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心 理 学 报 2007 ， 39 （ 3 ）：415~ Acta Psychologica Sinica
415
How Language Evolved
Michael C. Corballis
University of Auckland
Language, with its complex recursive structure, is almost certainly a uniquely human capacity. I argue that it evolved over the past 2 million years during the Pleistocene epoch, as part of a cognitive adaptation to deforestation and predation from dangerous killer animals on the African savanna. Rather than postulate any specific genes for language, I suggest that there was systematic selection for increase in brain size, allowing for more complex social cognition, including the “grammaticalization” of communication through learning and cultural pressures. Parallel to this development, the medium of communication changed gradually from a manual mode to a facial and then vocal mode, culmination in a mutation of the FOXP2 gene that gave our own species, Homo sapiens , the capacity for autonomous speech. This final switch may explain the so-called “human revolution,” leading to the dominance of humans on the planet, and the demise of other species of the genus Homo. Keywords: language, grammaticalization, the medium of communication.
的媒介依次变为手语模式、表情模式与语言模式。交流媒介的逐渐变化最终导致了 FOXP2 基因突变，
FOXP2 基因突变让智人具有了自主的言语能力。与地球上其它的人科动物相比，人类的语言能力使人类
分类号：B84-
Introduction The Cartesian background The evolution of language has long been contentious. The 17 th-century philosopher Réné Descartes (1647/1985) set the stage for much of the controversy when he contrasted humans with animals, suggesting that language was one attribute unique to humans. The distinctive characteristic of language was its open-ended quality, since there seemed to be no limits to what humans, even human “imbeciles,” could express. This freedom of expression, apparently denied to all other species, seemed to be inexplicable in terms of any mechanical principles, leading Descartes to declare that it must have been bestowed by God. Any such notions, though, were challenged by Darwin’s momentous book, On the Origin of Species , published in 1859. Although Darwin did not at first deal with the question of human evolution, or even with the evolution of language, his message was
Received 2006-06- Correspondence should be addressed to Michael C. Corballis, Department of Psychology, University of Auckland, Private Bag 92019, Auckland, New Zealand; e-mail: [email protected].
quickly made clear by Huxley (1863/2001): Humans had evolved through natural selection from the African apes. The Oxford philologist Friedrich Max Muller immediately took up the Cartesian challenge, declaring that language was critical proof of the gap between humans and “brutes” (Muller, 1861/1880). Darwin replied by suggesting that language emerged from the inarticulate cries of animals, which Muller in turn scornfully derided. In view of such vituperative exchanges, it is perhaps not surprising that in 1866 the Linguistic Society of Paris banned all discussion of the evolution of language. The London Philological Society followed suit in 1872. Curiously, that ban seems to have persisted until late in the 20th^ century, and the approach to language has been for the most part Cartesian rather than Darwinian. This was reinforced by the dominant linguist of the second half of the 20th^ century, Noam Chomsky. An avowed Cartesian, Chomsky has argued that human language is unrelated to any form of animal communication: “Modern studies of animal communication,” he once wrote, so far offer no counterevidence to the Cartesian assumption that human language is based on an entirely different
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principle” (Chomsky, 1966, p. 78). If this is so, one may question whether an evolutionary account is even possible. Not surprisingly, then, Chomsky (1975) suggested that language could not be explained in terms of natural selection, but may have arisen simply as a consequence of possessing an enlarged brain, without the assistance of natural selection: We know very little about what happens when 1010 neurons are crammed into something the size of a basketball, with further conditions imposed by the specific manner in which this system developed over time. It would be a serious error to suppose that all properties, or the interesting structures that evolved, can be ‘explained’ in terms of natural selection. Yet it is surely vacuous to appeal simply to a large brain, especially when the structures of language seem to imply intricate and dedicated programming. Some authors have proposed instead that language must have emerged as a result of some mutation, an idea sometimes referred to as the “big bang” theory of language evolution. Pinker (1994, p. 297) wrote of “the grammar gene,” with the implication that it was uniquely human. Bickerton (1995) asserted that “… true language, via the emergence of syntax, was a catastrophic event, occurring within the first few generations of Homo sapiens sapiens (p. 69).” Even more radically, Crow (2002) has proposed that a genetic mutation gave rise to the speciation of Homo sapiens , along with such uniquely human attributes as language, cerebral asymmetry, theory of mind, and a vulnerability to psychosis. Again, though, appeal to a single fortuitous event, such as a genetic mutation, smacks of the miraculous, and is little more enlightening than Descartes’ appeal to God. One of the arguments against an evolutionary account of language is that it is too complex to have evolved through natural selection in the period of some 6 million years since the hominid lineage split from that leading to the present-day chimpanzee and bonobo, and that human language is in any case too powerful to have been the product of natural selection. Premack (1985) put it like this: Human language is an embarrassment for evolutionary theory because it is vastly more powerful than one can account for in terms of selective fitness. A semantic language with simple mapping rules, of a kind one might suppose that the chimpanzee would have, appears to confer all the advantages one normally associates with discussion of mastodon hunting or the like. For discussions of that kind, syntactic classes, structure-dependent rules, recursion and the rest, are overly powerful devices, absurdly so (p. 282). This is the so-called “argument from incredulity.” It was also used in the 19th^ century to argue that the
eye could not have evolved from natural selection, but was soon dismissed (see, e.g., Chapter 5 of Dawkins, 1995). Indeed such arguments have never been persuasive to evolutionary biologists, since the gradual accumulation of small changes can lead to considerable complexity.
