- Language Acquisition Mirrors
- Biological adaptation for language
- Culture, Language Evolution and Child Language Acquisition
Hello and welcome to our Wikichapter. We are Ian, Hannah and Niki and we’re your typical caffeine dependant, sleep-starved linguistics students. With this little contribution, we hope to give an idea of how we can pick apart the things that make Language Evolution so complex, and how we can understand it by looking at our favourite nightmares – infants.
Why is Language Evolution Theory so complex?
Scientists constantly explore new ideas on how humans came to be what they are to they- but they would not be able to do all this without Language. Language evolution theory is complex because it involves the interaction of three systems – Biological adaptation, Individual learning and cultural transmission (Christiansen & Kirby, 2003).
Starting with biological adaptation, a favourable gene may allow a certain species to survive and thrive better- basically, natural selection. Natural selection means those with the favourable gene can pass it down, and in the case of language, this means that the mechanisms needed for language creation or learning are passed down and multiplied. These mechanisms for learning determine individual learning.
Your ability to learn drives linguistic evolution because if you cannot learn something, it is unlikely that you will pursue it. In this case, if you cannot make a particular noise, you will not make it- and subsequently, you will not teach others how to make the sound either. This affects your cultural transmission.
If a language is not used, no new words are created to express new concepts, and the language eventually goes out of use. Cultural transmission changes language because we change it to suit our needs, for example – “On Fleek”, a word/phrase that’s been created to express our satisfaction for well-sculpted eyebrows. The kind of language we use because of this cultural transmission can have an effect on shaping our brains – if everyone spoke fluent sarcasm, the brain would have to adjust to processing and understanding more and more sarcasm. If, for example, this inability to process sarcasm meant an inability to reproduce, natural selection would have occurred- bringing you back to biological adaptation.
With so many things interacting (and scientists being ever curious as to how these interact), conflict has arisen as a result of the complexity, which we will highlight in the next section.
Biggest Controversies of Language Evolution
As with most things in science, people don’t usually agree on things. In this case, the conflict is whether Language Evolution is a result of Biological Evolution, or Cultural Transmission. Another point of contention is if language originated from gestures or from vocalizations.
Biological, or cultural?
Scientists can agree on one thing about language evolution- that there was a pre-adaptation necessary. That ‘pre-adaptation’ is the ability to understand and use symbols- this is important because it means that the symbol may be arbitrary but we will still understand what it stands for (think the ‘peace’ sign you do with your hands; it has nothing to do with peace!)
However, they remain split as to how grammar came about. In one camp are the scientists who believe that grammar structures come from within, such as Noam Chomsky’s theory of Universal Grammar. This camp of scientists believe that it is a kind of specialization that allows individuals to understand complex ideas like who did what to whom, where, when, and why (Pinker, 2003). They also believe that grammar is a design so complex it could not possibly be anything but a biological adaptation. This is supported by the fact that there are heritable language disorders like stuttering and dyslexia, which have been associated to specific parts of the brain (Stromswold, 2001).
To say that biological evolution is the sole reason would be to detract from those who argue that it was a result of cultural transmission. Biology may have given us the vocal chords to speak with, but there must have been a motivation to speak– great apes (our nearest cousins) maintain social cohesion through mutual grooming (Dunbar & Dunbar, 1998). Assuming there became too many apes for this act to maintain social cohesion, perhaps this motivated the need for better methods at maintaining order.
Language systems could also be complex because of the transmission from generation to generation, where the new generation adapts language through a process called grammaticalization (think of how ‘going to’ became ‘gonna’) (Tomasello, 2003). The evidence of this stems from the appearance of languages upon contact, for example, Nicaraguan Sign Language, that only appeared when deaf signers in the area came in contact with each other.
With evidence well-established in literature, both camps have a reason to argue for their sides, and so the debate intensifies.
Gesture, or speech?
The other point of conflict is whether language emerged from vocalizations or from manual gestures. On one hand, by looking at our closest cousins again, it has been proposed that because vocal communication among them is largely a a result of emotions or as reactions, and are usually not voluntary (Corballis, 2002). This is unlike their gestures, but we will discuss that in detail later. Another consideration is increased tool use among these apes- with an increase of tool use, the hands became too occupied to be used for gesturing as well.
On the other hand, there is a theory that suggests that the basic structure of syllables derive from the succession of constrictions and openings of the mouth involved in chewing, sucking, and swallowing (MacNeilage, 1998). These sounds eventually were the things which developed into basic units of sound, also known as phonemes.
