From mouth to hand: gesture, speech, and the evolution of right-handedness

The strong predominance of right-handedness appears to be a uniquely human characteristic, whereas the left-cerebral dominance for vocalization occurs in many species, including frogs, birds, and mammals. Right-handedness may have arisen because of an association between manual gestures and vocalization in the evolution of language. I argue that language evolved from manual gestures, gradually incorporating vocal elements. The transition may be traced through changes in the function of Broca's area. Its homologue in monkeys has nothing to do with vocal control, but contains the so-called "mirror neurons," the code for both the production of manual reaching movements and the perception of the same movements performed by others. This system is bilateral in monkeys, but predominantly left-hemispheric in humans, and in humans is involved with vocalization as well as manual actions. There is evidence that Broca's area is enlarged on the left side in Homo habilis, suggesting that a link between gesture and vocalization may go back at least two million years, although other evidence suggests that speech may not have become fully autonomous until Homo sapiens appeared some 170,000 years ago, or perhaps even later. The removal of manual gesture as a necessary component of language may explain the rapid advance of technology, allowing late migrations of Homo sapiens from Africa to replace all other hominids in other parts of the world, including the Neanderthals in Europe and Homo erectus in Asia. Nevertheless, the long association of vocalization with manual gesture left us a legacy of right-handedness.     

The origins of modernity: was autonomous speech the critical factor?

Although Homo sapiens emerged in Africa some 170000 years ago, the origins of "modern" behavior, as expressed in technology and art, are attributed to people who migrated out of Africa around 50000 years ago, creating what has been called a human revolution in Europe and Asia. There is recent evidence that a mutation of the FOXP2 gene (forkhead box P2), important for the development of articulate speech, occurred some time within the past 100000 years. This event may have allowed speech to become fully autonomous, so that language no longer depended on a visuomanual component. The consequent freeing of the hands and development of pedagogy may have led to the technological advances that allowed H. sapiens to dominate and eventually replace all other hominids.     

语言如何进化

人类语言具有复杂多变的递归结构,漫长的物种进化过程中唯独人类精通语言。语言的进化始于大约两百万年前的“更新世时期”,语言在当时作为一种认知适应对于人类应对自然界带给人类的挑战（如动物掠食与森林毁坏）有很大帮助。人类进化过程中学习与文化因素形成一种选择压力促使人际交流语法化,人际交流语法化引发大脑容量增加,然而,最初的语言进化与基因无关。学习与文化压力也使交流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在进化中具有明显的优势     

Intelligence profiles in children and adolescents with left temporal lobe epilepsy: relationship to language laterality

Pre- and postoperative IQ profiles were examined in children and adolescents with left temporal-lobe epilepsy. Participants were grouped according to their pattern of speech dominance, as indicated by the intracarotid sodium amobarbital procedure. Seven participants with typical (left hemisphere) speech representation were matched for age and sex to seven participants with atypical (bilateral or right hemisphere) speech representation. The group with atypical speech representation tended to perform worse pre- and postoperatively on several verbal and nonverbal subtests. Unlike Strauss et al.'s (1990) sample, however, the groups did not differ significantly on pre- or postoperative Performance IQ. The results suggest that when individuals are matched closely for age and sex, there may only be limited support for a nonverbal "crowding" hypothesis. Atypical speech representation in children and adolescents with early temporal-lobe epilepsy appears to be associated with lowered performance on both verbal and nonverbal IQ subtests.     

Language as gesture

Language can be understood as an embodied system, expressible as gestures. Perception of these gestures depends on the “mirror system,” first discovered in monkeys, in which the same neural elements respond both when the animal makes a movement and when it perceives the same movement made by others. This system allows gestures to be understood in terms of how they are produced, as in the so-called motor theory of speech perception. I argue that human speech evolved from manual gestures, with vocal gestures being gradually incorporated into the mirror system in the course of hominin evolution. Speech may have become the dominant mode only with the emergence of Homo sapiens some 170,100 years ago, although language as a relatively complex syntactic system probably emerged over the past 2 million years, initially as a predominantly manual system. Despite the present-day dominance of speech, manual gestures accompany speech, and visuomanual forms of language persist in signed languages of the deaf, in handwriting, and even in such forms as texting.     