Evolutionary revival With respect to language, the argument from incredulity was challenged by Pinker and Bloom (1990). Contrary to Chomsky, they argue that, like the eye, human language evolved incrementally, in what they call a “cognitive arms race.” In this view, cognition is conceived primarily as a “social tool,” shaped by the complexities of social relationships, and involving such capacities as language and theory of mind (TOM) (e.g., Alexander, 1979; Flinn, Geary & Ward, 2005; Geary, 2005). Much of this arms race has to do with the competing principles of cooperation and the necessity to detect and remove freeloaders, who capitalize on the sacrifices made by others. This leads to ever more sophisticated means of cheating, and of detecting those who cheat. Language clearly plays a critical role, as anyone buying (or selling) a used car knows full well. Pinker and Bloom write: The ability to frame an offer so that it appears to present maximum benefit and minimum cost to the buyer, and the ability to see through such attempts and to formulate persuasive counterproposals, would have been a skill of inestimable value in primitive negotiations, as it is today (p. 725). Not all subsequent theorists have accepted Pinker and Bloom’s analysis, but their seminal article led to a revival, over the past dozen years or so, of speculation as to how and when language might have evolved. The first of a biennial series of conferences on the evolution of language was established in Edinburgh in 1996, and it was perhaps fitting that the third such conference, in 2000, was held in Paris, site of the original ban. One of the critical questions is whether language is indeed a uniquely human capacity, as held by Chomsky, Bickerton, and others in the Cartesian tradition.
What is unique about language? Although arguing that language evolved incrementally through natural selection, Pinker and Bloom were otherwise still fundamentally Chomskyan in their approach, accepting that syntax, perhaps the most distinctive quality of human language, is indeed uniquely human. Nevertheless there has been an erosion of the Chomskyan belief that human language is based on “an entirely different principle” from animal communication. Even Chomsky seems to have given ground; he was
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together, this capacity still falls far short of human syntax. There are no function words, no tenses, no recursion, and nothing resembling human conversation. The emergence of language as we humans know it must therefore have evolved at some time since the split, some 6 or 7 million years ago, between the hominid lineage and the lineage that led to present-day chimpanzees and bonobos.
Language, Cognition, and the Pleistocene One characteristic that distinguishes the hominids from the apes is that the apes are quadrupedal knuckle-walkers, whereas the hominids evolved a bipedal stance and method of locomotion. On the basis of overall brain size, there is little evidence that the early hominids differed substantially, at least in cognitive terms, from their ape predecessors, or indeed from present-day apes. The epoch that most clearly shows the transition from ape-like to human- like behavior is the Pleistocene, some 4 million years after the split between the hominid line and that leading to present-day chimpanzees and bonobos. The Pleistocene is formally dated from 1.81 million years ago to 11,500 years ago, though some have argued that it should be dated from as early as 2. million years ago (e.g., Suc, Bertini, Leroy, & Suballyova, 1997). It was characterized by a series of ice ages that reduced the forested areas in which the earlier primates, and probably the earlier hominids, had found safety and adequate sources of food. The forested terrain was replaced by more open savanna, and the hominids were forced into a hunter-gatherer mode of existence, initially scavenging for food but also gradually developing hunting techniques for the slaughter of animals. The Pleistocene also corresponds at least roughly with the emergence of the genus Homo. Evolutionary psychologists treat the Pleistocene as the cradle of humankind, relating present-day cognition to selective pressures arising from hunter-gatherer modes of subsistence (e.g., Barkow, Cosmides, & Tooby, 1992; Pinker, 1997). These pressures were climatic, ecological and social, and it was perhaps the ecological pressures that were at first instrumental in selecting for social skills, and then competition among conspecific that honed them further in the “cognitive arms race” referred to earlier (Flinn et al., 2005). Baumeister (2005) has also usefully distinguished between the social and the cultural, suggesting that it was culture that shaped human evolution. Other primates are social, with interrelationships, dominance patterns, and the like, but culture creates communities of like minds, with networks for the maintenance, transmission, and accumulation of information. Such networks imply at least some level of language. Tomasello et al. (2005) argue somewhat similarly that the critical element in
human evolution may have been not simply interpersonal understanding, demonstrable to a limited degree in other primates, but the further capacity to share intentions. The instinctive sharing of goals and ideals has no doubt been critical to the survival of our species in the face of hardship and predation, starting with the African savanna, but continuing in other regions as some members of early Homo migrated out of Africa into Europe and Asia. More generally, hominids adapted to their new conditions by inhabiting what has been called a “cognitive niche,” which Tooby and DeVore (1987) define as “conceptually abstracting from a situation a model of what manipulations are necessary to achieve proximate goals that correlate with fitness” (p. 209). For example, hunter-gatherers do not simply kill their prey, as other animals do, but precede each hunt with a plan based on earlier experiences and communication among band members, and follow each hunt with campfire debriefings (Lee, 1979).
