to be related to the emergence of broader motivational and cognitive abilities, including imitation processes. Besides, Tomasello (2008) argues that in the course of evolution, enhanced abilities for imitation played a crucial role in the emergence of protolanguage. Consequently, the absence of any declarative motive in communicative gestures of nonhuman primates (e.g., Camaioni, 1997) could be explained by their lack of some social-cognitive abilities, notably ability for imitation. In short, there is no doubt that declarative pointing gestures, through the development of the capacity to infer others’ goals and intentions have played a key role in the evolution of human communication. However, it is possible that the cognitive differences described above between humans and apes as well as between imperative and declarative pointings might not be so

2. Neural correlates of gestural communication Investigations of hemispheric asymmetries within the brain of nonhuman primates should provide some clues for discussing the hypothesis of a potential neural continuity between language areas in humans and the specific left-lateralized communicative gestural system suggested above in chimpanzees and baboons. As for humans, leftward neuroanatomical asymmetries have been frequently reported in great apes concerning the homologous regions of Broca’s area (i.e., Inferior Frontal Gyrus, IFG) and Wernicke’s area (i.e., Planum Temporale, PT) according to different assessment approaches such as post-mortem morphological analyses for the PT (Gannon et al., 1998), in vivo and post mortem imaging studies using traditional tracing of specific areas of interest for the IFG (Cantalupo & Hopkins, 2001) and for the PT (Cantalupo, Pilcher, & Hopkins, 2003) and voxel-based morphometry for both PT and IFG (Hopkins et al., 2008). For example, using structural MRI techniques to scan 20 brains of chimpanzees, Cantalupo and Hopkins (2001) revealed that Brodmann’s area 44 (i.e., homologous of Broca’s area) was larger in the left hemisphere than in the right one. Interestingly, the analysis of 56 MRI scans of chimpanzee brains revealed an association between the leftward asymmetries in the homologous of Broca area (IFG) and the predominance of right handedness for communicative gestures (Taglialatela, Cantalupo, & Hopkins, 2006) whereas, in a previous studies, Hopkins and Cantalupo (2004) showed that handedness for the non communicative bimanual tube task in chimpanzees was related to neuroanatomical asymmetries in the primary motor cortex but not to any of the homologous language areas. Such neuroanatomical correlates strengthen the hypothesis suggested above by the behavioural data (1) of a possible neural dissociation between gestural communication and manipulative motor functions and (2) that the left-lateralized specific communicative system may constitute a precursor of language areas.

V. Discussion This chapter stressed the major implication of gestural communication in infants in the development of speech as well as some potential continuities of features between human language and gestural communication in nonhuman primates, such as flexibility of learning and use, intentionality, left-hemispheric specialization and referential properties.

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corresponding social and external stimuli or knowledge (Premack, 1972; Seyfarth & Cheney, 2003). In agreement with this distinction drawn between the comprehension of calls (language-like) and their production (non language-like), it turns out that there is a corresponding shift between the neural circuits involved in the perception of vocalizations (language-like) and the neural substrate of vocal production (non language-like) in apes and monkeys. Contrary to human language and the production of gestures in chimpanzees and baboons, vocal control in nonhuman primates seems to imply non-lateralized subcortical structures (limbic or cingulate systems) but not homologous of language areas; this strengthens the argument for the emotional control of call productions (Aitken, 1981; reviewed in Jürgens, 2002; Ploog, 1981). In contrast, functional brain imaging (positron emission tomography, PET) showed that the passive listening of conspecifics’ vocalizations involves cerebral areas within the superior temporal gyrus in rhesus monkeys (e.g., Gil-da-Costa et al., 2006; Petkov et al., 2008; Poremba et al., 2004) and in chimpanzees (Taglialatela, Russell, Schaeffer, & Hopkins, 2009), that might be related to the areas that are involved in the comprehension of language in humans. However, whether processing conspecific vocalizations implies a functional left-hemispheric dominance in nonhuman primates, as for word processing in human language, is still unclear. Whereas research on macaques using cerebral lesions reported a functional dominance of the left hemisphere in the temporal lobe and auditory cortex (Dewson, 1977; Gaffan & Harrison, 1991; Heffner & Heffner, 1984), inconsistent findings in the direction of the functional brain lateralization have been reported in the processing of conspecific versus non-conspecific vocalizations in these brain imaging studies as well as indirect behavioural studies assessing head-orienting asymmetries in response to emission of vocal signals (Gil-da-Costa & Hauser, 2006; Hauser & Andersson, 1994; Petersen, Beecher, Zoloth, Moody, & Stebbins, 1978; Teufel, Hammerschmidt, & Fischer, 2007). Regarding these collective findings in nonhuman primates, we can not exclude that the neural substrate for extracting and categorizing information from vocal signals by listeners may be the precursor of the representational processes involved in the comprehension of language in humans (Gil-da-Costa et al., 2004; Russ, Lee, & Cohen, 2007; Zuberbühler, Cheney, & Seyfarth, 1999). However, these abilities in nonhuman primates might be better related to their remarkable capacities to understand and categorize the external world (Cheney & Seyfarth, 1990a; Seyfarth, Cheney, & Bergman, 2005) – that are at work also in the comprehension of human language – without having anything to do with the features of their specific vocal production system, and could not be thus particularly regarded as a direct precursor of the human speech production system (Meguerditchian & Vauclair, 2008).