Why look at infants?
Child language acquisition is actually a fascinating thing, to watch a child go from babbling to full articulation of thoughts and words.
Children go through a linear kind of evolution- from pre-speaking gestures, to babbling, and as they progress, to language mastery. As a child acquires language they display a similar co-dependence on biological endowments and on the cultural transmission from their parents. By looking at these long with their development, we may be able to better understand the things that we are still split over for Language Evolution.
Language Acquisition Mirrors
Gestures are an aspect of non-verbal communication that are essential in the transfer of meaning. While gestures are often used concurrently with speech, they are especially crucial for communication in the absence of speech. In essence, gestures refer to expressive movements using arms, hands and fingers, as well as other body parts, and can also include a myriad of facial expressions. Such gestures may include signing, pointing and other articulatory expressions.
Gesturing is evident in human communication as well as in non-human primate communication, albeit in distinctive manners. They are often regarded as a precursor to spoken language, in both the evolutionary sense and in Child Language Acquisition.
Gesturing in Humans
In human beings, gesturing plays an important role in child language acquisition. Human children begin to gesture in their early infancy as a means of communication before they acquire spoken language. The gestures that are made by human children during their infancy are a good indication of what will eventually be adapted into their lexicon. For example, the items that are alluded to via gesturing are likely to be the first words to enter the child’s verbal lexicon in the near future. (Iverson & Goldin-Meadow, 2005)The gestures that a child makes at approximately 14 months old can predict the child’s vocabulary at 42 months of age. Furthermore, gesturing allows for the prediction of when a child will begin speaking in two-word utterances. This usually occurs shortly after a child starts using a gesture along with a single-word utterance to convey an idea.
What gesturing does, in essence, is to serve a casual role in language learning, because children are able to practice expressing their ideas even though they lack the speech capabilities to do so verbally. ‘Listeners’ – or the people that observe the gestures made by children, such as their parents or caretakers etc. – then ‘translate’ their gestures into speech (Rowe & Goldin-Meadow, 2009). This immediate verbal input allows the child to develop links between speech and ideas and learn to express themselves via speech.
E.g. A child gestures towards a book → An adult repeats the word ‘book’ verbally →
The child learns over time how to say ‘book’ and also to link the spoken word to the item
Gesturing in Non-Human Primates
When comparing gesturing in humans and in non-human primates, it’s a plausible theory to think that gesturing may have been an early form of symbolic communication. Of course, the types and quantities of gestures made are varied across the different species of primates. However, these gestures are generally unaffected by the primates’ ecology, social structure, nor their cognitive skills. What this means is that, rather than primates learning how to gesture by imitation (i.e. looking at other primates gesturing and learning to gesture by copying them), they develop the skill of gesturing through ontogenic ritualization – meaning they are genetically predisposed to knowing these gestures. They are ingrained with the ability communicate in social contexts like play, grooming, nursing and sexual encounters through different kinds of manual and bodily actions. An example of this would be the ‘teeth baring’ gesture in chimps and bonobos, a gesture which indicates fear. Vervet monkeys, on the other hand, have different signals for different kinds of threat, showing that perhaps primates are also capable of using vocalization to some degree to communicate (Seyfarth, Cheney & Marler, 1980).
In some cases, these gestures show a complex kind of variability and flexibility. There even appears to be a sense of deliberateness to the gestures at times. These gestures can be used ‘intentionally’. Instead of having one gesture strictly having one meaning a single appropriate context, it appears that some of the same gestures can be used across different contexts, Conversely, different gestures can also be used in the same context and sometimes, those different gestures are used one after another in rapid succession (Piaget, 1952; Bates, 1976; Bruner, 1981).
Comparing Human and Non-Human Primate Gestures
Looking at both human and non-human primate gesturing allows us to see how exactly Child Language Acquisition mirrors Language Evolution. First of all, it provides insight into how humans acquired and evolved language. If you compare the two, it is clear that there are certain similarities between how humans and primates gesture – both species appears to have some sort of inherent ability to gesture, although human infants admittedly rely on learning through imitation as well. Both humans and primates use gesturing as a way to communicate in the absence of speech. Both types of gesturing, while sometimes flexible, also lack the ability to produce nuanced communication that is achievable through speech.
It is then valid to question if gesturing is the precursor to spoken language, because while primates occasionally use verbal cues in their communication, their vocalization abilities are fairly limited when compared to that of humans. Perhaps this is because primates lack the vital ‘next step’ in gaining the ability to develop speech, that is the biological adaptations required to develop speech.
