Editing Origin of language (section)

== Language origin hypotheses ==

=== Early speculations ===
{{quotation|I cannot doubt that language owes its origin to the imitation and modification, aided by signs and gestures, of various natural sounds, the voices of other animals, and man's own instinctive cries.|Charles Darwin, 1871. ''The Descent of Man, and Selection in Relation to Sex''<ref>Darwin, C. (1871). "The Descent of Man, and Selection in Relation to Sex, 2 vols. London: Murray, p. 56.</ref>}} In 1861, historical linguist [[Max Müller]] published a list of speculative theories concerning the origins of spoken language:<ref>Müller, F. M. 1996 [1861]. The theoretical stage, and the origin of language. Lecture 9 from Lectures on the Science of Language. Reprinted in R. Harris (ed.), ''The Origin of Language''. Bristol: Thoemmes Press, pp. 7–41.</ref>
* ''Bow-wow''. The ''bow-wow'', or ''cuckoo'', theory, which Müller attributed to the German philosopher [[Johann Gottfried Herder]], saw early words as imitations of the cries of beasts and birds.
* ''Pooh-pooh''. The ''pooh-pooh'' theory saw the [[Interjectional theory|first words as emotional interjections and exclamations]] triggered by pain, pleasure, surprise, etc.
* ''Ding-dong''. Müller suggested what he called the ''ding-dong'' theory, which states that all things have a vibrating natural resonance, echoed somehow by humans in their earliest words.
* ''Yo-he-ho''. The ''yo-he-ho'' theory claims that language emerged from collective rhythmic labor; that is, the attempt to synchronize muscular efforts resulting in sounds such as ''heave'' alternating with sounds such as ''ho''.
* ''Ta-ta''. The ''ta-ta'' theory did not feature in Max Müller's list, having been proposed in 1930 by Sir Richard Paget.<ref>Paget, R. 1930. ''Human speech: some observations, experiments, and conclusions as to the nature, origin, purpose and possible improvement of human speech''. London: Routledge & Kegan Paul.</ref> According to the ''ta-ta'' theory, humans made the earliest words by tongue movements that mimicked manual gestures, rendering them audible.

Most scholars today consider all such theories not so much wrong—they occasionally offer peripheral insights—as naïve and irrelevant.<ref>Firth, J. R. 1964. ''The Tongues of Men and Speech''. London: Oxford University Press, pp. 25–26.</ref><ref>Stam, J. H. 1976. ''Inquiries into the origins of language''. New York: Harper and Row, pp. 243–244.</ref> The problem with these theories is that they rest on the assumption that once early humans had discovered a workable ''mechanism'' for linking sounds with meanings, language would automatically have evolved.{{cn|date=December 2023}}

Much earlier, [[Islamic Golden Age|medieval Muslim scholars]] developed theories on the origin of language.<ref>{{Cite journal |last=Shah |first=Mustafa |date=January 2011 |title=Classical Islamic Discourse on the Origins of Language: Cultural Memory and the Defense of Orthodoxy |url=https://core.ac.uk/download/2793514.pdf |journal=[[Numen (journal)|Numen]] |volume=58 |issue=2–3 |pages=314–343 |doi=10.1163/156852711X562335 |s2cid=55165312 |via=CORE}}</ref><ref>{{Cite journal |last=Weiss |first=B. |author-link=Bernard G. Weiss |year=1987 |title='Ilm al-wad': An Introductory Account of a Later Muslim Philological Science |journal=Arabica |volume=34 |issue=1 |pages=339–356 |doi=10.1163/157005887X00054 |s2cid=161187751}}</ref> Their theories were of five general types:<ref>{{Cite journal |last=Weiss |first=B. |author-link=Bernard G. Weiss |year=1974 |title=Medieval Muslim discussions of the origin of language |url=https://www.jstor.org/stable/pdf/43370636.pdf |journal=Zeitschrift der Deutschen Morgenländischen Gesellschaft |volume=124 |issue=1 |pages=33–41 |doi=10.1163/156852711X562335 |jstor=43370636 |s2cid=55165312 }}</ref>
# ''Naturalist'': There is a natural relationship between expressions and the things they signify. Language thus emerged from a natural human inclination to imitate the sounds of nature.
# ''Conventionalist'': Language is a social convention. The names of things are [[Arbitrariness#Linguistics|arbitrary]] inventions of humans.
# ''Revelationist'': Language was gifted to humans by [[Allah|God]], and it was thus God—and not humans—who named everything.
# ''Revelationist-Conventionalist'': God revealed to humans a core base of language—enabling humans to communicate with each other—and then humans invented the rest of language.
#  ''Non-Committal'': The view that conventionalist and revelationist theories are equally plausible.

=== Problems of reliability and deception ===
{{further|Signalling theory}}

From the perspective of signalling theory, the main obstacle to the evolution of language-like communication in nature is not a mechanistic one. Rather, it is the fact that symbols—arbitrary associations of sounds or other perceptible forms with corresponding meanings—are unreliable and may as well be false.<ref>{{Cite journal |last=Zahavi |first=A. |date=May 1993 |title=The fallacy of conventional signalling |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=340 |issue=1292 |pages=227–230 |bibcode=1993RSPTB.340..227Z |doi=10.1098/rstb.1993.0061 |pmid=8101657}}</ref><ref>Zahavi, A. and A. Zahavi 1997. ''The Handicap Principle: A Missing Piece in Darwin's Puzzle''. New York and Oxford: Oxford University Press. {{ISBN|9780190284589}}</ref><ref>{{Cite journal |last=Smith |first=J. Maynard |year=1994 |title=Must reliable signals always be costly? |journal=Animal Behaviour |volume=47 |issue=5 |pages=1115–1120 |doi=10.1006/anbe.1994.1149 |issn=0003-3472 |s2cid=54274718}}</ref> The problem of reliability was not recognized at all by Darwin, Müller or the other early evolutionary theorists.

Animal vocal signals are, for the most part, intrinsically reliable. When a cat purrs, the signal constitutes direct evidence of the animal's contented state. The signal is trusted, not because the cat is inclined to be honest, but because it just cannot fake that sound. Primate vocal calls may be slightly more manipulable, but they remain reliable for the same reason—because they are hard to fake.<ref name="Goodall1986">{{Cite book |last=Goodall |first=Jane |url=https://archive.org/details/chimpanzeesofgom00good |title=The chimpanzees of Gombe: patterns of behavior |publisher=Belknap |year=1986 |isbn=978-0-674-11649-8 |location=Cambridge, MA}}</ref> Primate social intelligence is "[[Machiavellian intelligence|Machiavellian]]"; that is, [[self-serving]] and unconstrained by moral scruples. Monkeys, apes and particularly humans often attempt to deceive each other, while at the same time remaining constantly on guard against falling victim to deception themselves.<ref>{{Cite book |last1=Byrne |first1=Richard W. |title=Machiavellian intelligence : social expertise and the evolution of intellect in monkeys, apes, and humans |last2=Whiten |first2=Andrew. |publisher=Clarendon |year=1988 |isbn=978-0-19-852175-4 |location=Oxford}}</ref><ref>{{Cite journal |last=de Waal |first=Frans B. M. |year=2005 |title=Intentional Deception in Primates |journal=Evolutionary Anthropology |volume=1 |issue=3 |pages=86–92 |doi=10.1002/evan.1360010306 |s2cid=221736130}}</ref> Paradoxically, it is theorized that primates' resistance to deception is what blocks the evolution of their signalling systems along language-like lines. Language is ruled out because the best way to guard against being deceived is to ignore all signals except those that are instantly verifiable. Words automatically fail this test.<ref name="Knight1998" />

Words are easy to fake. Should they turn out to be lies, listeners will adapt by ignoring them in favor of hard-to-fake indices or cues. For language to work, listeners must be confident that those with whom they are on speaking terms are generally likely to be honest.<ref>{{Cite book |last=Power |first=Camilla |title=Approaches to the evolution of language: social and cognitive base |publisher=Cambridge University Press |year=1998 |isbn=978-0-521-63964-4 |editor-last=Hurford |editor-first=James R. |pages=111–129 |chapter=Old wives' tales: the gossip hypothesis and the reliability of cheap signals |editor-last2=Studdert-Kennedy |editor-first2=Michael |editor-last3=Chris Knight}}</ref> A peculiar feature of language is [[Displacement (linguistics)|displaced reference]], which means reference to topics outside the currently perceptible situation. This property prevents utterances from being corroborated in the immediate "here" and "now". For this reason, language presupposes relatively high levels of mutual trust in order to become established over time as an [[evolutionarily stable strategy]]. This stability is born of a longstanding mutual trust and is what grants language its authority. A theory of the origins of language must therefore explain why humans could begin [[Signalling theory|trusting cheap signals]] in ways that other animals apparently cannot.