From hand to mouth in the evolution of language: the influence of vocal behavior on lateralized hand use in manual gestures by chimpanzees (Pan troglodytes)

Studies in human subjects indicate that manual gestures accompanied by speech are produced more often by the right compared to the left hand. Additional studies indicate that the production of sign language is controlled by the same brain areas as speech, suggesting similar neurobiological substrates for language that are not modality specific. We report evidence that chimpanzees exhibit preferential use of the right hand in gestural communication. Moreover, use of the right hand in gestural communication is significantly enhanced when accompanied by a vocalization, particularly among human‐reared chimpanzees. Taken together, the data suggest that the lateralization of manual and speech systems of communication may date back as far as 5 million years ago.     

Cerebral asymmetry: motoring on

I argue that the phylogenetic and neurobiological bases for cerebral asymmetry in humans are likely to be found in motor systems rather than in perceptual systems. Current genetic models of human laterality suggest that a `dextral' allele might be responsible for right-handedness and left-cerebral dominance for speech in the majority of humans. The linking of handedness with language lateralization might reflect the early evolution of language as predominantly a system of manual gestures, perhaps switching to a mainly vocal system only with the emergence of Homo sapiens. So-called `mirror neurons' in the prefrontal cortex of the monkey, which fire both when the animal makes a grasping response and when it sees the same response performed by others, might be part of a circuit that is the precursor to language circuits in the brain. This circuit appears to be bilateral in monkeys, but left-hemispheric in humans.     

Archaeology and cognitive evolution

Archaeology can provide two bodies of information relevant to the understanding of the evolution of human cognition--the timing of developments, and the evolutionary context of these developments. The challenge is methodological. Archaeology must document attributes that have direct implications for underlying cognitive mechanisms. One example of such a cognitive archaeology is found in spatial cognition. The archaeological record documents an evolutionary sequence that begins with ape-equivalent spatial abilities 2.5 million years ago and ends with the appearance of modern abilities in the still remote past of 400,000 years ago. The timing of these developments reveals two major episodes in the evolution in spatial ability, one, 1.5 million years ago and the other, one million years later. The two episodes of development in spatial cognition had very different evolutionary contexts. The first was associated with the shift to an open country adaptive niche that occurred early in the time range of Homo erectus. The second was associated with no clear adaptive shift, though it does appear to have coincided with the invasion of more hostile environments and the appearance of systematic hunting of large mammals. Neither, however, occurred in a context of modern hunting and gathering.     

Persisting apraxia in two left-handed, aphasic patients with right-hemisphere lesions

Apraxia usually follows a left hemisphere lesion in right-handers with left hemisphere speech representation. Apraxia following a right hemisphere lesion in left-handers is rare, however, and not well documented in the literature. Two left-handed patients are described in whom apraxia and aphasia followed a right hemisphere lesion. Both the apraxic and the aphasic deficits improved but were still demonstrable 6 weeks following the infarct. The data are consistent with those for right-handers with left hemisphere lesions in suggesting some overlap of anatomical structures for the control of speech and praxis.     

Temporal integration of speech prosody is shaped by language experience: an fMRI study

Differences in hemispheric functions underlying speech perception may be related to the size of temporal integration windows over which prosodic features (e.g., pitch) span in the speech signal. Chinese tone and intonation, both signaled by variations in pitch contours, span over shorter (local) and longer (global) temporal domains, respectively. This cross-linguistic (Chinese and English) study uses functional magnetic resonance imaging to show that pitch contours associated with tones are processed in the left hemisphere by Chinese listeners only, whereas pitch contours associated with intonation are processed predominantly in the right hemisphere. These findings argue against the view that all aspects of speech prosody are lateralized to the right hemisphere, and promote the idea that varying-sized temporal integration windows reflect a neurobiological adaptation to meet the 'prosodic needs' of a particular language.     

A comparative analysis of serial ordering in ring-tailed lemurs (Lemur catta)

Research over the last 25 years has demonstrated that animals are able to organize sequences in memory and retrieve ordered sequences without language. Qualitative differences have been found between the serial organization of behavior in pigeons and monkeys. Here the authors test serial ordering abilities in ring-tailed lemurs, a strepsirrhine primate whose ancestral lineage diverged from that of monkeys, apes, and humans approximately 63 million years ago. Lemurs' accuracy and response times were similar to monkeys, thus suggesting that they may share mechanisms for serial organization that dates to a common primate ancestor.     