Biological adaptation for language
Without our the different organs in our bodies being adapted for spoken language, it is unlikely that humans would have been able to develop speech the same way that we have now. The comparison between humans and primates in this sense is truly fascinating, as it is able to provide a clear representation of how the biological makeup of humans may have once closely resembled that of primates in the past, and had, over a long period of time, evolved into the forms that humans have at the present.
The Importance of Speech
But first, it is important to understand why exactly speech in so important. It all boils down to communication – speech allows humans to communicate effectively and in a way that is efficient. With speech, humans are able to convey an infinite amount of ideas simply by rearranging the sounds that we produce, and varying tone and register etc.. This contrasts greatly to communicating solely through gestures. There is arguably a limit to the number to gestures that could be made and that, in turn, limits how humans are able to express themselves. And of course, there is an inherent need to express themselves because communication is essential to humans, be it for the transmission of information or merely to develop relationship that help us to function in a society.
The most salient differences when it comes to human and primate biological adaptations to speech is in their vocal tracts, as seen in the figure below:
Figure 1: Vocal tract of a chimpanzee (left) and an adult human (right)
While at first glance. Both vocal tracts may appear rather similar, they are actually very different from each other. Some of the main differences are a shortened muzzle and oral cavity (mouth) in humans as compared to the chimpanzee, and an elongated vocal tract in general that consists of a pharynx that is stretched out and a lower larynx (voice box) position, as well as a more flexible tongue that is able to move around the oral cavity and alter the sounds that are produced.
It is suggested that while the lowered larynx position may end up in an increased risk in choking, it has helped in vocalization by making more room for the tongue. The tongue is hence able to move more flexibly to produce a wider variety of perceptible sounds, aiding in speech (Belin, 2006). The lowered larynx could have also served an evolutionary advantage of exaggerating the size of humans to make them appear more threatening to listeners. Not only that, but the position of the vocal tract and tongue flexibility in humans also help in the production of distinct vowel sounds, an ability that most primates lack.
What is interesting is that the vocal tracts of human babies actually share a clear similarity to those of primates. Observe the diagram below:
The vocal tract of human babies have the larynx positioned closer to that of primates. The oral cavity also appears much more similar to a primate’s oral cavity. However, as the baby develops, the larynx begins to shift down to the adult position. This typically occurs at approximately 3 months of age and the relocation of the larynx is complete by the time the child is 4 years old. It is important to note that the age when the larynx begins to lower is also the age when infants begin to coo, while the age at when the larynx relocation is complete is when children’s speech becomes intelligible.
This aspect of Child Language Acquisition serves as an excellent mirror to Language Evolution because it shows that there is a link between the vocal tract biology and the ability to vocalize effectively: when the vocal tract resembles that of primates, there is limited vocalization, but when the vocal tract resembles that of human adults, speech is able to be perceived. Perhaps the primate vocal tract therefore resembled that of human ancestors’ vocal tracts before they evolved to resemble the vocal tracts of humans today.
Another difference between human and primate biology is in the area of brain development. Human brains have areas that are specifically dedicated to the learning of languages. Lateralization in the brain (i.e. how certain neural processes are apparently more dominant in a certain side of the brain) means that the left hemisphere of the brain is most often the dominant side for language processing. Humans also have developed Broca’s area – for motor control of speech – and Wernicke’s area – for language comprehension – which greatly increase human cognitive capability to suggest that these areas have existed in the brain structure of human and primate ancestors. However, it is likely that these areas did not originally have language functions, and that humans have, over time, come to develop specialization for language in these areas of the brain (Cantapulo & Hopkins, 2001; Gannon, 1998).
Why Can’t Primates Speak?
As a result, it is possible to infer the reasons behind why primates are unable to speak as humans do. For one, the biological structure of their vocal tracts are rather different from that of humans. The vocal tracts organs in primates are not able to move as freely and in a way that is as coordinated as those of humans. Their organs also lack the speed in which human vocal tracts are capable of moving at. It is also argued that primates lack the cognitive capability that is necessary for complex communication processes. Nevertheless, the brain structure that supports language development may very well have been present in the ancestors of humans and primates (Pinker, 1994).
Culture, Language Evolution and Child Language Acquisition
The debate regarding Language Evolution usually circles around what motivated Language Evolution in the first place. Did our body biologically evolve capabilities to produce language and speech to survive or did culture and primitive society consequate the need to communicate that resulted in our evolution of speech and language?