==== The "mother tongues" hypothesis ====
The "mother tongues" hypothesis was proposed in 2004 as a possible solution to this problem.<ref name="Fitch2004">{{Cite book |last=Fitch |first=W. T. |title=Evolution of communication systems: a comparative approach |publisher=MIT Press |year=2004 |isbn=978-0-262-15111-5 |editor-last=Griebel |editor-first=Ulrike |location=Cambridge, MA |pages=275–296 |chapter=Kin selection and 'mother tongues': a neglected component in language evolution |editor-last2=Oller |editor-first2=D. Kimbrough |chapter-url=https://homepage.univie.ac.at/tecumseh.fitch/media/files/FitchKin2004_large.pdf}}</ref> [[W. Tecumseh Fitch]] suggested that the Darwinian principle of "[[kin selection]]"<ref>{{Cite journal |last=Hamilton |first=W. D. |year=1964 |title=The genetical evolution of social behaviour. I, II |journal=Journal of Theoretical Biology |volume=7 |issue=1 |pages=1–52 |bibcode=1964JThBi...7....1H |doi=10.1016/0022-5193(64)90038-4 |pmid=5875341 |s2cid=5310280}}</ref>—the convergence of genetic interests between relatives—might be part of the answer. Fitch suggests that languages were originally "mother tongues". If language evolved initially for communication between mothers and their own biological offspring, extending later to include adult relatives as well, the interests of speakers and listeners would have tended to coincide. Fitch argues that shared genetic interests would have led to sufficient trust and cooperation for intrinsically unreliable signals—words—to become accepted as trustworthy and so begin evolving for the first time.<ref>{{Cite book |last=Knight |first=Chris |title=The Evolutionary Emergence of Language |publisher=Cambridge University Press |year=2000 |isbn=978-0-521-78157-2 |pages=99–120 |chapter=Play as Precursor of Phonology and Syntax |doi=10.1017/cbo9780511606441.007 |s2cid=56418139}}</ref>

Critics of this theory point out that kin selection is not unique to humans.<ref name="Tallerman2013">{{Cite book |last=Tallerman |first=Maggie |title=The evolutionary emergence of language: evidence and inference |publisher=Oxford University Press |year=2013 |isbn=978-0-19-965485-7 |editor-last=Botha |editor-first=Rudolf P. |pages=77–96 |chapter=Kin selection, pedagogy and linguistic complexity: whence protolanguage? |editor-last2=Everaert |editor-first2=Martin}}</ref> So even if one accepts Fitch's initial premises, the extension of the posited "mother tongue" networks from close relatives to more distant relatives remains unexplained.<ref name="Tallerman2013" /> Fitch argues, however, that the extended period of physical immaturity of human infants and the postnatal growth of the human brain give the human-infant relationship a different and more extended period of intergenerational dependency than that found in any other species.<ref name="Fitch2004" />

==== The "obligatory reciprocal altruism" hypothesis ====
Ib Ulbæk<ref name="Ulbæk1998" /> invokes another standard Darwinian principle—"[[reciprocal altruism]]"<ref>{{Cite journal |last=Trivers |first=R. L. |year=1971 |title=The evolution of reciprocal altruism |journal=Quarterly Review of Biology |volume=46 |pages=35–57 |doi=10.1086/406755 |s2cid=19027999}}</ref>—to explain the unusually high levels of intentional honesty necessary for language to evolve. "Reciprocal altruism" can be expressed as the principle that ''if you scratch my back, I'll scratch yours''. In linguistic terms, it would mean that ''if you speak truthfully to me, I'll speak truthfully to you''. Ordinary Darwinian reciprocal altruism, Ulbæk points out, is a relationship established between frequently interacting individuals. For language to prevail across an entire community, however, the necessary reciprocity would have needed to be enforced universally instead of being left to individual choice. Ulbæk concludes that for language to evolve, society as a whole must have been subject to moral regulation.

Critics point out that this theory fails to explain when, how, why or by whom "obligatory reciprocal altruism" could possibly have been enforced.<ref name="Knight2006" /> Various proposals have been offered to remedy this defect.<ref name="Knight2006" /> A further criticism is that language does not work on the basis of reciprocal altruism anyway. Humans in conversational groups do not withhold information to all except listeners likely to offer valuable information in return. On the contrary, they seem to want to [[Signalling theory|advertise to the world]] their access to socially relevant information, broadcasting that information without expectation of reciprocity to anyone who will listen.<ref>{{Cite book |last=Dessalles |first=Jean L. |title=Approaches to the evolution of language: social and cognitive base |publisher=Cambridge University Press |year=1998 |isbn=978-0-521-63964-4 |editor-last=James R. Hurford |pages=130–147 |chapter=Altruism, status and the origin of relevance |editor-last2=Michael Studdert-Kennedy |editor-last3=Chris Knight}}</ref>

==== The gossip and grooming hypothesis ====
Gossip, according to [[Robin Dunbar]] in his book ''[[Grooming, Gossip and the Evolution of Language]]'', language does for group-living humans what [[Social grooming|manual grooming]] does for other primates—it allows individuals to service their relationships and so maintain their alliances on the basis of the principle: ''if you scratch my back, I'll scratch yours''. Dunbar argues that as humans began living in increasingly larger social groups, the task of manually grooming all one's friends and acquaintances became so time-consuming as to be unaffordable.<ref name="Dunbar1996">{{Cite book |last=Dunbar |first=R. I. M. |title=Grooming, gossip and the evolution of language |publisher=Faber & Faber |year=1996 |isbn=978-0-571-17396-9 |location=London}}</ref> In response to this problem, humans developed "a cheap and ultra-efficient form of grooming"—''vocal grooming''. To keep allies happy, one now needs only to "groom" them with low-cost vocal sounds, servicing multiple allies simultaneously while keeping both hands free for other tasks. Vocal grooming then evolved gradually into vocal language—initially in the form of "[[Gossip|gossip".]]<ref name="Dunbar1996" /> Dunbar's hypothesis seems to be supported by adaptations, in the structure of language, to the function of narration in general.<ref>{{Cite book |last=von Heiseler |first=Till Nikolaus |url=https://www.academia.edu/9648129 |title=Evolution of Language |publisher=World Scientific |year=2014 |editor-last=Cartmill |editor-first=R. L. C. |location=London |pages=114–121 |chapter=Language evolved for storytelling in a super-fast evolution|doi=10.1142/9789814603638_0013 |isbn=978-981-4603-62-1 }}</ref>

Critics of this theory point out that the efficiency of "vocal grooming"—the fact that words are so cheap—would have undermined its capacity to signal commitment of the kind conveyed by time-consuming and costly manual grooming.<ref>{{Cite book |last=Power |first=C. |title=Approaches to the Evolution of Language: Social and Cognitive Bases |publisher=Cambridge University Press |year=1998 |editor-last=Hurford |editor-first=J. R. |pages=111–129 |chapter=Old wives' tales: the gossip hypothesis and the reliability of cheap signals |editor-last2=Studdert-Kennedy |editor-first2=M. |editor-last3=Knight |editor-first3=C.}}</ref> A further criticism is that the theory does nothing to explain the crucial transition from vocal grooming—the production of pleasing but meaningless sounds—to the cognitive complexities of syntactical speech.

==== Ritual/speech coevolution ====
The ritual/speech coevolution theory was originally proposed by social anthropologist [[Roy Rappaport]]<ref>Rappaport, R. A. 1999. "Ritual and Religion in the Making of Humanity." Cambridge University Press.</ref> before being elaborated by anthropologists such as Chris Knight,<ref>Knight, C. 1998. Ritual/speech coevolution: a solution to the problem of deception. In J. R. Hurford, M. Studdert-Kennedy and C. Knight (eds), Approaches to the Evolution of Language: Social and cognitive bases. Cambridge University Press, pp. 68–91.</ref> Jerome Lewis,<ref>Lewis, J. 2009. "As well as words: Congo Pygmy hunting, mimicry, and play." In R. Botha and C. Knight (eds), The Cradle of Language. Oxford: Oxford University Press, pp. 236–256.</ref> Nick Enfield,<ref>{{Cite journal |last=Enfield |first=N. J. |year=2010 |title=Without social context? |url=http://pubman.mpdl.mpg.de/pubman/item/escidoc:527132:11/component/escidoc:527220/Enfield_Science_Language%20Evolution_2010.pdf |journal=Science |volume=329 |issue=5999 |pages=1600–1601 |bibcode=2010Sci...329.1600E |doi=10.1126/science.1194229 |s2cid=143530707 |hdl-access=free |hdl=11858/00-001M-0000-0012-C777-5}}</ref> Camilla Power<ref>Power, C. 1998. "Old wives' tales: the gossip hypothesis and the reliability of cheap signals." In J. R. Hurford, M. Studdert Kennedy and C. Knight (eds), Approaches to the Evolution of Language: Social and Cognitive Bases. Cambridge University Press, pp. 111 29.</ref> and Ian Watts.<ref>Watts, I. 2009. Red ochre, body painting, and language: interpreting the Blombos ochre. In R. Botha and C. Knight (eds), The Cradle of Language. Oxford: Oxford University Press, pp. 62–92.</ref> Cognitive scientist and robotics engineer [[Luc Steels]]<ref>Steels, Luc. 2009. "Is sociality a crucial prerequisite for the emergence of language?" In Rudolf P. Botha and Chris Knight (eds), ''The prehistory of language''. Oxford: Oxford University Press. {{ISBN|978-0-19-954587-2}}</ref> is another prominent supporter of this general approach, as is biological anthropologist and neuroscientist [[Terrence Deacon]].<ref>{{Cite book |last=Deacon |first=Terrence William |url=https://archive.org/details/symbolicspeciesc00deac |title=The symbolic species: the co-evolution of language and the brain |publisher=W. W. Norton |year=1997 |isbn=978-0-393-03838-5 |location=New York}}</ref> A more recent champion of the approach is the Chomskyan specialist in [[linguistic syntax]], Cedric Boeckx.<ref name="Boeckx">Boeckx, C. (2023) What made us "hunter-gatherers of words". Front. Neurosci. 17:1080861. {{doi|10.3389/fnins.2023.1080861}}.</ref>

These scholars argue that there can be no such thing as a "theory of the origins of language". This is because language is not a separate adaptation, but an internal aspect of something much wider—namely, the entire domain known to anthropologists as human [[symbolic culture]].<ref>Knight, C. 2010. The origins of symbolic culture. In Ulrich J. Frey, Charlotte Störmer and Kai P. Willfuhr (eds) 2010. ''Homo Novus'' – A Human Without Illusions. Berlin, Heidelberg: Springer-Verlag, pp. 193–211.</ref> Attempts to explain language independently of this wider context have failed, say these scientists, because they are addressing a problem with no solution. Language would not work outside its necessary environment of confidence-building social mechanisms and institutions. For example, it would not work for a nonhuman ape communicating with others of its kind in the wild. Not even the cleverest nonhuman ape could make language work under such conditions.

{{quotation|Lie and alternative, inherent in language ... pose problems to any society whose structure is founded on language, which is to say all human societies. I have therefore argued that if there are to be words at all it is necessary to establish ''The Word'', and that The Word is established by the invariance of liturgy.|Roy Rappaport<ref>{{Cite book |last=Rappaport |first=Roy A. |title=Ecology, Meaning, and Religion |publisher=North Atlantic |year=1979 |isbn=978-0-913028-54-4 |location=Richmond, CA |pages=201–211}}</ref>}}

Advocates of this school of thought point out that words are cheap. Should an especially clever nonhuman ape, or even a group of articulate nonhuman apes, try to use words in the wild, they would carry no conviction. The primate vocalizations that do carry conviction—those they actually use—are unlike words, in that they are emotionally expressive, intrinsically meaningful, and reliable because they are relatively costly and hard to fake.