How the ventral pathway got lost: and what its recovery might mean

Textbooks dealing with the anatomical representation of language in the human brain display two language-related zones, Broca's area and Wernicke's area, connected by a single dorsal fiber tract, the arcuate fascicle. This classical model is incomplete. Modern imaging techniques have identified a second long association tract between the temporal and prefrontal language zones, taking a ventral course along the extreme capsule. This newly identified ventral tract connects brain regions needed for language comprehension, while the well-known arcuate fascicle is used for "sensorimotor mapping" during speech production. More than 130 years ago, Carl Wernicke already described a ventral connection for language, almost identical to the present results, but during scientific debate in the following decades either its function or its existence were rejected. This article tells the story of how this knowledge was lost and how the ventral connection, and in consequence the dual system, fits into current hypotheses and how language relates to other systems.     

Solving the Mystery of the Human Cerebellum

The mystery of the human cerebellum is this: Why did it enlarge so dramatically in the last million years of human evolution, concomitantly with the greater enlargement of the cerebral cortex? A solution to this mystery was proposed in the 20th century as a result of research by several groups of scientists who investigated the contributions of the cerebellum to the cerebral cortex. In contrast to the 19th century investigations, which were focused on the motor functions of the cerebellum, the focus of the subsequent investigations was expanded to include some mental functions because evidence was produced that the cerebellum contributes to cognition. It was proposed that the combination in the cerebellum of motor and mental capabilities enables the cerebellum to confer on humans some adaptive advantages of great value, and this ability would explain why the human cerebellum has continued to enlarge so dramatically. A valuable adaptive advantage that is included in the proposal is the possibility that the cerebellum couples the motor function of articulating speech to the mental function that selects the language to be spoken, thus helping to produce fluent human speech and language. The validity of this proposal about linguistic processing has not yet been verified. Therefore the mystery of cerebellar enlargement in humans is not yet solved and requires further research.     

The development of representational play in chimpanzees and bonobos: Evolutionary implications,pretense, and the role of interspecies communication

This research investigates the development of symbolic or representational play in two species of the genus Pan, bonobos (Pan paniscus) and chimpanzees (Pan troglodytes). The participants varied not only by species, but also as to whether they had become proficient in communicating with humans via a set of arbitrary visual symbols, called lexigrams. Using a developmental sequence of representational play based on McCune, we found every level that children manifest to be constructed by Pan. The most robust and regular ontogenetic sequence for both bonobos and chimpanzee was not McCune's five-level progression, but a three-step ontogenetic sequence: Level 1 (no representation, no pretense) precedes Levels 2–4 (representation but no pretense), which in turn precedes Level 5 (includes pretense as well as representation). A linguistic system for interspecies communication was necessary for Level 5 representational play and “true” pretense. Human scaffolding produced developmental progress within sequences for all the apes, except the bonobo who lacked a system of interspecies communication. This evidence suggests that the potential for representational play and its social stimulation were present in the common ancestor of bonobos, chimpanzees and humans five million years ago.     

The impact of age at seizure onset on the likelihood of atypical language representation in children with intractable epilepsy

Studies have suggested that early postnatal lesions are associated with a greater likelihood of atypical speech representation than lesions acquired later in life. Comparison groups have been defined differently across studies, with age typically being treated as a dichotomous (i.e., early versus late lesion onset) rather than continuous variable. Thus, little is known about the age at which children become less likely to exhibit atypical representation following a brain insult. This study examined the likelihood of typical versus atypical speech representation in children with intractable epilepsy (n = 75). Age of seizure onset was treated as a continuous variable to examine whether there was a naturally occurring cut-off point after which the rate of atypical speech representation decreased. A much higher proportion of children with seizure onset prior to the fifth year showed atypical speech representation as compared to children whose seizures began after 5 years of age.     

The role of the right hemisphere in the control of speech prosody in propositional and affective contexts

Sixteen right-handed adult males with localized insult to either the right or left hemisphere and five control subjects without brain damage read aloud target sentences embedded in paragraphs, while intoning their voices in either a declarative, interrogative, happy, or sad mode. Acoustical analysis of the speech wave was performed. Right-anterior (pre-Rolandic) and right-central (pre- and post-Rolandic) brain-damaged patients spoke with less pitch variation and restricted intonational range across emotional and nonemotional domains, while patients with right posterior (post-Rolandic) damage had exaggerated pitch variation and intonational range across both domains. No such deficits were found in patients with left posterior damage, whose prosody was similar to that of normal control subjects. It is suggested that damage to the right hemisphere alone may result in a primary disturbance of speech prosody that may be independent of the disturbances in affect often noted in right-brain-damaged populations.     