As mentioned in the previous section, we’ve dealt with the biological aspect of language evolution and how our anatomy resulted in humans having the abilities to produce language and speech. But, this still does not address the key problem of culture and social cohesion. If there is no one to talk to, would there ever be a need to talk?
Socially Coordinated Activities
The pinnacle of human success is the ability to socially coordinate activities. With society and teamwork, we have managed to create architectural and technological wonders. Dating back to the age of the first homo sapiens, humans formed groups to work together to accomplish tasks that an individual cannot accomplish, such as hunting larger sized preys (Knight, Studdert-Kennedy, & Hurford, 2000). This basic need of food for survival, together with the precision required to operate as a cohesive group in a hunt may have motivated the beginning of speech and language. Albeit rudimentary, the simple grunts and utterances created by our ancestors allowed them to cooperate and coordinate their hunting efforts to get food.
In the modern age, we see a similar process take place within our daily lives. When babies are hungry and stuck in their baby chairs with no access to food, they often cry and make noise to alert their mothers to bring them food. They continue to cry after being fed spoon after spoon of food until they are full. This continued process of making noise and getting fed until the baby is full is akin to the ancestral hunters’ primitive attempt at language to achieve coordination so as to be fed.
Biology At Work
We cannot safely assume that Culture is the sole motivator in speech. Biological and cultural factors goes hand in hand to create the modern miracle of language. The absence of either one of these factors would result in a significant failure in developing and acquiring language. By taking a look at modern examples, we can get a glimpse of how language evolution occurred, in this case, how languages would result decline.
Genie The Feral Child
A good example would be Genie. Genie was a feral child in the sense that she could not speak like children her age could. Instead, she would make animal noises as attempts of communication. This was due to her being held in isolation since birth (Curtiss, 2014).
If cultural motivations, such as the need to communicate to acquire food, was the sole motivation for language evolution and acquisition, we could teach her a language now and she would be able to acquire it completely, proper vocabulary and syntax competence. But that was not the case. She was unable to fully learn the grammar of english and had difficulties with complex vocabulary. She missed the “Critical Period” to learn language.
Critical Period, as defined by linguists, is the period when an infant is susceptible to learning any form of human language. During this period, infants are said to possess a form of universal grammar that can be shaped into any prevalent grammar system in the world. If language is not exposed and taught within this period, the child will lose the ability to learn languages to socially acceptable competencies (Snow & Hoefnagel-Höhle, 1978). Therefore, without cultural influence coming in at the right time, language acquisition will not take hold in an individual, which may eventually lead to a language declination.
Cultural Transmission At Play
This does not mean that cultural factors have no power in motivating language evolution and acquisition. There are situations where, instead of aiding language, biological factors impede the learning of spoken language. In these situation, the need for social bonding will motivate language to evolve when there is none to begin with.
Nicaraguan Sign Language
The Nicaraguan Sign Language is a good example of culturally motivated language evolution. In the beginning, deaf children in Nicaragua do not have a form of sign language that is native to them. This resulted in home signs that families created through simple gestures to communicate with the deaf child. These are highly individualised within families and not standardised within the community. This resulted in miscommunication within the deaf children.
Therefore, the deaf children of nicaragua in the school for deaf children decided to invent their own language and the Nicaraguan Sign Language was born. With the aid of the innate sense for syntax present in children, Nicaraguan Sign Language had a fully functional syntactic system (Senghas & Coppola, 2001). Through the standardised usage of signs by different cohorts of students, the Nicaraguan Sign Language now has diverse lexicon of signs for various tangible and abstract objects and ideas (Senghas, Kita, & Özyürek, 2004).
The need to socially communicate motivated the usage of language to take on a different biological medium. Hands, which were evolved to create tools, carry objects and self defense are now used as a medium of communication.
However, this creation of a new language requires several other factors such as having consistent and standardised behavioural practices that are transmitted through social learning. For example, the Budongo Forest Chimpanzees would use chewed leaves as sponges to collect honey while the Kibale Forest Chimpanzees used sticks (Whiten et al., 2001). The female chimpanzees would that use these tools more often than males to show their young how to use this tools. Without cultural transmission, the novelty of language will live and die with generation that created it. This might give us a glimpse at how preceding ancestral languages are also created and what factors could motivate such a creation.