Oral and gestural languages consist of pattern-making whose cost is essentially zero. As pure social conventions, signals of this kind cannot evolve in a Darwinian social world—they are a theoretical impossibility.<ref>Zahavi, A. 1993. "The fallacy of conventional signalling." ''Philosophical Transactions: Biological Sciences'' 340: 227–230, published by Royal Society.</ref> Being intrinsically unreliable, language works only if one can build up a reputation for trustworthiness within a certain kind of society—namely, one where symbolic cultural facts (sometimes called "institutional facts") can be established and maintained through collective social endorsement.<ref>Searle, J. R. 1996. ''The Construction of Social Reality''. London: Penguin.</ref> In any hunter-gatherer society, the basic mechanism for establishing trust in symbolic cultural facts is collective ''ritual''.<ref>Durkheim, E. 1947 [1915]. "Origins of these beliefs". Chapter VII. In É. Durkheim, ''The Elementary Forms of the Religious Life: A study in religious sociology''. Trans. J. W. Swain. Glencoe, Illinois: The Free Press, pp.&nbsp;205–239.</ref> Therefore, the task facing researchers into the origins of language is more multidisciplinary than is usually supposed. It involves addressing the evolutionary emergence of human ritual, kinship, religion and symbolic culture taken as a whole, with language an important but subsidiary component.

In a 2023 article, Cedric Boeckx<ref name="Boeckx" /> endorses the Rappaport/Searle/Knight way of capturing the "special" nature of human words. Words are symbols. This means that, from a standpoint in Darwinian signal evolution theory, they are "patently false signals." Words are facts, but "facts whose existence depends entirely on subjective belief".<ref>Knight, C. 2010. The origins of symbolic culture. In Ulrich J. Frey, Charlotte Störmer and Kai P. Willfuhr (eds) 2010. Homo Novus – A Human Without Illusions. Berlin, Heidelberg: Springer-Verlag, pp. 193–211.</ref> In philosophical terms, they are "institutional facts": fictions that are granted factual status within human social institutions<ref>Searle, J. R. 1996. "The Construction of Social Reality." London: Penguin.</ref> From this standpoint, according to Boeckx, linguistic utterances are symbolic to the extent that they are patent falsehoods serving as guides to communicative intentions. "They are communicatively useful untruths, as it were."<ref name="Boeckx" /> The reason why words can survive among humans despite being false is largely down to a matter of trust. The corresponding origins theory is that language can only have begun to evolve from the moment humans started reciprocally faking in communicatively helpful ways, i.e., when they became capable of upholding the levels of trust necessary for linguistic communication to work.

The point here is that an ape or other nonhuman must always carry at least some of the burden of generating the trust necessary for communication to work. That is, in order to be taken seriously, each signal it emits must be a patently reliable one, trusted because it is rooted in some way in the real world. But now imagine what might happen under social conditions where trust could be taken for granted. The signaller could stop worrying about reliability and concentrate instead on perceptual discriminability. Carried to its conclusion, this should permit digital signaling—the cheapest and most efficient kind of communication.

From this philosophical standpoint, animal communication cannot be digital because it does not have the luxury of being patently false. Costly signals of any kind can only be evaluated on an analog scale. Put differently, truly symbolic, digital signals become socially acceptable only under highly unusual conditions—such as those internal to a ritually bonded community whose members are not tempted to lie.{{citation needed|date=November 2024}}

Critics of the speech/ritual co-evolution idea theory include Noam Chomsky, who terms it the "non-existence" hypothesis—a denial of the very existence of language as an object of study for natural science.<ref>{{Cite journal |last=Chomsky |first=Noam |year=2011 |title=Language and Other Cognitive Systems. What is Special About Language? |journal=Language Learning and Development |volume=7 |issue=4 |pages=263–278 |doi=10.1080/15475441.2011.584041 |s2cid=122866773}}</ref> Chomsky's own theory is that language emerged in an instant and in perfect form,<ref>Chomsky, N. 2005. 'Three factors in language design.' ''Linguistic Inquiry'' 36(1): 1–22.</ref> prompting his critics in turn, to retort that only something that does not exist—a theoretical construct or convenient scientific fiction—could possibly emerge in such a miraculous way.<ref name="Knight2008" /> The controversy remains unresolved.

=== Tool resiliency, grammar and language production ===
[[Acheulean]] tool use began during the [[Lower Paleolithic]] approximately 1.75 million years ago. Studies focusing on the lateralization of Acheulean tool production and language production have noted similar areas of blood flow when engaging in these activities separately; this theory suggests that the brain functions needed for the production of tools across generations is consistent with the brain systems required for producing language. Researchers used functional transcranial Doppler ultrasonography (fTDC) and had participants perform activities related to the creation of tools using the same methods during the Lower Paleolithic as well as a task designed specifically for word generation.<ref name="Uomini2013">{{Cite journal |last1=Uomini |first1=Natalie Thaïs |last2=Meyer |first2=Georg Friedrich |date=30 August 2013 |editor-last=Petraglia |editor-first=Michael D. |title=Shared Brain Lateralization Patterns in Language and Acheulean Stone Tool Production: A Functional Transcranial Doppler Ultrasound Study |journal=PLOS ONE |volume=8 |issue=8 |pages=e72693 |bibcode=2013PLoSO...872693U |doi=10.1371/journal.pone.0072693 |issn=1932-6203 |pmc=3758346 |pmid=24023634 |doi-access=free}}</ref> The purpose of this test was to focus on the planning aspect of Acheulean tool making and cued word generation in language (an example of cued word generation would be trying to list all words beginning with a given letter). Theories of language developing alongside tool use has been theorized by multiple individuals;<ref>{{Cite journal |last1=Stout |first1=Dietrich |last2=Chaminade |first2=Thierry |date=12 January 2012 |title=Stone tools, language and the brain in human evolution |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=367 |issue=1585 |pages=75–87 |doi=10.1098/rstb.2011.0099 |pmc=3223784 |pmid=22106428}}</ref><ref>{{Cite journal |last1=Putt |first1=Shelby S. J. |last2=Anwarzai |first2=Zara |last3=Holden |first3=Chloe |last4=Ruck |first4=Lana |last5=Schoenemann |first5=P. Thomas |date=4 January 2022 |title=The evolution of combinatoriality and compositionality in hominid tool use: a comparative perspective |journal=International Journal of Primatology |volume=45 |issue=3 |pages=589–634 |doi=10.1007/s10764-021-00267-7 |issn=1573-8604 |s2cid=245654206}}</ref><ref>{{Cite journal |last1=Barham |first1=Lawrence |last2=Everett |first2=Daniel |date=June 2021 |title=Semiotics and the Origin of Language in the Lower Palaeolithic |journal=Journal of Archaeological Method and Theory |volume=28 |issue=2 |pages=535–579 |doi=10.1007/s10816-020-09480-9 |issn=1072-5369 |s2cid=225509049 |doi-access=free}}</ref> however, until recently, there has been little empirical data to support these hypotheses. Focusing on the results of the study performed by Uomini ''et al.'' evidence for the usage of the same brain areas has been found when looking at cued word generation and Acheulean tool use. The relationship between tool use and language production is found in working and planning memory respectively and was found to be similar across a variety of participants, furthering evidence that these areas of the brain are shared.<ref name="Uomini2013" /> This evidence lends credibility to the theory that language developed alongside tool use in the Lower Paleolithic.

=== Humanistic theory ===
The [[Humanism|humanistic]] tradition considers language as a human invention. [[Renaissance philosophy|Renaissance philosopher]] [[Antoine Arnauld]] gave a detailed description of his idea of the origin of language in [[Port-Royal Grammar]]. According to Arnauld, people are social and rational by nature, and this urged them to create language as a means to communicate their ideas to others. Language construction would have occurred through a slow and gradual process.<ref>{{Cite book |last1=Arnauld |first1=Antoine |author-link1=Antoine Arnauld |url=https://archive.org/details/portroyalgrammar0000lanc |title=General and Rational Grammar: The Port-Royal Grammar |last2=Lancelot |first2=Claude |publisher=Mouton |year=1975 |isbn=902793004X |location=The Hague |url-access=registration |orig-year=1660}}</ref> In later theory, especially in [[functional linguistics]], the primacy of communication
is emphasised over psychological needs.<ref name="Daneš1987">{{Cite book |last=Daneš |first=František |author-link=František Daneš |title=Functionalism in Linguistics |publisher=John Benjamins |year=1987 |isbn=9789027215246 |editor-last=Dirven |editor-first=R. |pages=3–38 |chapter=On Prague school functionalism in linguistics |editor-last2=Fried |editor-first2=V.}}</ref>

The exact way language evolved is however not considered as vital to the study of languages. [[Structural linguistics|Structural linguist]] [[Ferdinand de Saussure]] abandoned [[evolutionary linguistics]] after having come to the firm conclusion that it would not be able to provide any further revolutionary insight after the completion of the major works in [[historical linguistics]] by the end of the 19th century. Saussure was particularly sceptical of the attempts of [[August Schleicher]] and other Darwinian linguists to access prehistorical languages through series of reconstructions of [[proto-language]]s.<ref>{{Cite book |last=Aronoff |first=Mark |url=https://langsci-press.org/catalog/book/151 |title=On Looking into Words (and Beyond): Structures, Relations, Analyses |publisher=SUNY Press |year=2017 |isbn=978-3-946234-92-0 |editor-last=Bowern |pages=443–456 |chapter=Darwinism tested by the science of language |access-date=3 March 2020 |editor-last2=Horn |editor-last3=Zanuttini}}</ref>

