Interaction Studies - Volume 5, Issue 3, 2004

In this study of the functionally referential alarm calls in the meerkats (Suricata suricatta), we tested the hypothesis that the ability to refer to a specific location was an important factor in the evolution of discrete vocalizations. We investigated what information receivers gained about the location of the predator from alarm calls with high stimulus specificity compared to alarm calls with low stimulus specificity. Furthermore, we studied whether visual cues about the localization of the predator may be available from the posture of the caller. We described the general behaviour of the caller, the caller’s posture, and in particular its gaze direction. We then observed receivers responding to the different call types, to determine whether the acoustic structure of the calls was enough for them to respond in the appropriate way, or whether they used additional visual cues from the caller. We tested this with specific manipulation experiments, using three set ups of playback experiments: (1) no caller visible; (2) model guard with specific gaze direction; and (3) live sentinel. Natural observations and experiments confirmed that in high urgency situations the meerkats have enough information from the acoustic structure of the call to respond appropriately. When hearing low urgency calls that are less stimuli specific, meerkats used visual cues as an additional source of information in a few cases. This may indicate that functionally referential calls evolved to denote the location of the predator, rather than the predator type or its velocity of approach. However, when discussing this result in comparison to other functionally referential calls, such as the food associated calls and recruitment calls, this localization hypothesis does not appear to apply to the functionally referential calls in general.

Different theories have been proposed for explaining the evolution of language. One of this maintains that gestural communication has been the precursor of human speech. Here we present a series of neurophysiological evidences that support this hypothesis. Communication by gestures, defined as the capacity to emit and recognize meaningful actions, may have originated in the monkey motor cortex from a neural system whose basic function was action understanding. This system is made by neurons of monkey’s area F5, named mirror neurons, activated by both execution and observation of goal-related actions. Recently, two new categories of mirror neurons have been described. Neurons of one category respond to the sound of an action, neurons of the other category respond to the observation of mouth ingestive and communicative actions. The properties of these neurons indicate that monkey’s area F5 possesses the basic neural mechanisms for associating gestures and meaningful sounds as a pre-adaptation for the later emergence of articulated speech. The homology and the functional similarities between monkey area F5 and Broca’s area support this evolutionary scenario.

This article argues for the gestural origins of speech and language based on the available evidence gathered in humans and nonhuman primates and especially from ape studies. The strong link between motor functions (hand use and manual gestures) and speech in humans is reviewed. The presence of asymmetrical cerebral organization in nonhuman primates along with functional asymmetries in the perception and production of vocalizations and in intentional referential gestural communication is then emphasized. The nature of primate communicatory systems is presented, and the similarities and differences between these systems and human speech are discussed. It is argued that recent findings concerning neuroanatomical asymmetries in the chimpanzee brain and the existence of both mirror neurons and lateralized use of hands and vocalizations in communication necessitate a reconsideration of the phylogenic emergence of the cerebral and behavioral prerequisites for human speech.

Pointing by apes is near-ubiquitous in captivity, yet rare in their natural habitats. This has implications for understanding both the ontogeny and heritability of pointing, conceived as a behavioral phenotype. The data suggest that the cognitive capacity for manual deixis was possessed by the last common ancestor of humans and the great apes. In this review, nonverbal reference is distinguished from symbolic reference. An operational definition of intentional communication is delineated, citing published or forthcoming examples for each of the defining criteria from studies of manual gestures in apes. Claims that chimpanzees do not point amongst themselves or do not gesture declaratively are refuted with published examples. Links between pointing and cognitive milestones in other domains relating means to ends are discussed. Finally, an evolutionary scenario of pointing as an adaptation to changes in hominid development is briefly sketched.

Potential speech abilities constitute a key component in the description of the Neandertals and their relations with modern Homo Sapiens. Since Lieberman & Crelin postulated in 1971 the theory that “Neanderthal man did not have the anatomical prerequisites for producing the full range of human speech” their speech capability has been a subject of hot debate for over 30 years, and remains a controversial question. In this study, we first question the methodology adopted by Lieberman and Crelin, and we point out articulatory and acoustic flaws in the data and the modeling. Then we propose a general articulatory-acoustic framework for testing the acoustic consequences of the trade-off between oral and pharyngeal cavities. Specifically, following Honda & Tiede (1998), we characterize this trade-off by a Laryngeal Height Index (LHI) corresponding to the length ratio of the pharyngeal cavity to the oral cavity. Using an anthropomorphic articulatory model controlled by lips, jaw, tongue and larynx parameters, we can generate the Maximal Vowel Space (MVS), which is a triangle in the F1 / F2 plane, the three point vowels /a/, /i/, and /u/ being located at its three extremities. We sample the evolution of the position of the larynx from birth to adulthood with four different LHI values, and we show that the associated MVS are very similar. Therefore, the MVS of a given vocal tract does not depend on the LHI: gestures of the tongue body, lips and jaw allow compensations for differences in the ratio between the dimensions of the oral cavity and pharynx. We then infer that the vowel space of Neandertals (with high or low larynx) was potentially no smaller than that of a modern human and that Neandertals could produce all the vowels of the world’s languages. Neandertals were no more vocally handicapped than children at birth are. Therefore, there is no reason to believe that the lowering of the larynx and a concomitant increase in pharynx size are necessary evolutionary pre-adaptations for speech. However, since our study is strictly limited to the morphological and acoustic aspects of the vocal tract, we cannot offer any definitive answer to the question of whether Neandertals could produce human speech or not.