Seeking the neurobiological bases of speech perception

One of the most highly developed human abilities is communication by speech. Throughout the years, research on speech perception has demonstrated that humans are well adapted to extract highly encoded linguistic information from the speech signal. The sophisticated nature of these capacities and their early appearance during development suggest the existence of a rich biological substrate for speech perception. In the present paper, we describe some of these important capacities and examine research from different domains that may help illuminate the nature of their biological foundations.     

The ecological rationality of delay tolerance: insights from capuchin monkeys

Both human and non-human animals often face decisions between options available at different times, and the capacity of delaying gratification has usually been considered one of the features distinguishing humans from other animals. However, this characteristic can widely vary across individuals, species, and types of task and it is still unclear whether it is accounted for by phylogenetic relatedness, feeding ecology, social structure, or metabolic rate. To disentangle these hypotheses, we evaluated temporal preferences in capuchin monkeys, South-American primates that, despite splitting off from human lineage approximately 35 million years ago, show striking behavioural analogies with the great apes. Then, we compared capuchins’ performance with that of the other primate species tested so far with the same procedure. Overall, capuchins showed a delay tolerance significantly higher than closely related species, such as marmosets and tamarins, and comparable to that shown by great apes. Capuchins’ tool use abilities might explain their comparatively high preference for delayed options in inter-temporal choices. Moreover, as in humans, capuchin females showed a greater delay tolerance than males, possibly because of their less opportunistic foraging style. Thus, our results shed light on the evolutionary origins of self-control supporting explanations of delay tolerance in terms of feeding ecology.     

WOMEN: A MORE BALANCED BRAIN?

Abstract. On the basis of knowledge prior to 1988, Ashbrook pointed out that whereas most men are primarily dependent on the left cerebral hemisphere (“dominant hemisphere”) for verbally related functions, women show a greater hemispheric balance in this respect. For men, he argues, their possession of a “speaking” and a “non-speaking hemisphere” results in a positive-negative, bipolar way of thinking that may be characterized as dualistic and dialectically hierarchical. In contrast, the greater balance of hemispheric function in women appears to promote greater generalization and synthesis in their thinking. In this article I cite more recent neuroanatomical and brain imaging studies that provide further evidence of disparities in structure and function of the brains of men and women. As background for an attempt to explain these differences, I give a brief review of the triune evolution of the mammalian brain leading up to the human cerebrum. It is of major significance that the female has played a central role in mammalian evolution for more than 180 million years and that the evolutionary transition from reptilian therapsids to mammals is characterized by the develoment of (1) nursing conjoined with maternal care; (2) audiovocal communication or aiding mother-offspring contact; and (3) play. In human beings, the infant-carrying hypothesis suggests one means by which, over generations, certain parts of a woman's right hemisphere could undergo functional and eventual anatomical expansion. The mother-infant communication of basic mammalian sounds with vowel and consonant components suggests a basis for the origination of speech. Finally, in an expansion of Ashbrook's original thesis, we arrive at the rovocative consideration that, unlike men, with their dialectical reasoning, women, with their more balanced brain, are provided with a trialectial ladder for climbing to achieve knowledge. In terms of quantum mechanics, the particle and wave would correspond to the substance and strength of the sides of the ladder, and the derived psychic information (which is neither matter nor energy) would compare to the rungs in between.     

Prelinguistic Infants, but Not Chimpanzees, Communicate About Absent Entities

ABSTRACT— One of the defining features of human language is displacement , the ability to make reference to absent entities. Here we show that prelinguistic, 12-month-old infants already can use a nonverbal pointing gesture to make reference to absent entities. We also show that chimpanzees—who can point for things they want humans to give them—do not point to refer to absent entities in the same way. These results demonstrate that the ability to communicate about absent but mutually known entities depends not on language, but rather on deeper social-cognitive skills that make acts of linguistic reference possible in the first place. These nonlinguistic skills for displaced reference emerged apparently only after humans' divergence from great apes some 6 million years ago.