Saussure's solution to the problem of language evolution involves dividing [[theoretical linguistics]] in two. Evolutionary and historical linguistics are renamed as [[diachronic linguistics]]. It is the study of [[language change]], but it has only limited explanatory power due to the inadequacy of all of the reliable research material that could ever be made available. [[Synchronic linguistics]], in contrast, aims to widen scientists' understanding of language through a study of a given contemporary or historical language stage as a system in its own right.<ref>{{Cite book |last=de Saussure |first=Ferdinand |author-link=Ferdinand de Saussure |url=https://monoskop.org/images/0/0b/Saussure_Ferdinand_de_Course_in_General_Linguistics_1959.pdf |title=Course in general linguistics |publisher=Philosophy Library |year=1959 |isbn=978-0-231-15727-8 |location=New York |access-date=6 May 2020 |archive-url=https://web.archive.org/web/20190808231716/https://monoskop.org/images/0/0b/Saussure_Ferdinand_de_Course_in_General_Linguistics_1959.pdf |archive-date=8 August 2019 |url-status=dead |orig-year=1916}}</ref>

Although Saussure put much focus on diachronic linguistics, later structuralists who equated structuralism with the synchronic analysis were sometimes criticised of ahistoricism. According to [[Structural anthropology|structural anthropologist]] [[Claude Lévi-Strauss]], language and meaning—in opposition to "knowledge, which develops slowly and progressively"—must have appeared in an instant.<ref>{{Cite book |last=Lévi-Strauss |first=Claude |title=Introduction to the work of Marcel Mauss |publisher=Routledge |year=1987 |isbn=0-7100-9066-8 |pages=59–60}}</ref>

Structuralism, as first introduced to [[sociology]] by [[Émile Durkheim]], is nonetheless a type of humanistic evolutionary theory which explains diversification as necessitated by growing complexity.<ref>{{Cite book |last=Hejl |first=P. M. |title=Biology as Society, Society as Biology: Metaphors |publisher=Springer |year=2013 |isbn=9789401106733 |editor-last=Maasen |editor-first=Sabine |pages=155–191 |chapter=The importance of the concepts of 'organism' and 'evolution' in Emile Durkheim's division of social labor and the influence of Herbert Spencer |editor-last2=Mendelsohn |editor-first2=E. |editor-last3=Weingart |editor-first3=P.}}</ref> There was a shift of focus to functional explanation after Saussure's death. Functional structuralists including the [[Prague Linguistic Circle|Prague Circle]] linguists and [[André Martinet]] explained the growth and maintenance of structures as being necessitated by their functions.<ref name="Daneš1987" /> For example, novel technologies make it necessary for people to invent new words, but these may lose their function and be forgotten as the technologies are eventually replaced by more modern ones.

=== Chomsky's single-step theory ===
According to Chomsky's single-mutation theory, the emergence of language resembled the formation of a crystal; with [[digital infinity]] as the [[seed crystal]] in a super-saturated primate brain, on the verge of blossoming into the human mind, by physical law, once [[Universal Darwinism|evolution]] added a single small but crucial keystone.<ref>Chomsky, N. (2004). ''Language and Mind: Current thoughts on ancient problems''. Part&nbsp;I & Part&nbsp;II. In Lyle Jenkins (ed.), ''Variation and Universals in Biolinguistics''. Amsterdam: Elsevier, pp.&nbsp;379–405.</ref><ref>{{Cite journal |last=Chomsky |first=N. |year=2005 |title=Three factors in language design |journal=Linguistic Inquiry |volume=36 |issue=1 |pages=1–22 |doi=10.1162/0024389052993655 |s2cid=14954986}}</ref> Thus, in this theory, language appeared rather suddenly within the history of human evolution. Chomsky, writing with computational linguist and computer scientist Robert C.&nbsp;Berwick, suggests that this scenario is completely compatible with modern biology. They note that "none of the recent accounts of human language evolution seem to have completely grasped the shift from conventional Darwinism to its fully [[stochastic]] modern version—specifically, that there are stochastic effects not only due to sampling like directionless drift, but also due to directed stochastic variation in fitness, migration, and heritability—indeed, all the "forces" that affect individual or gene frequencies{{Nbsp}}... All this can affect evolutionary outcomes—outcomes that as far as we can make out are not brought out in recent books on the evolution of language, yet would arise immediately in the case of any new genetic or individual innovation, precisely the kind of scenario likely to be in play when talking about language's emergence."

Citing evolutionary geneticist [[Svante Pääbo]], they concur that a substantial difference must have occurred to differentiate ''[[Homo sapiens]]'' from [[Neanderthal]]s to "prompt the relentless spread of our species, who had never crossed open water, up and out of Africa and then on across the entire planet in just a few tens of thousands of years.{{Nbsp}}... What we do not see is any kind of 'gradualism' in new tool technologies or innovations like fire, shelters, or figurative art." Berwick and Chomsky therefore suggest language emerged approximately between 200,000 years ago and 60,000 years ago (between the appearance of the first anatomically modern humans in southern Africa and the last exodus from Africa respectively). "That leaves us with about 130,000 years, or approximately 5,000–6,000 generations of time for evolutionary change. This is not 'overnight in one generation' as some have (incorrectly) inferred—but neither is it on the scale of geological eons. It's time enough—within the ballpark for what Nilsson and Pelger (1994) estimated as the time required for the full evolution of a [[vertebrate]] eye from a single cell, even without the invocation of any 'evo-devo' effects."<ref>{{Cite book |last1=Berwick |first1=Robert |title=Why Only Us: Language and Evolution |last2=Chomsky |first2=Noam |publisher=MIT Press |year=2016 |isbn=978-0-262-03424-1 |location=Cambridge, MA}}</ref>

The single-mutation theory of language evolution has been directly questioned on different grounds. A formal analysis of the probability of such a mutation taking place and going to fixation in the species has concluded that such a scenario is unlikely, with multiple mutations with more moderate fitness effects being more probable.<ref>{{Cite journal |last1=de Boer |first1=Bart |last2=Thompson |first2=Bill |last3=Ravignani |first3=Andrea |last4=Boeckx |first4=Cedric |date=16 January 2020 |title=Evolutionary Dynamics Do Not Motivate a Single-Mutant Theory of Human Language |journal=Scientific Reports |volume=10 |issue=1 |page=451 |bibcode=2020NatSR..10..451D |doi=10.1038/s41598-019-57235-8 |issn=2045-2322 |pmc=6965110 |pmid=31949223}}</ref> Another criticism has questioned the logic of the argument for single mutation and puts forward that from the formal simplicity of [[Merge (linguistics)|Merge]], the capacity Berwick and Chomsky deem the core property of human language that emerged suddenly, one cannot derive the (number of) evolutionary steps that led to it.<ref>{{Cite journal |last1=Martins |first1=Pedro Tiago |last2=Boeckx |first2=Cedric |date=27 November 2019 |title=Language evolution and complexity considerations: The no half-Merge fallacy |journal=PLOS Biology |volume=17 |issue=11 |pages=e3000389 |doi=10.1371/journal.pbio.3000389 |issn=1545-7885 |pmc=6880980 |pmid=31774810 |doi-access=free}}</ref>

=== The Romulus and Remus hypothesis ===
{{See also|Recursion#In language|Prefrontal synthesis}}

The Romulus and Remus hypothesis, proposed by neuroscientist [[Andrey Vyshedskiy]], seeks to address the question as to why the modern speech apparatus originated over 500,000 years before the earliest signs of modern human imagination. This hypothesis proposes that there were two phases that led to modern recursive language. The phenomenon of [[Recursion#In language|recursion]] occurs across multiple linguistic domains, arguably most prominently in [[syntax]] and [[Morphology (linguistics)|morphology]]. Thus, by nesting a structure such as a sentence or a word within themselves, it enables the generation of potentially ([[Countable set|countably]]) infinite new variations of that structure. For example, the base sentence [Peter likes apples.] can be nested in [[Irrealis mood|irrealis]] clauses to produce [Mary said [Peter likes apples.]], [Paul believed [Mary said [Peter likes apples.]]] and so forth.<ref>{{Cite book |last=Carnie |first=Andrew |author-link=Andrew Carnie |url=https://books.google.com/books?id=MFZ1UV3YGtgC |title=Syntax: A Generative Introduction |publisher=Wiley-Blackwell |year=2012 |isbn=978-0-470-65531-3 |edition=3rd |location=West Sussex |pages=20–21}}</ref>

The first phase includes the slow development of non-recursive language with a large vocabulary along with the modern speech apparatus, which includes changes to the hyoid bone, increased voluntary control of the muscles of the diaphragm, and the evolution of the FOXP2 gene, as well as other changes by 600,000&nbsp;years ago.<ref>{{Cite journal |last1=Dediu |first1=Dan |last2=Levinson |first2=Stephen C. |year=2013 |title=On the antiquity of language: the reinterpretation of Neandertal linguistic capacities and its consequences |journal=Frontiers in Psychology |volume=4 |page=397 |doi=10.3389/fpsyg.2013.00397 |issn=1664-1078 |pmc=3701805 |pmid=23847571 |doi-access=free}}</ref> Then, the second phase was a rapid [[#Chomsky's single-step theory|Chomskian single step]], consisting of three distinct events that happened in quick succession around 70,000&nbsp;years ago and allowed the shift from non-recursive to recursive language in early hominins.
# A genetic mutation that slowed down the [[prefrontal synthesis]] (PFS) critical period of at least two children that lived together.
# This allowed these children to create recursive elements of language such as spatial prepositions.
# Then this merged with their parents' non-recursive language to create recursive language.<ref name="Vyshedskiy2019">{{Cite journal |last=Vyshedskiy |first=Andrey |date=29 July 2019 |title=Language evolution to revolution: the leap from rich-vocabulary non-recursive communication system to recursive language 70,000 years ago was associated with acquisition of a novel component of imagination, called Prefrontal Synthesis, enabled by a mutation that slowed down the prefrontal cortex maturation simultaneously in two or more children&nbsp;– the Romulus and Remus hypothesis |journal=Research Ideas and Outcomes |volume=5 |doi=10.3897/rio.5.e38546 |issn=2367-7163 |doi-access=free}}</ref>

It is not enough for children to have a modern prefrontal cortex (PFC) to allow the development of PFS; the children must also be mentally stimulated and have recursive elements already in their language to acquire PFS. Since their parents would not have invented these elements yet, the children would have had to do it themselves, which is a common occurrence among young children that live together, in a process called [[cryptophasia]].<ref>{{Cite journal |last=Bakker |first=Peter |date=July 1987 |title=Autonomous Languages of Twins |journal=Acta Geneticae Medicae et Gemellologiae: Twin Research |volume=36 |issue=2 |pages=233–238 |doi=10.1017/S0001566000004463 |issn=0001-5660 |pmid=3434134 |doi-access=free}}</ref> This means that delayed PFC development would have allowed more time to acquire PFS and develop recursive elements.