Short + Language = Slangs
Language constantly change, with teenagers leading most of the novel word changes with Slang words. Slang words are non-standardised words and phrases developed to navigate social constructs. This means that they are words the cool kids say so if people want to hang out with the cool kids, they need to learn these non-standardised words (Nelsen & Rosenbaum, 1972).
But as teenagers grow up to be the adults, they lead the standardisation of words in their language. Words that were Slang in their youths will become words that get taught to their children. The next generation will assume that slang words are part of the standard vocabulary. This is how meaning changes over the years to become something completely different from the original meaning.
Example of Nice
The word “Nice” originally meant ignorant or foolishness in an individual. Across time, this word has changed its meaning to a more positive connotation to signify a sense of liking and appreciation by the ladies. In the Information Era, the slang and abbreviations culture of the internet has morphed the word once more, although not in meaning but in form (“Online Etymology Dictionary,” 2017). An alternative, unofficially standardised way of spelling the word “Nice” is Naise. The word also carries the old meaning of a sense of liking and appreciation, regardless of gender. Motivated by elaborate wordplay on how each consonant and vowel are pronounced individually, Naise should be the more accurate way of spelling “Nice”. However, this was not motivated by any survival needs and was simply out of jest and social cohesion. This allows us to delve deeper into the investigation of mutations and variations in Language Evolution to consider the various non-survival factors that may come into play when language evolves.
In conclusion, we would like to establish parallels between Child Language Acquisition (CLA) and Language Evolution (LE) to better understand why Child Language Acquisition will help us understand Language Evolution better.
3 Parallels in Child Language Acquisition (CLA) & Language Evolution (LE)
There are 3 parallels discussed previously that links Child Language Acquisition to Language Evolution.
Origins of Speech Theory
The origins of speech has always been a mystery to linguists in the field of language evolution. The debate between nature and nurture and which motivated the creation of language is uncertain. By referring to child language acquisition and how infant develop speech from gestures, we can hypothesize the same for the initial change from gesture to speech by our homo sapien ancestors.
Mimesis to Speech Theory
There is an existing theory that seems to fit well with the idea of using Child Language Acquisition to explain Language Evolution, and that is the Mimesis to Speech Theory (Zlatev, 2014). It explains the various reasons why there would be a transition in the evolution of humans from primate ancestors from gestural communication to verbal communication. One of the possible theories is that spoken communication was advantageous in the sense that it helped free up the hands to enable them to be used for other functions, such as using tools. Another possibility is that speech allowed for communication in the dark. But it is most likely that speech was a natural progression from gesturing because of the inherently multimodal nature of human communication, where communication was more effective when humans used more that just arbitrary sounds or gestures.
However, the purpose in which our anatomy evolve for speech is unclear. It could be to facilitate a more efficient system of breathing or to actually accommodate a system of rudimentary speech. The transition of the human vocal tract from birth to adulthood is uncannily similar to that of the evolution of early primates to current humans. Using Child Language Acquisition, we know the anatomy of the infant evolves to facilitate speech, therefore, by extension, we could also hypothesize the same for the beginning of language evolution.
Cultural influences that affects Child Language Acquisition mirrors the behavioural practices transmitted among societies in primates. By looking at how children are motivated by cultural forces to create and modify their language, we can hypothesize a similar model of cultural influence that affects language evolution.
Compatibility of CLA-LE Model
There are no definitive ways to know how language evolved and originated due to the temporal nature of language and utterances. Records of languages from the past only involved languages with a written form and the written form was only invented just 5,000 years ago. Everything beyond that is speculated inference.
Therefore, the usage of the Child Language Acquisition – Language Evolution model would allow us to understand language evolution through the empirical understanding of Child Language Acquisition due to their many uncanny similarities. The hypotheses and inferences would then be supported by both historical and fossilised evidence in Language Evolution, together with real-time observable studies in Child Language Acquisition.
Bates, E. (1987). Language and context: the acquisition of pragmatics. Orlando: Acad. Press.
Belin, P. (2006). Voice processing in human and non-human primates. Philosophical Transactions of the Royal Society B: Biological Sciences,361(1476), 2091-2107. doi:10.1098/rstb.2006.1933
Bruner, J. (1981). The social context of language acquisition. Language & Communication,1(2-3), 155-178. doi:10.1016/0271-5309(81)90010-0
Cantalupo, C., & Hopkins, W. D. (2001). Asymmetric Broca’s area in great apes. Nature,414(6863), 505-505. doi:10.1038/35107134
Christiansen, M. H., & Kirby, S. (2003). Language evolution: consensus and controversies. Trends in Cognitive Sciences, 7(7), 300–307. https://doi.org/10.1016/S1364-6613(03)00136-0
Corballis, M. C. (2002). From hand to mouth: the origins of language. Princeton, N.J.: Princeton University Press.
