Language Evolution and Human Development

systematization. To the degree that gestures could be made up “on the fly” there would be little evolutionary advantage supporting systematiziation. Darwin (1877) thought it unlikely that a system such as language could have emerged directly from gesture. He believed that this would require the shifting of a function from one organ to another and then back again, as if flying had moved from the wing to the stomach and then back to the wing. However there is no reason to think that Darwin The Gradual Emergence of Language 10 would have excluded the possibility that gesture and vocalization underwent coevolution. Today, speech and gesture complement each other during communication (McNeill, 1985). Some messages are conveyed through speech, but others are conveyed through the hands and the posture. Still other messages are conveyed by changes in the tone of voice. It is likely that each of these channels of communication were also available to Homo erectus and that they functioned in an interactive and complementary fashion, much as they do now. Advantages of Mimesis Donald (1991) has emphasized the central role of what he calls “mimesis” in the communicative world of Homo erectus. Mimesis involves communication about actions, plans, feelings, and objects through iconic depiction of parts of those actions, states, and objects (see also Nelson, this volume). The depiction may conjure up an image of an object by gesturing its shape or imitating its sound. Actions can be depicted by repetition of their central components. In general, mimesis achieves reference through partonymy, or mention of a part to express the whole. Mimesis can be expressed through signing, chant, song, drama, ritual, and basic forms of costume. To be maximally effective, these systems must occur within fixed social contexts that guarantee attention and some traditional form. As conventionalization advances, mimetic systems can support group solidarity, planning, and socialization in an increasingly structured way. Through conventionalized chant and dance, the male society can plan hunts and battles and reenact past struggles. At the same time, mothers can use song, sound play, and chant to hold the attention of their young children while socializing them into the practices of the community. Mimetic processes are linked to the emotional use of language that was elaborated during the previous evolutionary period. As a result, mimetic communications can be used to move people emotionally, preparing them for war, migration, or other major group activities. It appears that mimesis was enormously successful in providing social cohesion and shared planning. As a result, Homo erectus was able to expand its territory to all of Africa, eventually leading to the extinction of all other hominid species. It also allowed Homo erectus to migrate successfully out of Africa to all parts of Asia, including the Middle East, China, and Indonesia. We should attribute this successful expansion to two basic processes. The first was an ability to eliminate competitors, particularly in Africa. To do this, erectus must have relied on group solidarity as a support for warfare. Second, to support the migration to new territories, erectus must have been extremely adaptive. This adaptivity could not have been grounded in some simple physical change. Instead, it must have resulted from a general improvement in cognitive capacity, particularly as reflected in group problem solving and adaptation. MacWhinney (1999a) and Tomasello (1999) have developed parallel accounts of these changes that emphasize the importance of being able to take the perspective of another human being as an intentional agent. In both accounts, this ability is a precondition to the full successful use of language. Tomasello locates the emergence of this ability in the period after 200,000 years ago. However, like Donald (1991), MacWhinney sees the ability arising as early as 2MYA with ongoing developments during more recent periods. The account developed here emphasizes the initial role of The Gradual Emergence of Language 11 social cohesion as an evolutionary support for communication. The movement to an upright position allowed us to engage more directly with our conspecifics both gesturally and vocally. The rise of cortical control over vocalization led to further improvements. However, to take full advantage of face-to-face communication, we needed to expand our image of the other. Supports for this include neural control of imitation (Decety, Chaminade, Grezes, & Meltzoff, 2002), face recognition (Moskovitch, Winocur, & Behrmann, 1997), and the construction of a full projectable body image (Ramachandran & Hubbard, 2001). These new developments could rely on some parallel evolutions that had been occurring outside of the social realm. Because the movement to an upright posture freed the hands for increased use of tools, hominids derived additional evolutionary advantage from the formation of mental models of plans for tool usage and action schemas. These schemas relied on the development of a mental model for the self’s activities. This same mental model could them eventually be projected onto the other, allowing hominids to develop increasingly sophisticated ideas about the future actions of others. However, without further support from language, this level of theory of mind construction could only go so far. Neuronal Adaptations The brain Homo erectus tripled in size during the period between 2MYA and 100,000 years ago. Some of this increase is allometrically related to the overall growth in stature during this period. Thus, it might be better to speak of a doubling in size, rather than a tripling in size. The brain expanded in size because the preconditions for a successful expansion were now all in place. In particular, previous evolution had already produced at least these five abilities: 1. The shift to bipedalism had freed the hands for gestural communication. 