Delayed PFC development also comes with negative consequences, such as a longer period of reliance on one's parents to survive and lower survival rates. For modern language to have occurred, PFC delay had to have an immense survival benefit in later life, such as PFS ability. This suggests that the mutation that caused PFC delay and the development of recursive language and PFS occurred simultaneously, which lines up with evidence of a genetic bottleneck around 70,000&nbsp;years ago.<ref>{{Cite journal |last1=Amos W. |last2=Hoffman J. I. |date=7 January 2010 |title=Evidence that two main bottleneck events shaped modern human genetic diversity |journal=Proceedings of the Royal Society B: Biological Sciences |volume=277 |issue=1678 |pages=131–137 |doi=10.1098/rspb.2009.1473 |pmc=2842629 |pmid=19812086}}</ref> This could have been the result of a few individuals who developed PFS and recursive language which gave them significant competitive advantage over all other humans at the time.<ref name="Vyshedskiy2019" />

=== Gestural theory ===
The gestural theory states that human language developed from [[gesture]]s that were used for simple communication.

Two types of evidence support this theory.
# Gestural language and vocal language depend on similar neural systems. The regions on the [[Cerebral cortex|cortex]] that are responsible for mouth and hand movements border each other.
# Nonhuman [[primates]] can use gestures or symbols for at least primitive communication, and some of their gestures resemble those of humans, such as the "begging posture", with the hands stretched out, which humans share with chimpanzees.<ref>Premack, David & Premack, Ann James. ''The Mind of an Ape'', {{ISBN|0-393-01581-5}}.</ref><ref>{{Cite journal |last1=Pollick |first1=A. S. |last2=de Waal |first2=F. B. |date=May 2007 |title=Ape Gestures and Language Evolution |journal=Proceedings of the National Academy of Sciences |volume=104 |issue=19 |pages=8184–8189 |bibcode=2007PNAS..104.8184P |doi=10.1073/pnas.0702624104 |pmc=1876592 |pmid=17470779 |doi-access=free}}</ref>

Research has found strong support for the idea that [[oral communication]] and sign language depend on similar neural structures. Patients who used sign language, and who suffered from a left-[[Cerebral hemisphere|hemisphere]] [[lesion]], showed the same disorders with their sign language as vocal patients did with their oral language.<ref>{{Cite book |last=Kimura |first=Doreen |title=Neuromotor mechanisms in human communication |publisher=Oxford University Press |year=1993 |isbn=978-0-19-505492-7 |location=New York}}</ref> Other researchers found that the same left-hemisphere brain regions were active during sign language as during the use of vocal or written language.<ref>{{Cite journal |last=Newman |first=A. J. |display-authors=etal |year=2002 |title=A Critical Period for Right Hemisphere Recruitment in American Sign Language Processing |journal=Nature Neuroscience |volume=5 |issue=1 |pages=76–80 |doi=10.1038/nn775 |pmid=11753419 |s2cid=2745545}}</ref>

Primate gesture is at least partially genetic: different nonhuman apes will perform gestures characteristic of their species, even if they have never seen another ape perform that gesture. For example, gorillas beat their breasts. This shows that gestures are an intrinsic and important part of primate communication, which supports the idea that language evolved from gesture.<ref name="Arbib2008">{{Cite journal |last1=Arbib |first1=M. A. |last2=Liebal |first2=K |last3=Pika |first3=S. |date=December 2008 |title=Primate vocalization, gesture, and the evolution of human language |journal=Current Anthropology |volume=49 |issue=6 |pages=1053–1076 |doi=10.1086/593015 |pmid=19391445 |s2cid=18832100}}</ref>

Further evidence suggests that gesture and language are linked. In humans, manually gesturing has an effect on concurrent vocalizations, thus creating certain natural vocal associations of manual efforts. Chimpanzees move their mouths when performing fine motor tasks. These mechanisms may have played an evolutionary role in enabling the development of intentional vocal communication as a supplement to gestural communication. Voice modulation could have been prompted by preexisting manual actions.<ref name="Arbib2008" />

From infancy, gestures both supplement and predict speech.<ref>{{Cite journal |last1=Capone |first1=Nina C. |last2=McGregor |first2=Karla K. |year=2004 |title=Gesture Development |journal=Journal of Speech, Language, and Hearing Research |volume=47 |issue=1 |pages=173–186 |doi=10.1044/1092-4388(2004/015) |pmid=15072537 |s2cid=7244799}}</ref><ref>{{Cite journal |last1=Ozçalişkan |first1=S. |last2=Goldin-Meadow |first2=S. |date=July 2005 |title=Gesture is at the cutting edge of early language development |journal=Cognition |volume=96 |issue=3 |pages=B101–B113 |doi=10.1016/j.cognition.2005.01.001 |pmid=15996556 |s2cid=206863317}}</ref> This addresses the idea that gestures quickly change in humans from a sole means of communication (from a very young age) to a supplemental and predictive behavior that is used despite the ability to communicate verbally. This too serves as a parallel to the idea that gestures developed first and language subsequently built upon it.

Two possible scenarios have been proposed for the development of language,<ref name="Rizzolatti">Rizzolatti, G. (2008). Giacomo Rizzolatti on the Evolution of Language. Retrieved from http://gocognitive.net/interviews/evolution-language-gestures{{full citation needed|date=January 2015}}</ref> one of which supports the gestural theory:
# Language developed from the calls of human ancestors.
# Language was derived from gesture.
The first perspective that language evolved from the calls of human ancestors seems logical because both humans and animals make sounds or cries. One evolutionary reason to refute this is that, anatomically, the centre that controls calls in monkeys and other animals is located in a completely different part of the brain than in humans. In monkeys, this centre is located in the depths of the brain related to emotions. In the human system, it is located in an area unrelated to emotion. Humans can communicate simply to communicate—without emotions. So, anatomically, this scenario does not work.<ref name="Rizzolatti" /> This suggests that language was derived from gesture<ref name="Kendon2017">{{Cite journal |last=Kendon |first=Adam |date=February 2017 |title=Reflections on the "gesture-first" hypothesis of language origins |journal=Psychonomic Bulletin & Review |volume=24 |issue=1 |pages=163–170 |doi=10.3758/s13423-016-1117-3 |pmc=5325861 |pmid=27439503}}</ref>(humans communicated by gesture first and sound was attached later).

The important question for gestural theories is why there was a shift to vocalization. Various explanations have been proposed:
# Human ancestors started to use more and more tools, meaning that their hands were occupied and could no longer be used for gesturing.<ref>{{Cite book |last=Corballis |first=Michael C. |title=The transition to language |publisher=Oxford University Press |year=2002 |isbn=978-0-19-925066-0 |editor-last=Wray |editor-first=Alison |pages=161–179}}</ref>
# Manual gesturing requires that speakers and listeners be visible to one another. In many situations, they might need to communicate, even without visual contact—for example after nightfall or when foliage obstructs visibility.
# A composite hypothesis holds that early language took the form of part gestural and part vocal [[mimesis]] (imitative 'song-and-dance'), combining modalities because all signals (like those of nonhuman apes and monkeys) still needed to be costly in order to be intrinsically convincing. In that event, each multi-media display would have needed not just to disambiguate an intended meaning but also to inspire confidence in the signal's reliability. The suggestion is that only once community-wide contractual understandings had come into force<ref>{{Cite book |last=Knight |first=Chris |url=http://www.chrisknight.co.uk/wp-content/uploads/2008/01/knight-springer-online-fulltext.pdf |title=The evolution of language: proceedings of the 6th international conference (EVOLANG6), Rome, Italy, 12–15 April 200 |publisher=World Scientific |year=2006 |isbn=9789812566560 |editor-last=Cangelosi |editor-first=Angelo |volume=7 |location=New Jersey |pages=109–128 |chapter=Language co-evolved with the rule of law |journal=Mind & Society |doi=10.1007/s11299-007-0039-1 |editor-last2=Smith |editor-first2=Andrew D. M. |editor-last3=Smith |editor-first3=Kenny |s2cid=143877486}}</ref> could trust in communicative intentions be automatically assumed, at last allowing ''Homo sapiens'' to shift to a more efficient default format. Since vocal distinctive features (sound contrasts) are ideal for this purpose, it was only at this point—when intrinsically persuasive body-language was no longer required to convey each message—that the decisive shift from manual gesture to the current primary reliance on ''spoken'' language occurred.<ref name="Knight2008" /><ref name="Knight1998" /><ref>{{Cite book |last=Knight |first=Chris |title=The Evolutionary emergence of language: social function and the origins of linguistic for |publisher=Cambridge University Press |year=2000 |isbn=978-0-521-78157-2 |editor-last=Chris Knight |pages=99–1119 |chapter=Play as precursor of phonology and syntax |editor-last2=Michael Studdert-Kennedy |editor-last3=James R. Hurford}}</ref>
A comparable hypothesis states that in 'articulate' language, gesture and vocalisation are intrinsically linked, as language evolved from equally intrinsically linked dance and song.<ref name="Vaneechoutte2014" />

Humans still use manual and facial gestures when they speak, especially when people meet who have no language in common.<ref>{{Cite book |last1=Kolb, Bryan |title=Fundamentals of Human Neuropsychology |last2=Ian Q. Whishaw |publisher=Worth Publishers |year=2003 |isbn=978-0-7167-5300-1 |edition=5th |name-list-style=amp}}</ref> There are also a great number of [[sign language]]s still in existence, commonly associated with Deaf communities. These sign languages are equal in complexity, sophistication, and expressive power, to any oral language.<ref>Sandler, Wendy; & Lillo-Martin, Diane. (2006). Sign Language and Linguistic Universals. Cambridge University Press.</ref> The cognitive functions are similar and the parts of the brain used are similar. The main difference is that the "phonemes" are produced on the outside of the body, articulated with hands, body, and facial expression, rather than inside the body articulated with tongue, teeth, lips, and breathing.<ref>{{Cite book |last=Meena |first=Ram Lakhan |title=Current Trends of Applied Linguistics |publisher=K. K. Publications |year=2021 |page=48}}</ref> (Compare the [[motor theory of speech perception]].)