Tool manufacture One characteristic of the genus Homo is the manufacture and use of tools. The earliest stone tool industry, known as the Oldowan industry, has been dated from around 2.5 million years ago, and associated with the earliest member of our genus, Homo rudolfensis (Semaw et al., 1997). This industry, known as the Oldowan, was primitive compared with the later Acheulian tool industry associated with the larger-brained Homo erectus around 1.8 million years ago (Foley & Lahr, 1997). Although there was something of a rise in manufacturing sophistication from around 300,000 years ago (Ambrose, 2001), the Acheulian industry remained fairly static for over a million years, and even persisted into the culture of early Homo sapiens some 125,000 years ago (Walter et al., 2000). Indeed, the most significant advances did not occur until within the past 100,000 years in what is known as the “human revolution” (Mellars & Stringer, 1989), which is discussed in more detail in a later section. While the manufacture of stone tools clearly marks a cognitive advance, the static nature of tool development throughout most of the Pleistocene suggests that tools do not really tell us much about how the human mind evolved through that period. It is of course possible that there were more sophisticated tools made from perishable materials. Indeed other primates use sticks and stones as tools, and capuchin monkeys in particular are especially well known for tool use, and occasionally modify twigs for more efficient use as diggers (Moura & Lee, 2004), and it would be surprising if the early hominids who preceded the genus Homo did not also make use of sticks and stones as tools. Even crows
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have a capacity to fashion tools out of leaves and twigs to forage for grubs in holes (Hunt, 1996; Hunt, Corballis, & Gray, 2001). Consequently, recent accounts of how cognition and language might have evolved over the Pleistocene have placed less emphasis on tools than on other marks of cognitive development.
The expanding brain Perhaps the clearest evidence that cognitive capacity grew during the Pleistocene is the dramatic increase in brain size associated with the genus Homo. The earlier hominids had brains that differed little in size from those of present day apes, at least if body size is taken into account. For example, according to estimates based on fossil skulls, the average brain size of Australopithecus afarensis was around 433 cc, compared with averages of 393 cc for the chimpanzee, 418 for the orangutan, and 465 for the gorilla (Martin, 1990). It rose to some 612 cc in Homo habilis , 854 cc in early Homo erectus (also known as Homo ergaster ), 1016 cc in later Homo erectus. After a period of stasis, there appears to have been a secondary increase from about 500,000 years ago, reaching about 1552 cc in the Neanderthals ( Homo neanderthalensis ), and a slightly smaller 1355 cc in Homo sapiens (Wood & Collard, 1999). Brain size depends partly on body size, which probably explains why the Neanderthals, being slightly larger than modern humans, also had slightly larger brains. One way to take body size into account is to use an index called the encephalization quotient (EQ), which is based on the regression of brain size on body size (Martin, 1981), and it has been estimated that the EQ was slightly smaller in Neanderthals than in early humans (Ruff, Trinkaus, & Holliday, 1997). This mercifully restores humans to the top of the pile. The increase in brain size, rather than the emergence of one or more so-called grammar genes, may well have been the vehicle for the evolution of language, and perhaps of more general cognitive developments. Nevertheless the increase in brain size itself was surely dependent on genetic changes. One gene that is a specific regulator of brain size is the abnormal spindle-like microcephaly associated (ASPM) gene, and phylogenetic analysis suggests strong positive selection of this gene in the lineage leading to Homo sapiens (Evans, et al., 2004). Indeed, a selection sweep appears to have occurred as recently as 5,800 years ago, suggesting that the human brain is still undergoing rapid evolution (Mekel-Bobrov, et al., 2005). Interestingly, two other genes appear to have resulted in increased brain size through inactivation rather than positive selection. One of these encodes the enzyme CMP- N - acetylneuraminic acid (CMP-Neu5Ac) hydroxylase (CMAH). An inactivating mutation of this gene has resulted in a deficiency in humans of the mammalian
sialic acid N-glycolylneuraminic acid (Neu5Gc). This appears to have been the end result of a process of down-regulation throughout mammalian evolution, since the acid is absent in Neanderthal fossils as well as in humans, and is only weakly present in chimpanzees relative to other primates and mammals. Chou et al. (2002) speculate that inactivation of the CMAH gene may have removed a constraint on brain growth in human ancestry. Molecular-clock analysis indicated that the inactivating mutation probably occurred some 2.7 million years ago, leading up to the expansion in brain size from around 2.1 million years ago. The other inactivating mutation that may also have contributed to the increase in brain size occurred on a gene that encodes for the myosene heavy chain MYH16. This chain is responsible for the heavy masticatory muscles in most primates, including chimpanzees and gorillas, as well as the early hominids. Molecular-clock analysis suggests that the inactivation dates from around 2.4 million years ago, leading to speculation that the diminution of jaw muscles and their supporting bone