pronounced (see Leavens, Racine, & Hopkins, 2009). Given that apes have some understanding of the importance of the visual modality on the part of the recipient for conveying their gestural signals (see Call & Tomasello, 1994; Gomez, 1996; Hostetter, Russell, Freeman, & Hopkins, 2007) and of what others can and cannot see (Hare, Call, Agnetta, & Tomasello, 2000; Hare, Call, & Tomasello, 2001, 2006; Tomasello, Call, & Hare, 2003), we believe that the potential involvement of such high level cognitive processes in apes should not be completely excluded during the production of imperative pointing. Then, these behaviours might constitute a cognitive substrate for the phylogenetic emergence of declarative communication. Thus, such pointing behaviours in apes may be considered as a perfect prerequisite for referential communication that is at work in human language.

2. Comparison of features between vocal and gestural communication in nonhuman primates The properties of the vocal system in nonhuman primates seem less convincing as the best prerequisite for the emergence of speech. In fact, a degree of audience effect (e.g., Mitani & Nishida, 1993; Wich & de Vries, 2006) and some plasticity in the vocal structure between or within social groups have been described and they are related to social, environmental and contextual changes in the group. These features reveal a probable influence of a learning component during the individual’s lifetime as well as some control of the production of vocal signals (e.g., for reviews: Roian-Egnor & Hauser, 2004; Meguerditchian & Vauclair, 2008; see also the chapter by Lemassson). However, in contrast to the gestural modality, despite this flexibility, there is still poor evidence that monkeys and apes are able to generate new vocal signals and that the production of vocalizations might be dissociated from their appropriate social context and emotional state (e.g., Goodall, 1986; Pollick & De Wall, 2007). The vocal features in the repertoire of nonhuman primates seems mostly genetically determined (RoianEgnor & Hauser, 2004) and not under an intentional control. Nevertheless, the investigation on the perception of vocal signals in nonhuman primates revealed interesting continuities with the comprehension of language. Then a contrast might be drawn between the production and the perception of vocalizations within a comparative framework with human language. In effect, it is well documented by playback studies that vocal signals are meaningful and referential for monkey and ape listeners as the recipients are able to extract information from calls such as the category of predators and of food, the identity of the caller, the matrilineal kinship, the nature of the social relationships among conspecifics, and their respective dominance rank (e.g., Cheney, & Seyfarth, 1980, 1990a, 1990b, 1999; Hauser, 1991; Seyfarth, Cheney, & Marler, 1980; Slocombe & Zuberbühler, 2005; Zuberbühler, 2000, 2001, 2003; see also the chapter by Zuberbühler, Arnold and Slocombe). Such representational abilities in listeners may result from progressive associations, at an individual level during its lifetime, between the vocal signals emitted in the social group and their

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system, that differs from the system controlling the purely motor functions of object manipulations, may have further turned bimodal with the progressive insertion of intentional vocalizations and oro-facial expressions into the gestural system in the course of evolution to finally become, as it is currently, under the dominance of the vocal modality (speech) in humans (see Corballis, 2003). In this view manual gestures during speech production in humans might constitute the residual part of this bimodal communicatory system for language (McNeill, 1992). This theory of an integrated communication system (vocal + gestural) is consistent with the results of observations and experiments on gestural communication in adults (e.g., Gentilucci, & Dalla Volta, 2008), in infants and children (e.g., Bernardis, Bello, Pettenati, Stefanini, & Gentilucci, 2008 and see parts I & II) which argue strongly for the view that that a single integrated communication system in the left cerebral hemisphere might be in charge of both vocal and gestural communication and that communicative gestures play an active role in the development of children’s communicative skills. Within the theoretical framework of continuity between humans, chimpanzees and baboons concerning hemispheric specialization for language, neither neuroanatomical nor neurofunctional data related to gestural communication are available so far in monkeys such as baboons. However, neurobiological studies in monkeys provide some elements to the discussion about the tight connexion between the mouth, the hand and Broca’s area. For example, electrical stimulation of Brodmann’s area 44 in rhesus monkeys induced hand and lip movements, suggesting the existence of a neural connexion between the manual and the oro-facial motor system in relation to Broca’s area (Petrides, Cadoret, & Mackey, 2005). Studies of macaque monkeys have also demonstrated the existence of mirror-neurons in area F5 of the brain, i.e., the homologous to Broca’s area (see the chapter by Ferrari and Fogassi). These neurons are activated not only when the monkey is performing a manual action, e.g. cracking nuts, but also during the observation of these actions (Gallese, Fadiga, Fogassi, & Rizzolati, 1996), their passive listening (Kohler et al., 2002) as well as the observations of the use of tools (Ferrari, Rozzi, & Fogazzi, 2005) and of communicative facial actions (“lip-smacking” and lip protrusion) carried out by the experimenter standing in front of the monkey (Ferrari, Gallese, Rizzolatti, & Fogassi, 2003). It thus seems that, in the monkey brain, area F5 is predisposed to control and recognize visuo-gestural manual actions as well as oro-facial communication, supporting the hypothesis that these neurons might constitute an ideal substrate for the emergence of imitation, theory of mind and language as well (e.g., Arbib, 2005; Rizzolati & Arbib, 1998). These combined findings in monkeys and chimpanzees question notably the role of the oro-facial system during the evolution of language compared to the manual and vocal systems. Given that speech implies complex sequential oral gestures, we suggest that the oro-facial system might constitute a relevant mediator between the gestural communicatory system and speech. On the assumption that the basic structure of syllables derives from the succession of constrictions and mouth openings involved in