Critics of gestural theory note that it is difficult to name serious reasons why the initial pitch-based vocal communication (which is present in primates) would be abandoned in favor of the much less effective non-vocal, gestural communication.<ref>{{Cite journal |last1=Hewes |first1=Gordon W. |last2=Andrew |first2=R. J. |last3=Carini |first3=Louis |last4=Choe |first4=Hackeny |last5=Gardner |first5=R. Allen |last6=Kortlandt |first6=A. |last7=Krantz |first7=Grover S. |last8=McBride |first8=Glen |last9=Nottebohm |first9=Fernando |last10=Pfeiffer |first10=John |last11=Rumbaugh |first11=Duane G. |last12=Steklis |first12=Horst D. |last13=Raliegh |first13=Michael J. |last14=Stopa |first14=Roman |last15=Suzuki |first15=Akira |year=1973 |title=Primate Communication and the Gestural Origin of Language [and Comments and Reply] |journal=Current Anthropology |volume=14 |issue=1/2 |pages=5–24 |doi=10.1086/201401 |jstor=2741093 |s2cid=146288708 |last16=Washburn |first16=S. L. |last17=Wescott |first17=Roger W.}}</ref> However, [[Michael Corballis]] has pointed out that it is supposed that primate vocal communication (such as alarm calls) cannot be controlled consciously, unlike hand movement, and thus it is not credible as precursor to human language; primate vocalization is rather homologous to and continued in involuntary reflexes (connected with basic human emotions) such as screams or laughter (the fact that these can be faked does not disprove the fact that genuine involuntary responses to fear or surprise exist).<ref name="Kendon2017" /> Also, gesture is not generally less effective, and depending on the situation can even be advantageous, for example in a loud environment or where it is important to be silent, such as on a hunt. Other challenges to the "gesture-first" theory have been presented by researchers in [[psycholinguistics]], including [[David McNeill]].<ref>{{Cite journal |last1=McNeill |first1=David |last2=Bertenthal |first2=Bennett |last3=Cole |first3=Jonathan |last4=Gallagher |first4=Shaun |date=April 2005 |title=Gesture-first, but no gestures? |journal=Behavioral and Brain Sciences |volume=28 |issue=2 |pages=138–139 |doi=10.1017/S0140525X05360031 |s2cid=51753637}}</ref>

=== Tool-use associated sound in the evolution of language ===
Proponents of the motor theory of language evolution have primarily focused on the visual domain and communication through observation of movements. The ''Tool-use sound hypothesis'' suggests that the production and perception of sound also contributed substantially, particularly ''incidental sound of locomotion'' (''ISOL'') and ''tool-use sound'' (''TUS'').<ref name="Larsson2015">{{Cite journal |last=Larsson |first=M |year=2015 |title=Tool-use-associated sound in the evolution of language |journal=Animal Cognition |volume=18 |issue=5 |pages=993–1005 |doi=10.1007/s10071-015-0885-x |pmid=26118672 |s2cid=18714154}}</ref> Human bipedalism resulted in rhythmic and more predictable ''ISOL''. That may have stimulated the evolution of musical abilities, auditory working memory, and abilities to produce complex vocalizations, and to mimic natural sounds.<ref>{{Cite journal |last=Larsson |first=M |year=2014 |title=Self-generated sounds of locomotion and ventilation and the evolution of human rhythmic abilities |journal=Animal Cognition |volume=17 |issue=1 |pages=1–14 |doi=10.1007/s10071-013-0678-z |pmc=3889703 |pmid=23990063}}</ref> Since the human brain proficiently extracts information about objects and events from the sounds they produce, ''TUS'', and mimicry of ''TUS'', might have achieved an iconic function. The prevalence of sound symbolism in many extant languages supports this idea. Self-produced ''TUS'' activates multimodal brain processing ([[motor neuron]]s, hearing, [[proprioception]], touch, vision), and ''TUS'' stimulates primate audiovisual mirror neurons, which is likely to stimulate the development of association chains. Tool use and auditory gestures involve motor-processing of the forelimbs, which is associated with the evolution of vertebrate vocal communication. The production, perception, and mimicry of ''TUS'' may have resulted in a limited number of vocalizations or protowords that were associated with tool use.<ref name="Larsson2015" /> A new way to communicate about tools, especially when out of sight, would have had selective advantage. A gradual change in acoustic properties, meaning, or both could have resulted in arbitrariness and an expanded repertoire of words. Humans have been increasingly exposed to ''TUS'' over millions of years, coinciding with the period during which spoken language evolved.

=== Mirror neurons and language origins ===
In humans, [[functional magnetic resonance imaging|functional MRI]] studies have reported finding areas homologous to the monkey [[mirror neuron]] system in the [[frontal lobe|inferior frontal cortex]], close to [[Broca's area]], one of the language regions of the brain. This has led to suggestions that human language evolved from a gesture performance/understanding system implemented in mirror neurons. Mirror neurons have been said to have the potential to provide a mechanism for action-understanding, imitation-learning, and the simulation of other people's behavior.<ref>[[John Skoyles (scientist)|Skoyles, John R.]], ''Gesture, Language Origins, and Right Handedness'', Psychology: 11,#24, 2000</ref> This hypothesis is supported by some [[Cytoarchitectonics|cytoarchitectonic]] homologies between monkey premotor area F5 and human Broca's area.<ref name="Petrides2005">{{Cite journal |last1=Petrides |first1=M. |last2=Cadoret |first2=G. |last3=Mackey |first3=S. |date=June 2005 |title=Orofacial somatomotor responses in the macaque monkey homologue of Broca's area |journal=Nature |volume=435 |issue=7046 |pages=1235–1238 |bibcode=2005Natur.435.1235P |doi=10.1038/nature03628 |pmid=15988526 |s2cid=4397762}}</ref>

Rates of vocabulary expansion link to the ability of children to vocally mirror non-words and so to acquire the new word pronunciations. Such [[speech repetition]] occurs automatically, quickly<ref>{{Cite journal |last1=Porter |first1=R. J. |last2=Lubker |first2=J. F. |date=September 1980 |title=Rapid reproduction of vowel-vowel sequences: evidence for a fast and direct acoustic-motoric linkage in speech |journal=Journal of Speech and Hearing Research |volume=23 |issue=3 |pages=593–602 |doi=10.1044/jshr.2303.593 |pmid=7421161}}</ref> and separately in the brain to [[speech perception]].<ref>{{Cite journal |last1=McCarthy |first1=R. |last2=Warrington |first2=E. K. |date=June 1984 |title=A two-route model of speech production. Evidence from aphasia. |journal=Brain |volume=107 |issue=2 |pages=463–485 |doi=10.1093/brain/107.2.463 |pmid=6722512 |doi-access=free}}</ref><ref>{{Cite journal |last1=McCarthy |first1=R. A. |last2=Warrington |first2=E. K. |year=2001 |title=Repeating without semantics: surface dysphasia? |journal=Neurocase |volume=7 |issue=1 |pages=77–87 |doi=10.1093/neucas/7.1.77 |pmid=11239078 |s2cid=12988855}}</ref> Moreover, such vocal imitation can occur without comprehension such as in [[speech shadowing]]<ref>{{Cite journal |last=Marslen-Wilson |first=W. |year=1973 |title=Linguistic structure and speech shadowing at very short latencies |journal=Nature |volume=244 |issue=5417 |pages=522–523 |bibcode=1973Natur.244..522M |doi=10.1038/244522a0 |pmid=4621131 |s2cid=4220775}}</ref> and [[echolalia]].<ref name="Petrides2005" /><ref>{{Cite journal |last1=Fay |first1=W. H. |last2=Coleman |first2=R. O. |date=July 1977 |title=A human sound transducer/reproducer: temporal capabilities of a profoundly echolalic child |journal=Brain and Language |volume=4 |issue=3 |pages=396–402 |doi=10.1016/0093-934x(77)90034-7 |pmid=907878 |s2cid=29492873}}</ref> Further evidence for this link comes from a recent study in which the brain activity of two participants was measured using fMRI while they were gesturing words to each other using hand gestures with a game of [[charades]]—a modality that some have suggested might represent the evolutionary precursor of human language. Analysis of the data using [[Granger Causality]] revealed that the mirror-neuron system of the observer indeed reflects the pattern of activity of in the motor system of the sender, supporting the idea that the motor concept associated with the words is indeed transmitted from one brain to another using the mirror system.<ref>{{Cite journal |last1=Schippers |first1=M. B. |last2=Roebroeck |first2=A |last3=Renken |first3=R. |last4=Nanetti |first4=L. |last5=Keysers |first5=C. |year=2010 |title=Mapping the Information flow from one brain to another during gestural communication |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=107 |issue=20 |pages=9388–9393 |bibcode=2010PNAS..107.9388S |doi=10.1073/pnas.1001791107 |pmc=2889063 |pmid=20439736 |doi-access=free}}</ref>

Not all linguists agree with the above arguments, however. In particular, supporters of Noam Chomsky argue against the possibility that the mirror neuron system can play any role in the hierarchical recursive structures essential to syntax.<ref>{{Cite book |last=Moro |first=Andrea |title=The boundaries of Babel: the brain and the enigma of impossible language |publisher=MIT Press |year=2008 |isbn=978-0-262-13498-9 |location=Cambridge, MA}}{{page needed|date=March 2017}}</ref>