structure removed a further constraint on brain growth (Stedman et al., 2004). It is a matter of further speculation as to why this seemingly deleterious mutation became fixed in the ancestral human population. It may have had to do with the change from a predominantly vegetable diet to a meat-eating one, or it may have had to do with the increasing use of the hands rather than the jaws to prepare food (Currie, 2004). As a spin-off, though, it may have allowed brain size to increase in the face of selective pressures that favored more complex cognition, such as theory of mind and recursive language. Curiously, though, it is not entirely clear that the increase in the size of particular areas of the brain was driven by selection for the particular functions subserved by those areas. Rather, the increases in subareas of the brain are tightly constrained by the fact that the brain grows as a covarying whole. Different regions do grow at different rates, but according to a fairly inflexible rule, with structures that emerge late in development increasing more than structures that emerge early. The disproportionate growth in the cortex relative to other parts of the brain might therefore have arisen, not because the cortex is intrinsically specialized for “higher-order” functions, but because the cortex is the last area to undergo neurogenesis. According to this view, structure preceded function, and cognitive traits like language found representation in the cortex because the cortex grew disproportionately relative to subcortical structures (Finlay, Darlington, & Nicastro, 2001). This notion is somewhat controversial, since there is also some evidence for selective pressures on the sizes of different brain regions, leading to the notion
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more complex in the Pleistocene, with numerous activities to be labeled and perhaps communicated. Suppose, for example, that there were some ten meaningful objects (such as lion, tree, stone chopper, child, etc) and six actions (such as run, climb, throw, carry, etc). This gives rise to 60 possible combinations (some admittedly unlikely), and to attach a single label to each would therefore require 60 distinct utterances. There is clear benefit in attaching distinct symbols, which we can now call “words,” to each object and to each action, requiring only 16 words. Hence the first step toward grammar may have the distinction between different classes of words, and the discovery that meanings could be generated by combining them, resulting in a considerable gain in economy (Nowak et al., 2000) A real-life example of the “discovery” of a combinatorial principle is provided by Nicaraguan Sign Language (NSL), which first emerged some 25 years ago when a school was established for deaf children. In an experimental study, users of NSL were asked to describe the action of rolling down a slope. Those from the first cohort mimicked both the rolling and the down motions in a single gesture. The majority of those in the second and third cohorts indicated the motion in two gestures, one to indicate a rolling motion and the other to indicate downward motion (Senghas, Kita, & Özyürek, 2004). This is a living example of grammaticalization. Nowak (2001) has extended this approach theoretically to indicate how universal grammar might have evolved. In their computational modelling, Nowak and his collaborators assume an evolutionary perspective, but it is perhaps equally plausible to suppose that grammaticalization was shaped by culture and communicative demands rather than, or as well as, by genetic mutation and natural selection. A combinatorial structure is efficient in the sense that it cuts down the number of elements from which to build a message. Speech, for example, is built from a relatively small number of phonemes—44 in American English—that are combined hierarchically into morphemes, words, phrases, and sentences, and thence into stories, epistles, contracts, and the like. Even so, the generation of complex structures requires extensive working memory, and it may have been the demands on processing capacity rather than the emergence of combinatorial rules, or any kind of universal grammar, that drove the increase in brain size. Indeed, Baddeley, Gathercole, and Papagno (1998) have argued that the phonological loop—a component of working memory—evolved primarily as a device for learning new words. One of the final stages in the emergence of a fully syntactic language may have been the incorporation of recursive, embedded structures, which imposes heavy demands on working memory. This may have coincided with
the secondary spurt in brain size that occurred from about 500,000 years ago (Wood & Collard, 1999). Striedler (2006) points out that an increase in brain size of itself provides for better neural connectivity, which he calls the rule of “large equals well- connected” (p. 6). This may have been the final change that provided fore the precision and intricate programming necessary for syntactic language, although again there is likely to have been some regional selectivity within the brain (Geary, 2005). In summary, the hierarchical, combinatorial structure of language probably evolved during the Pleistocene, as an adaptation toward increased efficiency of communication in the face of increased complexity of social life. Language probably did not evolve as an isolated skill, but was rather linked to other capacities involving recursive thought, such as enhanced theory of mind, and perhaps mental time travel—the ability to project one’s self mentally forward and backward in time (Suddendorf & Corballis, 1997).