Curtiss, S. (2014). Genie: A Psycholinguistic Study of a Modern-Day Wild Child. Academic Press.
Dunbar, R., & Dunbar, R. I. M. (1998). Grooming, Gossip, and the Evolution of Language. Harvard University Press.
Gannon, P. J. (1998). Asymmetry of Chimpanzee Planum Temporale: Humanlike Pattern of Wernicke’s Brain Language Area Homolog. Science,279(5348), 220-222. doi:10.1126/science.279.5348.220
Iverson, J. M., & Goldin-Meadow, S. (2005). Gesture Paves the Way for Language Development. Psychological Science,16(5), 367-371. doi:10.1111/j.0956-7976.2005.01542.x
Knight, C., Studdert-Kennedy, M., & Hurford, J. (2000). The Evolutionary Emergence of Language: Social Function and the Origins of Linguistic Form. Cambridge University Press.
Kuhl, P. K. (2011). Social Mechanisms in Early Language Acquisition: Understanding Integrated Brain Systems Supporting Language. Oxford Handbooks Online. doi:10.1093/oxfordhb/9780195342161.013.0043
Lieberman, P. (1968). Primate Vocalizations and Human Linguistic Ability. The Journal of the Acoustical Society of America,44(6), 1574-1584. doi:10.1121/1.1911299
MacNeilage, P. F. (1998). The frame/content theory of evolution of speech production. The Behavioral and Brain Sciences, 21(4), 499-511-546.
Nelsen, E. A., & Rosenbaum, E. (1972). LANGUAGE PATTERNS WITHIN THE YOUTH SUBCULTURE: DEVELOPMENT OF SLANG VOCABULARIES. Merrill-Palmer Quarterly of Behavior and Development, 18(3), 273–285.
Online Etymology Dictionary. (2017). Retrieved April 24, 2017, from http://www.etymonline.com/index.php?term=nice
Piaget, J. (1952). The origins of intelligence in children. New York: International Universities Press.
Pinker, S. (1994). The language instinct: the new science of language and mind. London: Penguin.
Pinker. (2003). Language evolution: States of the Art. New York: Oxford University Press.
Rowe, M. L., & Goldin-Meadow, S. (2009). Early gestureselectivelypredicts later language learning. Developmental Science,12(1), 182-187. doi:10.1111/j.1467-7687.2008.00764.x
Senghas, A., & Coppola, M. (2001). Children Creating Language: How Nicaraguan Sign Language Acquired a Spatial Grammar. Psychological Science, 12(4), 323–328. https://doi.org/10.1111/1467-9280.00359
Senghas, A., Kita, S., & Özyürek, A. (2004). Children Creating Core Properties of Language: Evidence from an Emerging Sign Language in Nicaragua. Science, 305(5691), 1779–1782. https://doi.org/10.1126/science.1100199
Seyfarth, R. M., Cheney, D. L., & Marler, P. (1980). Vervet monkey alarm calls: Semantic communication in a free-ranging primate. Animal Behaviour,28(4), 1070-1094. doi:10.1016/s0003-3472(80)80097-2
Snow, C. E., & Hoefnagel-Höhle, M. (1978). The Critical Period for Language Acquisition: Evidence from Second Language Learning. Child Development, 49(4), 1114–1128. https://doi.org/10.2307/1128751
Stromswold, K. (2001). The Heritability of Language: A Review and Metaanalysis of Twin, Adoption, and Linkage Studies. Language, 77(4), 647–723.
Tomasello, M. (2003). On the Different Origins of Symbols and Grammar. In M. H. Christiansen & S. Kirby (Eds.), Language Evolution (pp. 94–110). Oxford University Press. https://doi.org/10.1093/acprof:oso/9780199244843.003.0006
Whiten, A., Goodall, J., McGrew, W. C., Nishida, T., Reynolds, V., Sugiyama, Y., … Boesch, C. (2001). CHARTING CULTURAL VARIATION IN CHIMPANZEES. Behaviour, 138(11), 1481–1516. https://doi.org/10.1163/156853901317367717
Zlatev, J. (2014). Bodily Mimesis and the Transition to Speech. The Evolution of Social Communication in Primates Interdisciplinary Evolution Research, 165-178. doi:10.1007/978-3-319-02669-5_9