=== Putting-down-the-baby theory ===
According to [[Dean Falk]]'s "putting-down-the-baby" theory, vocal interactions between early hominid mothers and infants began a sequence of events that led, eventually, to human ancestors' earliest words.<ref name="Falk2004">{{Cite journal |last=Falk |first=D. |date=August 2004 |title=Prelinguistic evolution in early hominins: whence motherese? |url=http://www.cogsci.ucsd.edu/~johnson/COGS260/Falk2004.pdf |url-status=dead |journal=Behavioral and Brain Sciences |volume=27 |issue=4 |pages=491–583 |doi=10.1017/s0140525x04000111 |pmid=15773427 |s2cid=39547572 |archive-url=https://web.archive.org/web/20140104205636/http://www.cogsci.ucsd.edu/~johnson/COGS260/Falk2004.pdf |archive-date=4 January 2014 |access-date=4 January 2014}}</ref> The basic idea is that evolving human mothers, unlike their counterparts in other primates, could not move around and forage with their infants clinging onto their backs. Loss of fur in the human case left infants with no means of clinging on. Frequently, therefore, mothers had to put their babies down. As a result, these babies needed to be reassured that they were not being abandoned. Mothers responded by developing 'motherese'—an infant-directed communicative system embracing facial expressions, body language, touching, patting, caressing, laughter, tickling, and emotionally expressive contact calls. The argument is that language developed out of this interaction.<ref name="Falk2004" />

In ''[[The Mental and Social Life of Babies]]'', psychologist [[Kenneth Kaye]] noted that no usable adult language could have evolved without interactive communication between very young children and adults. "No symbolic system could have survived from one generation to the next if it could not have been easily acquired by young children under their normal conditions of social life."<ref>{{Cite book |last=Kaye |first=K. |url=https://archive.org/details/mentalsociallife0000kaye_a5t8/page/186 |title=The Mental and Social Life of Babies |publisher=University of Chicago Press |year=1982 |isbn=0-226-42848-6 |pages=[https://archive.org/details/mentalsociallife0000kaye_a5t8/page/186 186]}}</ref>

=== From-where-to-what theory ===
[[File:From where to what.png|thumb|An illustration of the "from where to what" model of language evolution]]
The "from where to what" model is a language evolution model that is derived primarily from the organization of [[language processing in the brain]] into two structures: the auditory dorsal stream and the auditory ventral stream.<ref>{{Cite journal |last=Poliva |first=Oren |date=20 September 2017 |title=From where to what: a neuroanatomically based evolutionary model of the emergence of speech in humans |journal=F1000Research |volume=4 |page=67 |doi=10.12688/f1000research.6175.3 |issn=2046-1402 |pmc=5600004 |pmid=28928931 |doi-access=free}}</ref><ref>{{Cite journal |last=Poliva |first=Oren |date=30 June 2016 |title=From Mimicry to Language: A Neuroanatomically Based Evolutionary Model of the Emergence of Vocal Language |journal=Frontiers in Neuroscience |volume=10 |page=307 |doi=10.3389/fnins.2016.00307 |issn=1662-453X |pmc=4928493 |pmid=27445676 |doi-access=free}}</ref> It hypothesizes seven stages of language evolution (see illustration). Speech originated for the purpose of exchanging contact calls between mothers and their offspring to find one another in the event they became separated (illustration part&nbsp;1). The contact calls could be modified with intonations in order to express either a higher or lower level of distress (illustration part&nbsp;2). The use of two types of contact calls enabled the first question-answer conversation. In this scenario, the child would emit a low-level distress call to express a desire to interact with an object, and the mother would respond with either another low-level distress call (to express approval of the interaction) or a high-level distress call (to express disapproval) (illustration part&nbsp;3). Over time, the improved use of intonations and vocal control led to the invention of unique calls (phonemes) associated with distinct objects (illustration part&nbsp;4). At first, children learned the calls (phonemes) from their parents by imitating their lip-movements (illustration part&nbsp;5). Eventually, infants were able to encode into long-term memory all the calls (phonemes). Consequentially, mimicry via lip-reading was limited to infancy and older children learned new calls through mimicry without lip-reading (illustration part&nbsp;6). Once individuals became capable of producing a sequence of calls, this allowed multi-syllabic words, which increased the size of their vocabulary (illustration part&nbsp;7). The use of words, composed of sequences of syllables, provided the infrastructure for communicating with sequences of words (i.e. sentences).

The theory's name is derived from the two auditory streams, which are both found in the brains of humans and other primates. The auditory ventral stream is responsible for sound recognition, and so it is referred to as the auditory ''what'' stream.<ref>{{Cite journal |last=Scott |first=S. K. |date=1 December 2000 |title=Identification of a pathway for intelligible speech in the left temporal lobe |journal=Brain |volume=123 |issue=12 |pages=2400–2406 |doi=10.1093/brain/123.12.2400 |issn=1460-2156 |pmc=5630088 |pmid=11099443}}</ref><ref>{{Cite journal |last1=Davis |first1=Matthew H. |last2=Johnsrude |first2=Ingrid S. |date=15 April 2003 |title=Hierarchical Processing in Spoken Language Comprehension |journal=The Journal of Neuroscience |volume=23 |issue=8 |pages=3423–3431 |doi=10.1523/jneurosci.23-08-03423.2003 |issn=0270-6474 |pmc=6742313 |pmid=12716950 |doi-access=free}}</ref><ref>{{Cite journal |last1=Petkov |first1=Christopher I. |last2=Kayser |first2=Christoph |last3=Steudel |first3=Thomas |last4=Whittingstall |first4=Kevin |last5=Augath |first5=Mark |last6=Logothetis |first6=Nikos K. |date=10 February 2008 |title=A voice region in the monkey brain |journal=Nature Neuroscience |volume=11 |issue=3 |pages=367–374 |doi=10.1038/nn2043 |issn=1097-6256 |pmid=18264095 |s2cid=5505773}}</ref> In primates, the auditory dorsal stream is responsible for [[sound localization]], and thus it is called the auditory ''where'' stream. Only in humans (in the left hemisphere) is it also responsible for other processes associated with language use and acquisition, such as speech repetition and production, integration of phonemes with their lip movements, perception and production of intonations, phonological [[long-term memory]] (long-term memory storage of the sounds of words), and phonological working memory (the temporary storage of the sounds of words).<ref>{{Cite journal |last1=Buchsbaum |first1=Bradley R. |last2=Baldo |first2=Juliana |last3=Okada |first3=Kayoko |last4=Berman |first4=Karen F. |last5=Dronkers |first5=Nina |last6=D'Esposito |first6=Mark |last7=Hickok |first7=Gregory |date=December 2011 |title=Conduction aphasia, sensory-motor integration, and phonological short-term memory – An aggregate analysis of lesion and fMRI data |journal=Brain and Language |volume=119 |issue=3 |pages=119–128 |doi=10.1016/j.bandl.2010.12.001 |issn=0093-934X |pmc=3090694 |pmid=21256582}}</ref><ref>{{Cite journal |last1=Warren |first1=Jane E. |last2=Wise |first2=Richard J.S. |last3=Warren |first3=Jason D. |date=December 2005 |title=Sounds do-able: auditory–motor transformations and the posterior temporal plane |journal=Trends in Neurosciences |volume=28 |issue=12 |pages=636–643 |doi=10.1016/j.tins.2005.09.010 |issn=0166-2236 |pmid=16216346 |s2cid=36678139}}</ref><ref>{{Cite journal |last=Campbell |first=Ruth |date=12 March 2008 |title=The processing of audio-visual speech: empirical and neural bases |journal=Philosophical Transactions of the Royal Society of London B: Biological Sciences |volume=363 |issue=1493 |pages=1001–1010 |doi=10.1098/rstb.2007.2155 |issn=0962-8436 |pmc=2606792 |pmid=17827105}}</ref><ref>{{Cite journal |last1=Kayser |first1=Christoph |last2=Petkov |first2=Christopher I. |last3=Logothetis |first3=Nikos K. |date=December 2009 |title=Multisensory interactions in primate auditory cortex: fMRI and electrophysiology |journal=Hearing Research |volume=258 |issue=1–2 |pages=80–88 |doi=10.1016/j.heares.2009.02.011 |issn=0378-5955 |pmid=19269312 |s2cid=31412246}}</ref><ref>{{Cite journal |last1=Hickok |first1=Gregory |last2=Buchsbaum |first2=Bradley |last3=Humphries |first3=Colin |last4=Muftuler |first4=Tugan |date=1 July 2003 |title=Auditory–Motor Interaction Revealed by fMRI: Speech, Music, and Working Memory in Area Spt |journal=Journal of Cognitive Neuroscience |volume=15 |issue=5 |pages=673–682 |doi=10.1162/089892903322307393 |issn=1530-8898 |pmid=12965041}}</ref><ref>{{Cite journal |last1=Schwartz |first1=M. F. |last2=Faseyitan |first2=O. |last3=Kim |first3=J. |last4=Coslett |first4=H. B. |date=20 November 2012 |title=The dorsal stream contribution to phonological retrieval in object naming |journal=Brain |volume=135 |issue=12 |pages=3799–3814 |doi=10.1093/brain/aws300 |issn=0006-8950 |pmc=3525060 |pmid=23171662}}</ref><ref>{{Cite journal |last=Gow |first=David W. |date=June 2012 |title=The cortical organization of lexical knowledge: A dual lexicon model of spoken language processing |journal=Brain and Language |volume=121 |issue=3 |pages=273–288 |doi=10.1016/j.bandl.2012.03.005 |issn=0093-934X |pmc=3348354 |pmid=22498237}}</ref><ref>{{Cite journal |last1=Buchsbaum |first1=Bradley R. |last2=D'Esposito |first2=Mark |date=May 2008 |title=The Search for the Phonological Store: From Loop to Convolution |journal=Journal of Cognitive Neuroscience |volume=20 |issue=5 |pages=762–778 |doi=10.1162/jocn.2008.20501 |issn=0898-929X |pmid=18201133 |s2cid=17878480}}</ref> Some evidence also indicates a role in recognizing others by their voices.<ref>{{Cite journal |last1=Lachaux |first1=Jean-Philippe |last2=Jerbi |first2=Karim |last3=Bertrand |first3=Olivier |last4=Minotti |first4=Lorella |last5=Hoffmann |first5=Dominique |last6=Schoendorff |first6=Benjamin |last7=Kahane |first7=Philippe |date=31 October 2007 |title=A Blueprint for Real-Time Functional Mapping via Human Intracranial Recordings |journal=PLOS ONE |volume=2 |issue=10 |pages=e1094 |bibcode=2007PLoSO...2.1094L |doi=10.1371/journal.pone.0001094 |issn=1932-6203 |pmc=2040217 |pmid=17971857 |doi-access=free}}</ref><ref>{{Cite journal |last1=Jardri |first1=Renaud |last2=Houfflin-Debarge |first2=Véronique |last3=Delion |first3=Pierre |last4=Pruvo |first4=Jean-Pierre |last5=Thomas |first5=Pierre |last6=Pins |first6=Delphine |date=April 2012 |title=Assessing fetal response to maternal speech using a noninvasive functional brain imaging technique |journal=International Journal of Developmental Neuroscience |volume=30 |issue=2 |pages=159–161 |doi=10.1016/j.ijdevneu.2011.11.002 |issn=0736-5748 |pmid=22123457 |s2cid=2603226}}</ref> The emergence of each of these functions in the auditory dorsal stream represents an intermediate stage in the evolution of language.