The emergence of speech Attempts to communicate with the great apes have taught us at least one thing—these animals are a long way from being able to speak. For example Viki, a chimpanzee raised from infancy in a human household, could never utter more than about four indistinct words (Hayes, 1952). The capacity to speak must itself have evolved relatively slowly, since considerable anatomical and neural modifications were necessary in order to make articulate speech possible. These modifications, which were probably largely independent of increases in brain size, probably also took place during the Pleistocene, but may not have reached the stage at which autonomous, articulate speech was possible until the emergence of our own species. Articulate speech required radical change in the neural control of vocalization. The species-specific and largely involuntary calls of primates depend on an evolutionarily ancient system that originates in the limbic system, but in humans this is augmented by a separate neocortical system operating through the pyramidal tract, and synapsing directly with the brainstem nuclei for the vocal cords and tongue (Ploog, 2002). The evidence suggests that voluntary control of vocalization in the chimpanzee is extremely limited, at best (e.g., Goodall, 1986). The development of cortical control must surely have occurred gradually, rather than in all-or-none fashion, and perhaps reached its final level of development only in anatomically modern humans. An adaptation unique to H. sapiens is neurocranial globularity, defined as the roundness of the cranial vault in the sagittal, coronal, and transverse planes, which is likely to have increased the relative size of the
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temporal and/or frontal lobes relative to other parts of the brain (D. E. Lieberman, McBratney, & Krovitz, 2002). These changes may reflect more refined control of articulation and/or more accurate perceptual discrimination of articulated sounds. Speech also required anatomical changes to the vocal tract. While this too must have been gradual, P. Lieberman (1998; Lieberman, Crelin, & Klatt, 1972) has argued that the lowering of the larynx, an adaptation that increased the range of speech sounds, was incomplete even in the Neanderthals of 30, years ago. Perhaps, then, it was this, rather than the absence of language itself, that kept them separate from H. sapiens , leading to their eventual extinction. Lieberman’s work remains controversial (e.g., Gibson & Jessee, 1999), but there is other evidence that the cranial structure underwent critical changes subsequent to the split between anatomically modern and earlier “archaic” Homo , such as the Neanderthals, Homo heidelbergensis , and Homo rhodesiensis. One such change is the shortening of the sphenoid, the central bone of the cranial base from which the face grows forward, resulting in a flattened face (D. E. Lieberman, 1998). D. E. Lieberman speculates that this is an adaptation for speech, contributing to the unique proportions of the human vocal tract, in which the horizontal and vertical components are roughly equal in length—a configuration, he argues, that improves the ability to produce acoustically distinct speech sounds. Also critical to articulate speech was an increase in the innervation of the tongue. The hypoglossal nerve is much larger in humans than in great apes, probably because of the important role of the tongue in speech. Fossil evidence suggests that the size of the hypoglossal canal in early australopithecines, and perhaps in Homo habilis , was within the range of that in modern great apes, whereas that of the Neanderthal and early H. sapiens skulls contained was well within the modern human range (Kay, Cartmill, & Barlow, 1998), although this has been disputed (DeGusta, Gilbert, & Turner, 1999). Changes in the control of breathing were also important for speech, and this is at least partly reflected in the fact that the thoracic region of the spinal cord is larger in humans than in nonhuman primates, probably because breathing during speech involves extra muscles of the thorax and abdomen. Fossil evidence indicates that this enlargement was not present in the early hominids or even in Homo ergaster , dating from about 1.6 million years ago, but was present in several Neanderthal fossils (MacLarnon & Hewitt, 1999, 2004). The culmination of changes required for articulate speech may well have occurred very late in the evolution of Homo , perhaps even with the arrival of our own species. Some have taken this as evidence that language itself emerged only in Homo sapiens.
Yet such radical changes must have taken place slowly, at least over the duration of the Pleistocene. This suggests that there must have been a prior form of communication that was shaped in two parallel ways, toward more sophisticated syntax, and both toward a vocal form. There are compelling reasons to suppose that this communication was initially based on manual gestures, but increasingly incorporated movements of the face, and finally articulate vocalization.
The gestural origins of language The hypothesis that language evolved from manual gestures has a long pedigree, dating back at least to the 18th^ century philosopher Condillac (1971/1746). It was revived by Hewes (1973), and has more recently been revised and extended by several authors (e.g., Arbib, 2005; Armstrong, 1999; Armstrong et al., 1995; Corballis, 1992, 1999, 2002, 2003a; Givón, 1995; Place, 2000; Rizzolatti & Arbib, 1998; Skoyles, 2000). As we have seen, it has become abundantly clear that great apes, our closest relatives, cannot acquire speech, but they have achieved a moderate level of success using manual gestures (e.g., Gardner & Gardner, 1969; Savage-Rumbaugh et al., 1998). This suggests that the common ancestor of Homo sapiens and the chimpanzee and bonobo would not have been equipped for a vocal form of language, but might well have begun to develop a form of communication based on manual gestures. This development may well have been enhanced in the hominid lineage by the emergence of bipedalism, which would have freed the hands from any involvement in locomotion. The idea that language may have evolved from manual gestures is further supported by other lines of evidence, some of them recent.