2. The freeing of the hands for tool use had led to further elaboration of primate mechanisms, such as “mirror” neurons, for representation of the actions of others. 3. During several millennia of migration and population movements across open ground and through the bush, our ancestors had developed systems for tracking their own spatial positions in the present, past, and future, as well as the spatial positions and perspectives of others. 4. Our ancestors could use their visual system to generate images of past actions and spatial configurations involving themselves and others. 5. During the period before 2MYA, our ancestors had developed a tight linkage of attention to vocalization processes during face-to-face interaction and imitation. These developments provide preconditions for the evolution of mimesis. To further support mimesis, the brain needed to provide complete episodic storage for combinations of gesture and vocalization. It had to store whole chants or gestures as they expressed particular events of importance to individuals and the group. Some of these chants and gestures might be rather extended. For example, there might be a dance that represented the time of ripening of the mongongo nut or one that outlined the components of the hunt for the eland. At the same time, these larger mimetic sequences would be composed of smaller pieces that had achieved other cognitive-social grounding. For example, by The Gradual Emergence of Language 12 pointing to a place in the area around the group’s encampment, the dancer could signal the direction of a hunt. By holding his arm in a particular stance, the dancer could mime the attack on the game. These component gestures, movements of the body, and vocal chants could then be reused with other components in a relatively unsystematic fashion. Unlike the evolutionary pressures of earlier periods, the storage and retrieval of conventionalized mimetic sequences cannot be achieved simply by linking up older areas or by reusing earlier connections. Instead, the brain must add new computational space to store the multitude of new visual and auditory images. In addition, the brain needs to expand the role of the frontal areas for storing and switching between perspectives. Because this system grew up in a haphazard way from earlier pieces of lip smacking, pointing, gesture, and rhythm, it would be difficult to extract a core set of elements from mimetic communications. Instead, patterns and forms must be learned and stored as holistic unanalyzed sequences. This Gestalt-like shape of early mimetic patterns corresponds well with the Gestalt-like cognitions that we develop through our interactions with objects (Gibson, 1977). For example, when we chop wood, there is a complete interpenetration of muscle actions, visual experiences, hand positions, and sounds. We can think of this as a single merged form such as I-hands-back-lift-axe-dropsplit-chips-wood-cut. Mimetic forms have this same unanalyzed quality. This lack of analysis is not the result of chunking or automatization, since the Gestalt is not constructed by a system of combinatorial semantics. Instead, each chunk is a raw, unanalyzed whole that is fully grounded on direct action and perception. Because they are highly grounded on our direct perceptions and actions, they communicate in a basic way. However, they provide little support for cognitive organization. The growth of the brain in response to these pressures was so rapid that it is typically assumed that it involved a single genetic mechanism. One such mechanism might involve regulatory genes (Allman, 1999) that control the development of structures in the fetus. Changes in the timing of the expression of these genes can lead to the observed acrossthe-board increase in size for the cortex and cerebellum that we see in Homo erectus. However, the expansion of the cortex placed additional adaptive pressures on Homo erectus. The bigger brain required a much greater level of caloric intake. This pressure could be met through changes in diet and modifications to the digestive system. A more fundamental pressure was the fact that increases in the size of the infant brain produce problems for the birth process. The width of the hips had narrowed in both men and women as a response to bipedalism. As long as the skull was not much larger than that found in the great apes, this did not cause major problems for the birth process. However, the expansion of the skull in Homo erectus ran directly into this evolutionary barrier. To deal with this, the infant is born at a time when it is still fairly immature and the skull is relatively pliable. The increasingly organized shape of the society facilitates the survival of the child. In addition, women have had to sacrifice their ability to run quickly so that the hips could widen, permitting births with larger infant heads. The slowing of infant development not only helps in the birth process, but also helps the child maintain cortical plasticity (Elman, Bates, Plunkett, Johnson, & Karmiloff-Smith, 1996; Julesz & Kovacs, 1995) even into adolescence, thereby further enhancing the ability of the group to construct accepted mimetic patterns. The Gradual Emergence of Language