3. A bimodal system rather a gestural system? Regarding the limitations of the vocal production system and the continuities discussed in this chapter between the gestural modality in our primate cousins and the major features of language, we believe that gestural communication constitutes a better candidate than vocalizations for investigating the precursors of language. However, recent exceptional reports may provide some new clues to the evolution of the vocal system as well as an evolutionary scenario of the emergence of language. Hopkins, Taglialatela and Leavens (2007) described two atypical “learned” sounds produced by several chimpanzees among the captive groups from the Yerkes Primate Research Center (see also the chapter of Hopkins, Taglialatela and Leavens): an “extended grunt” involving the vocal tract (i.e., vocal system) and a splutter called ‘‘raspberry” involving only the lips with the air of the mouth (i.e., oro-facial system). The authors showed that, contrary to the rest of the species-typical vocal repertoire, the production of these sounds is under voluntary control and is used exclusively in the presence of both a human and an out-of-reach food in order to attract the human’s attention (see Hopkins, Taglialatela, & Leavens, 2007). Interestingly, it turns out that these signals not only share the same communicative intent as the ‘‘food beg” gestures in captive chimpanzees but also, when produced simultaneously with these gestures, induce a stronger right-hand preference than when the gesture is produced alone (Hopkins & Cantero, 2003), indicating that the left hemisphere may be more activated when producing both gestures and these atypical vocal and lip sounds simultaneously. Moreover, in contrast to the left-sided oro-facial asymmetries (i.e., right-hemispheric dominance), detected for the species-typical vocal repertoire of chimpanzees, the use of the atypical attention-getting sounds involved an asymmetry toward the right-side of the mouth, i.e. left-hemispheric dominance (Reynolds Losin, Russell, Freeman, Meguerditchian, & Hopkins, 2008). Thus, we might support the view that the specific left-lateralized communicative system suggested above in baboons and chimpanzees by the reports of specific patterns of right-handedness for gestures, may be involved for both gestures and ‘‘learned” attention-getting sounds in chimpanzees but only for gestural communication in baboons (i.e., neither the intentional use of vocalizations nor any association between vocal and gestural signalling for transmitting the same intents have been observed in this species). The existence of such a bimodal intentional communicative system in chimpanzees has been recently illustrated in a brain imaging study (PET) conducted in 3 captive individuals. Impressively, communicative signalling for begging food from a human by using either gestures, atypical attention-getting sounds, or both of them simultaneously, activated a homologous region of Broca’s area (IFG) predominantly in the left hemisphere (Taglialatela, Russell, Schaeffer, & Hopkins, 2008). In other words, precursors of a left-hemispheric cerebral substrate for language production might have emerged first with the use of communicative gestures in the common ancestor of humans, chimpanzees and baboons. Then, this communicative