Signed languages First, there is no question that true language can be accomplished using manual and facial gesture, without voicing. It is now well established that the signed languages of the deaf display all of the essential linguistic properties of spoken language (Emmorey, 2002; Neidle, Kegl, MacLaughlin, Bahan & Lee, 2000; Stokoe, 1960). Signs are fundamentally different from gestures of the sort that occur in everyday life, independently of any linguistic function, and which tend to be iconic (i.e., pictorial or mimed) rather than symbolic. Signs, in contrast, tend to be symbolic, although there is also an analogue component, suggesting a link to a more pictorial form of communication. In the course of evolution, then, pantomimes of actions might have incorporated gestures that are analogue representations of objects or actions (Donald, 1991), but through time these gestures may have lost the analogue features and become abstract. In modern American Sign Language,
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“mirror neurons,” which respond both when the animal makes a particular reaching and grasping movement and when it observes the same movement made by others (Rizzolatti, Fadiga, Gallese & Fogassi, 1996). This implies a motor theory of grasp perception. Mirror neurons are located in area F5 of the prefrontal cortex, which is the homolog of Broca’s area, an area critical to the production of speech in humans (Rizzolatti & Arbib, 1998). Primates have little if any cortical control over vocalization (Ploog, 2002), but it appears that Broca’s area, now considered part of a more general “mirror system” involving the understanding of biological action (Rizzolatti, Fogassi, & Gallese, 2001), eventually incorporated vocalization. As we shall see below, this may not have been fully accomplished until the emergence of Homo sapiens. Even in primates, there are neural links between hand and mouth. In monkeys, the mirror system responds to movements of the mouth as well as to movements of the hands (Ferrari, Gallese, Rizzolatti & Fogassi, 2003). In humans, the link between hand and mouth can be demonstrated behaviourally as well. Gentilucci, Benuzzi, Gangitano, and Grimaldi (2001) showed that when subjects were instructed to open their mouths while grasping objects, the size of the mouth opening increased with the size of the grasped object, and conversely, when they open their hands while grasping objects with their mouths, the size of the hand opening also increased with the size of the object. Grasping movements of the hand also affect the kinematics of speech itself. Grasping larger objects (Gentilucci et al., 2001) and bringing them to the mouth (Gentilucci, Santunione, Roy & Stefanini,
- induces selective increases in parameters of lip kinematics and voice spectra of syllables pronounced simultaneously with action execution. Even observing another individual grasping or bringing to the mouth larger objects affects the lip kinematics and the voice spectra of syllables simultaneously pronounced by the viewer (Gentilucci, 2003). In the course of evolution, this mechanism of double command to hand and mouth could have been instrumental in the transfer of a communication system, based on the mirror system, from movements of the hand to movements of the mouth (Gentilucci & Corballis,, 2006). This evolutionary sequence of events may be paralleled by those in the development of language in children, in which gestures are intimately tied to vocalizations (e.g., Bates & Dick, 2002). For example, canonical babbling in children aged from 6-8 months is accompanied by rhythmic hand movements (Masataka, 2001). Manual gestures predate early development of speech in children, and predict later success even up to the two-word level (Iverson & Goldin-Meadow, 2005). Word comprehension in children between 8 and 10 months and word
productions between 11-13 months are accompanied by gestures of pointing and showing and gestures indicating recognition, respectively (Bates & Snyder, 1987). Even in adults, it is well known that manual gestures accompany speech, to form a single integrated system (McNeill, 1985, 1992). This means, of course, that the transition from manual gesture to speech was not complete, although it was nevertheless sufficient to enable effective communication through an acoustic signal alone, as when we communicate by telephone or radio. The visual component is nevertheless informative. Many people with impaired hearing develop lipreading as an effective alternative, and watching people’s lips while they talk can influence what they report hearing. This is illustrated by the McGurk effect, in which dubbing a syllable (e.g., “ba”) onto a mouth that is saying something different (e.g., “ga”) shifts the perception to some intermediate syllable (e.g., “da”) (McGurk & MacDonald, 1976). Preuss, Qi, and Kaas (1999) have also documented greater differentiation in the visual cortex in humans than in apes (including the chimpanzee), which may conceivably relate to the emergence of lipreading in hominid evolution. The connections between hand and mouth may have been established initially in the context ingestion, and the acts of grasping and bringing food to the mouth, but adapted later for communication. MacNeilage (1998) has suggested that speech itself originated from repetitive ingestive movements of the mouth. This may well be correct, but it is perhaps only half the story, since it neglects the important role, in primates at least, of hand and arm movements in eating.
