Invited commentary on: Arbib, Michael; Liebal, Katja & Pika, Simone 'Primate Vocalization, Gesture, and the Evolution of Human Language'

Arbib, Liebal, and Pika have produced an ambitious and timely elaboration of the mirror system hypothesis of language origins. In their review, apes stand out among primates in their capacities for intentional communication with manual gestures. To the degree that the last common ancestor (LCA) of (the other) apes and humans displayed flexibility in manual signaling, over and above that seen in facial expressions and vocalizations, is the degree to which manual signaling gains allure as a prelinguistic substrate for the later complex story of language evolution. Their review concludes that great apes have moderately flexible, typically dyadic gestural communicative habits, whereas modern humans have extraordinary flexibility in triadic signaling in numerous domains. Here, I will elaborate on the significance of pointing and symbol acquisition by apes for reconstructing the triadic competencies of the LCA. Arbib and colleagues frequently invoke the notion of flexibility in signaling. But what does "flexibility" mean in this context? In one sense, it means that an individual can use a number of different communicative tactics toward attainment of a goal and can use the same signal to different ends (Tomasello and Call 1997, 243). An example of this kind of flexibility is that chimpanzees communicate in different sensory modalities depending on whether an observer can see them (e.g., Hostetter, Cantero, and Hopkins 2001; Leavens et al. 2004). In this sense, signalers are seen as tacticians. In a second sense, flexibility is a characteristic of dyads. In ontogenetic ritualization, pairs of social partners develop their own pair-specific patterns of nonverbal communication (Tomasello and Call 1997, 301). An example of this kind of dyadic flexibility occurs when infants ritualize their signals for being picked up: no longer straining with their whole bodies, bouncing rhythmically on their substrates, reaching with both arms fully extended, raising their arms while looking at the caregiver. Examples from adulthood include dancing and love-making in these and other contexts, couples develop dyad-specific repertoires of nonverbal signals. Here, signalers are seen as dancers. In a third sense, flexibility means that signals are acquired during development and shaped through common social experiences of signal consequences (Tomasello and Call 1997, 243). Whether humans typically point with their lips or fingers, for example, is a function not so much of individual tactics or dyadic accommodations but of cultural traditions (Wilkins 2003). This flexibility is manifested in patterns of within-cultural similarities as contrasted with systematic cross-cultural differences. Here, signalers are seen as being plastic in an ontogenetic sense