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From gesture to language Arbib, M. (2005). From monkey-like action recognition to human language: an evolutionary framework for neurolinguistics. Behavioral and Brain Sciences, 28, 105–167. Arbib, M., Liebal K., & Pika, S. (2008). Primate vocalization, gesture, and the evolution of human language. Current Anthropology, 49, 1503–1076. Bard, K. A. (1992). Intentional behavior and intentional communication in young free-ranging orangutans. Child Development, 62, 1186–97. Baron-Cohen, S. (1999). The evolution of a theory of mind. In M. C. Corballis & S. E. G. Lea (Eds.), The descent of mind: Psychological perspectives on hominid evolution (pp. 261–277). Oxford: Oxford University Press. Bates, E., Camaioni, L., & Volterra, V. (1975). The acquisition of performatives prior to speech. Merrill-Palmer Quarterly, 21, 205–226. Bates, E., O’Connell, B., Vaid, J., Sledge, P., & Oakes, L. (1986). Language and hand preference in early development. Developmental Neuropsychology, 2, 1–15. Bellugi, U. (1991). The link between hand and brain: Implications from a visual language. In D. Martin (Ed.), Advances in cognition, education and deafness (pp. 11–35). Washington, DC: Gallaudet University Press. Bernardis, P., Bello, A., Pettenati, P., Stefanini, S., & Gentilucci, M. (2008). Manual actions affect vocalizations of infants. Experimental Brain Research, 184, 599–603. Bernardis, P., & Gentilucci, M. (2006). Speech and gesture share the same communication system. Neuropsychologia, 44, 178–190. Blake, J., O’Rourke, P., & Borzellino, G. (1994). Form and function in the development of pointing and reaching gestures. Infant Behavior and Development, 17, 195–203. Bonvillian, J.D., Richards, H.C., & Dooley, T.T. (1997). Early sign language acquisition and the development of hand preference in young children. Brain & Language, 58, 1–22. Broca, P. (1865). Sur le siège de la faculté du langage articulé. Bulletin de la Société d’Anthropologie de Paris, 6, 377–393. Brooks, R., & Meltzoff, A. N. (2008). Infant gaze following and pointing predict accelerated vocabulary growth through two years of age: A longitudinal, growth curve modeling study. Journal of Child Language, 35, 207–220. Butterworth, G. (2003). Pointing is the royal road to language for babies. In S. Kita (Ed.), Pointing: Where language, culture, and cognition meet (pp. 9–34). Mahwah, NJ: Lawrence Erlbaum Associates. Butterworth, G., & Morissette, P. (1996). Onset of pointing and the acquisition of language in infancy. Journal of Reproductive and Infant Psychology, 14, 219–231. Call, J., & Tomasello, M. (1994). Production and comprehension of referential pointing by orangutans (Pongo pygmaeus). Journal of Comparative Psychology, 108, 307–317. Camaioni, L. (1997). The emergence of intentional communication in ontogeny, phylogeny and pathology. European Psychologist, 2, 216–225. Camaioni, L., Perucchini, P., Bellagamba, F., & Colonnesi, C. (2004). The role of declarative pointing in developing a theory of mind. Infancy, 5, 291–308. Cantalupo, C., & Hopkins, W. D. (2001). Asymmetrical Broca’s area in great apes. Nature, 414, 505. Cantalupo, C., Pilcher, D., & Hopkins, W. D. (2003). Are planum temporale and sylvian fissure asymmetries directly related? A MRI study in great apes. Neuropsychologia, 41, 1975–1981. Cartmill, E. A., &, Byrne, R. W. (2007). Orangutans modify their gestural signalling according to their audience’s comprehension. Current Biology, 17, 1345–1348. Cheney, D. L., & Seyfarth, R. M. (1980). Vocal recognition in free-ranging vervet monkeys. Animal Behaviour, 28, 362–367.

chewing, sucking, swallowing and visuo-facial communicative cyclicities, such as lipsmacks, MacNeilage (1998) proposed the “frame-content” theory of speech. According to this theory, the basic components of speech – an oscillatory one (frame) and a segmental one (content) – have their source in cyclic activities of ingestion in our ancestors. Thus, it might be hypothesised that ingestive behaviours were progressively ritualized in oro-facial (lipsmacking) and gestural communication in monkeys (Arbib, 2005). Gentilucci and Corballis (2006) have speculated that facial elements were gradually introduced with vocal elements into the gestural system during language evolution.

Conclusion A comparative perspective between apes and humans is likely to provide some answers concerning the evolutionary origins of language. The studies reviewed in this chapter reveal the existence of a dynamic interplay between speech and gestures in human children, from the early stages of development. Infants’ gestural communication provides an ontogenetic foundation for verbal communicative behaviours. In nonhuman primates, features of gestural communication as well as the comparison between their vocal and gestural communication, support the hypothesis that the first phylogenetic precursors of human language were communicative gestures. Nevertheless, in the common ancestor of human and chimpanzee, this gestural system would progressively evolve into a bimodal system in including the first use of intentional vocalizations. Studies investigating asymmetries of communicative behaviours reinforce this hypothesis, and the different patterns of manual preference between communicative gestures and manipulative actions observed in both human and non human primates can serve to emphasize the existence of a bimodal communicative system in the left cerebral hemisphere. We have also shown that this view is strengthened in the light of some new findings from the neuroimaging literature, providing very convincing arguments in favour of a gestural hypothesis of language origin.

Acknowledgments The research reported in this chapter is supported by a French National Research Agency (ANR) grant reference ANR-08-BLAN-0011_01.

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