Clicks before vocal articulation? MacNeilage (1998) also drew attention to the similarity between human speech and primate sound- producing facial gestures such as lip smacks, tongue smacks, and teeth chatters. Ferrari et al. (2003) recorded discharge both from mirror neurons in monkeys during the lip smack, which is the most common facial gesture in monkeys, and from other mirror neurons in the same area during mouth movements related to eating. These observations raise the possibility that the earliest audible language was composed of nonvocalized sounds. This idea receives some support from click languages. Aside from a now extinct click language in Australia, click languages are confined to Africa. Two of the many groups that make extensive use of click sounds are the Hadzabe and San, who are separated geographically by some 2000 kilometers, and genetic evidence suggests that the most recent common ancestor of these groups goes back to the root of present-day mitochondrial DNA lineages, perhaps as early as 100,000 years ago (Knight et al., 2003).
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This is not to say that click languages are in any way primitive or linguistically deficient. Moreover modern click languages do incorporate vocalizations. It is also likely that the earliest click languages, and indeed gestural languages generally, included vocalization, perhaps initially in the form of grunts, but with gradual shaping toward articulate sounds. Nevertheless the critical event that resulted in fully articulate vocalization may have emerged after the earliest click languages were formed, and may have depended on a genetic mutation.
The FOXP2 gene Evidence for a genetic component underlying articulate vocal speech came initially from studies of a speech disorder afflicting an extended family, known as the KE family, in England. Over three generations, half of the members of this family have been affected by the disorder, which persists from the affected child’s first attempts to speak through adulthood (Vargha-Khadem, Watkins, Alcock, Fletcher, & Passingham, 1995). Some have argued that the deficit is primarily linguistic, mainly (but not exclusively) affecting the ability to use inflectional morphosyntactic rules, such as changing the endings of words to mark tense or number (Gopnik, 1990); indeed Pinker (1994) explicitly identified the deficit as a loss of the “grammar gene.” Other, more recent work suggests, though, that the core deficit is one of articulation rather than syntax, with morphosyntax a secondary casualty (Alcock, Passingham, Watkins & Vargha-Khadem, 2000; Vargha-Khadem et al., 1998; Watkins, Dronkers, & Vargha-Khadem, 2002). The disorder is now known to be due to a point mutation on the FOXP2 gene (forkhead box P2) on chromosome 7, and for normal speech to be acquired, two functional copies of this gene seem to be necessary (Fisher, Vargha-Khadem, Watkins, Monaco, & Pembrey, 1998). FOXP2 has been sequenced in humans, chimpanzees, gorillas, orangutans, rhesus monkeys, and mice (Enard et al., 2002). The sequences reveal changes in amino-acid encoding and the pattern of nucleotide polymorphism that emerged after the split between human and chimpanzee lineages, and were therefore probably selected for their beneficial effect on vocal communication. The FOXP2 gene is involved in the development of several structures, including the lungs, intestinal system, and cardiovascular system (Shu, Yang, Zhang, Lu, & Morrisey (2001), as well as several brain areas. Nevertheless the mutation of the gene in the speech- affected members of the KE family may have specifically influenced the functioning of the mirror system. Liégeois et al. (2003) used fMRI to record brain activity in both affected and unaffected members of the KE family while they covertly generated verbs in response to nouns. Whereas
unaffected members showed the expected activity concentrated in Broca’s area in the left hemisphere, affected members showed relative under activation in both Broca’s area and its right-hemisphere homologue, as well as in other cortical language areas, such as Wernicke’s area and the left supramarginal gyrus. They also showed over activation bilaterally in regions not associated with language. However, there was bilateral activation in the posterior superior temporal gyrus; on the left, this area overlaps Wernicke’s area, important in the comprehension of language. This suggests that affected members may have generated words in terms of their sounds, rather than in terms of articulatory patterns. Their deficits were not attributable to any difficulty with verb generation itself, since affected and unaffected members did not differ in their ability to generate verbs overtly, and the patterns of brain activity were similar to those recorded during covert verb generation. Another study based on structural MRI showed morphological abnormalities in affected members in the same areas (Watkins, Vargha-Khadem et al., 2002). Enard et al. (2002) have estimated the date of the most recent mutation as occurring within the past 100,000 years, although the standard error was sufficiently large to make it conceivable that it was a defining event in the speciation of Homo sapiens as long as 200,000 years ago. Either way, it suggests that the mutation may have been the critical event that allowed language to become autonomously vocal. This may have had profound consequences for the subsequent development of our species.
Homo sapiens and the “human revolution” Human society underwent a profound transformation some time since the emergence of Homo sapiens. In what has been terms a “human revolution” (Mellars & Stringer, 1989), a sudden flowering of art and technology took place in Europe around 30,000 to 40,000 years ago, and included a dramatic expansion of manufactured objects to include projectiles, harpoons, awls, buttons, needles, and ornaments (Ambrose, 2001). Cave drawings in France and Northern Italy, depicting a menagerie of horses, rhinos, bears, lions, and horses, date from the same period (Knecht, Pike-Tay, & White, 1993). The first unequivocal musical instruments are bird-bone flutes from the early Upper Paleolithic in Germany (Hahn & Münzel, 1995), and there is widespread evidence across Russia, France, and Germany for the weaving of fibers into clothing, nets, bags, and ropes, dating from some 29,000 years ago (Soffer, Adovasio, Illingworth, Amirkhanov, Praslov, & Street, 2000). Further, the Neanderthals, who had adapted to the glacial climate of northwestern Eurasian for at least 200,000 years, abruptly disappeared between 30,
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Not only stories, but great stores of knowledge are exchanged around the fire among the !Kung and the dramatizations—perhaps best of all— bear knowledge critical to survival. A way of life that is difficult enough would, without such knowledge, become simply impossible. (p. 171) Pinker and Bloom (1990) suggest that vocal oratory (especially at night, one is tempted to think) might have been subject to sexual selection, citing Symons’s (1979) observation that tribal chiefs are often both gifted orators and highly polygynous. That is, women may have found men who speak a lot to be sexually attractive, leading to the propagation of genes favoring oratory—although this scarcely accords with the alternative romantic ideal of the strong silent male. The possible role of sexual selection in shaping the human mind, including language, is discussed by Miller (2000). The switch from a manuo-visual to a vocal-auditory form of language was probably gradual, and language has probably always been a combination of the two modes. Even today, we gesture manually and facially as we speak. But was the final achievement of a vocal mode that could carry a verbal message more or less autonomously really sufficient to explain the human revolution, leading to the extinction of other extant hominids? Even small changes in the efficiency of communication can have momentous effects. This is illustrated by the later emergence of writing and literacy, and more recently by the emergence of computer technology, leading to the Internet. Communication systems also permit the accumulation of culture, so that advances feed on advances, in a ratchet-like way. It may therefore not be too far- fetched to suppose that we did talk our way into the human revolution, and our hominid cousins out of existence. In other words, so to speak, language aided emerging humans to organize their groups and coordinate their behaviour (e.g., hunts, warfare) in ways that enable them to outcompete related species.
Conclusion The evolution of language remains a controversial topic, no less so perhaps than it was when the Linguistic Society of Paris imposed the ban in 1866. Although few would now maintain that language was a gift from God, or is somehow not amenable to scientific study, fundamental issues remain. One issue has to do with the extent to which language is shaped by genes or by culture. I have argued that the cultural influence is more important than implied by Chomsky’s notion of a biologically determined universal grammar, although the fact that no other species has demonstrated anything resembling true human language indicates that natural selection must have played a role. A related issue is whether language evolved as a distinct system, as implied by
the notion of universal grammar, or whether it co- evolved with other sociocultural capacities, such as theory of mind or mental time travel. Although these capacities depend in part on different brain regions, they may possess properties in common. I have argued that the most salient of these may be recursion—the ability to embed phrases in phases, ideas in ideas, scenarios in scenarios. Indeed, recursion may be the principle feature that distinguishes the human mind from that of other animals (Corballis, 2003b). Issues about the evolution of language can be clarified somewhat if a clear distinction is made between language and speech. Although language itself may be partly determined by culture, speech itself clearly depended on biological changes, including alterations to the vocal tract, mechanism of breathing, and neural control of vocalization. These changes were no doubt the result of natural selection. The distinction between language and speech is supported by the fact that signed languages have all of the essential properties of true language, and I have argued further that language itself evolved first as a visuomanual system, only gradually incorporating movements of the face and voicing. Indeed speech may not have become autonomous until the appearance of our own species, Homo sapiens , within the past 200,000 years, and possibly even later, as suggested earlier. The notion that language evolved from manual gestures is itself controversial, although increasingly supported by developmental and neurophysiological evidence. It is also beginning to make sense of some of the archaeological evidence. There are strong reasons to believe that the essentials of the human mind evolved during the Pleistocene, from around 1. million years ago, and it is difficult to imagine that these essential would not have included language. Other evidence, including the so-called “human revolution,” has led a number of authors to argue that language did not evolve until the emergence of our own species, perhaps even within the past 100, years. According to the gestural-origins theory, it was not language, but rather speech, that emerged as an autonomous system, and that explains the dramatic developments in culture and manufacture that have taken place over the past 100,000 years. This is not to say that the switch from manual gesture to speech was sudden; rather, facial gesture and vocalization were probably gradually introduced, with vocalization eventually assuming dominance. We should not forget that manual gestures are still a prominent accompaniment of speech. In summary, then, I have argued that the unique properties of human language probably evolved over the past 2 million years, along with other unique characteristics of the human mind. I have proposed
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that two developments occurred in parallel. One was the grammaticalization of communication, made possible through an increase in brain size, and made necessary by the pressures toward more effective social cognition. This probably took place gradually throughout the Pleistocene. The other was the gradual shift from a form of communication dominated by manual gestures, toward a form of communication focused on the face, with the eventual incorporation of vocalization. By freeing the hands from communication, by allowing communication at night, and by removing much of the metabolic load, this final adjustment may have been at least partially responsible for the human revolution, leading to the dominance of Homo sapiens on the planet, and the extinction of all other hominid species.
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