A contact call origin for human language is consistent with animal studies, as like human language, contact call discrimination in monkeys is lateralised to the left hemisphere.<ref>{{Cite journal |last1=Petersen |first1=M. |last2=Beecher |first2=M. |last3=Zoloth |last4=Moody |first4=D. |last5=Stebbins |first5=W. |date=20 October 1978 |title=Neural lateralization of species-specific vocalizations by Japanese macaques (Macaca fuscata) |journal=Science |volume=202 |issue=4365 |pages=324–327 |bibcode=1978Sci...202..324P |doi=10.1126/science.99817 |issn=0036-8075 |pmid=99817}}</ref><ref>{{Cite journal |last1=Heffner |first1=H. |last2=Heffner |first2=R. |date=5 October 1984 |title=Temporal lobe lesions and perception of species-specific vocalizations by macaques |journal=Science |volume=226 |issue=4670 |pages=75–76 |bibcode=1984Sci...226...75H |doi=10.1126/science.6474192 |issn=0036-8075 |pmid=6474192}}</ref> Mice with knock-out to language related genes (such as [[FOXP2]] and [[SRPX2]]) also resulted in the pups no longer emitting contact calls when separated from their mothers.<ref>{{Cite journal |last1=Shu |first1=W. |last2=Cho |first2=J. Y. |last3=Jiang |first3=Y. |last4=Zhang |first4=M. |last5=Weisz |first5=D. |last6=Elder |first6=G. A. |last7=Schmeidler |first7=J. |last8=De Gasperi |first8=R. |last9=Sosa |first9=M. A. G. |date=27 June 2005 |title=Altered ultrasonic vocalization in mice with a disruption in the Foxp2 gene |journal=Proceedings of the National Academy of Sciences |volume=102 |issue=27 |pages=9643–9648 |bibcode=2005PNAS..102.9643S |doi=10.1073/pnas.0503739102 |issn=0027-8424 |pmc=1160518 |pmid=15983371 |doi-access=free}}</ref><ref>{{Cite journal |last1=Sia |first1=G. M. |last2=Clem |first2=R. L. |last3=Huganir |first3=R. L. |date=31 October 2013 |title=The Human Language-Associated Gene SRPX2 Regulates Synapse Formation and Vocalization in Mice |journal=Science |volume=342 |issue=6161 |pages=987–991 |bibcode=2013Sci...342..987S |doi=10.1126/science.1245079 |issn=0036-8075 |pmc=3903157 |pmid=24179158}}</ref> Supporting this model is also its ability to explain unique human phenomena, such as the use of intonations when converting words into commands and questions, the tendency of infants to mimic vocalizations during the first year of life (and its disappearance later on) and the protruding and visible [[human lip]]s, which are not found in other apes. This theory could be considered an elaboration of the putting-down-the-baby theory of language evolution.

=== Grammaticalisation theory ===
"[[Grammaticalization]]" is a continuous historical process in which free-standing words develop into grammatical appendages, while these in turn become ever more specialized and grammatical. An initially "incorrect" usage, in becoming accepted, leads to [[unforeseen consequence]]s, triggering knock-on effects and extended sequences of change. Paradoxically, grammar evolves because, in the final analysis, humans care less about grammatical niceties than about making themselves understood.<ref>Sperber, D. and D. Wilson 1986. ''Relevance. Communication and cognition''. Oxford: Blackwell.</ref> If this is how grammar evolves today, according to this school of thought, similar principles at work can be legitimately inferred among distant human ancestors, when grammar itself was first being established.<ref name="Deutscher2005">{{Cite book |last=Deutscher |first=Guy |url=https://archive.org/details/unfoldingoflangu00deut |title=The unfolding of language: an evolutionary tour of mankind's greatest invention |publisher=Metropolitan |year=2005 |isbn=978-0-8050-7907-4 |location=New York}}</ref><ref>Hopper, P. J. 1998. Emergent grammar. In M. Tomasello (ed.), ''The New Psychology of Language''. Mahwah, NJ: Lawrence Erlbaum, 155–175.</ref><ref name="Heine2007">{{Cite book |last1=Heine |first1=Bernd |title=The genesis of grammar : a reconstructio |last2=Kuteva |first2=Tania |publisher=Oxford University Press |year=2007 |isbn=978-0-19-922777-8}}</ref>

In order to reconstruct the evolutionary transition from early language to languages with complex grammars, it is necessary to know which hypothetical sequences are plausible and which are not. In order to convey abstract ideas, the first recourse of speakers is to fall back on immediately recognizable concrete imagery, very often deploying [[metaphor]]s rooted in shared bodily experience.<ref name="Lakoff">Lakoff, G. and M. Johnson 1980. ''Metaphors We Live By''. Chicago: University of Chicago Press.</ref> A familiar example is the use of concrete terms such as "belly" or "back" to convey abstract meanings such as "inside" or "behind". Equally metaphorical is the strategy of representing temporal patterns on the model of spatial ones. For example, English speakers might say "It is going to rain", modelled on "I am going to London." This can be abbreviated colloquially to "It's gonna rain." Even when in a hurry, English speakers do not say "I'm gonna London"—the contraction is restricted to the job of specifying tense. From such examples it can be seen why grammaticalisation is consistently unidirectional—from concrete to abstract meaning, not the other way around.<ref name="Deutscher2005" />

Grammaticalization theorists picture early language as simple, perhaps consisting only of nouns.<ref name="Heine2007" /><sup>p.&nbsp;111</sup> Even under that extreme theoretical assumption, however, it is difficult to imagine what would realistically have prevented people from using, say, "spear" as if it were a verb ("Spear that pig!"). People might have used their nouns as verbs or their verbs as nouns as occasion demanded. In short, while a noun-only language might seem theoretically possible, grammaticalization theory indicates that it cannot have remained fixed in that state for any length of time.<ref name="Deutscher2005" /><ref name="Heine2012">{{Cite book |last1=Heine |first1=Bernd |title=The Oxford handbook of language evolution |last2=Kuteva |first2=Tania |publisher=Oxford University Press |year=2012 |isbn=978-0-19-954111-9 |editor-last=Maggie Tallerman |pages=512–527 |chapter=Grammaticalization theory as a tool for reconstructing language evolution |editor-last2=Kathleen R. Gibson}}</ref>

Creativity drives grammatical change.<ref name="Heine2012" /> This presupposes a certain attitude on the part of listeners. Instead of punishing deviations from accepted usage, listeners must prioritise imaginative mind-reading. Imaginative creativity—emitting a leopard alarm when no leopard was present, for example—is not the kind of behaviour which, say, [[vervet monkey]]s would appreciate or reward.<ref>{{Cite journal |last1=Cheney |first1=Dorothy L. |last2=Seyfarth |first2=Robert M. |year=2005 |title=Constraints and preadaptations in the earliest stages of language evolution |url=http://www.psych.upenn.edu/~seyfarth/Publications/LinguisticReview.pdf |journal=The Linguistic Review |volume=22 |issue=2–4 |pages=135–159 |doi=10.1515/tlir.2005.22.2-4.135 |s2cid=18939193}}</ref> Creativity and reliability are incompatible demands; for "[[Machiavellian intelligence hypothesis|Machiavellian]]" primates as for animals generally, the overriding pressure is to demonstrate reliability.<ref>{{Cite book |last1=Maynard Smith |first1=John |title=Animal signals |last2=Harper |first2=David |publisher=Oxford University Press |year=2003 |isbn=978-0-19-852684-1 |location=New York}}</ref> If humans escape these constraints, it is because in their case, listeners are primarily interested in mental states.

To focus on mental states is to accept fictions—inhabitants of the imagination—as potentially informative and interesting. An example is metaphor: a metaphor is, literally, a false statement.<ref>Davidson, R. D. 1979. What metaphors mean. In S. Sacks (ed.), ''On Metaphor''. Chicago: University of Chicago Press, pp.&nbsp;29–45.</ref> In ''[[Romeo and Juliet]]'', Romeo declares "Juliet is the sun!". Juliet is a woman, not a ball of plasma in the sky, but human listeners are not (or not usually) pedants insistent on point-by-point factual accuracy. They want to know what the speaker has in mind. Grammaticalisation is essentially based on metaphor. To outlaw its use would be to stop grammar from evolving and, by the same token, to exclude all possibility of expressing abstract thought.<ref name="Lakoff" /><ref>Lakoff, G. and R. Núñez 2000. ''Where mathematics comes from''. New York: Basic Books.</ref>

A criticism of all this is that while grammaticalization theory might explain language change today, it does not satisfactorily address the really difficult challenge—explaining the initial transition from primate-style communication to language as it is known today. Rather, the theory assumes that language already exists. As [[Bernd Heine]] and [[Tania Kuteva]] acknowledge: "Grammaticalisation requires a linguistic system that is used regularly and frequently within a community of speakers and is passed on from one group of speakers to another".<ref name="Heine2007" /> Outside modern humans, such conditions do not prevail.

=== Evolution-progression model ===

<!-- Should this heading be capitalised in this way? Should the hyphen be an en dash instead? (i.e. – instead of -)
— DesertPipeline 2021-05-08  -->Human language is used for self-expression; however, expression displays different stages. The consciousness of self and feelings represents the stage immediately prior to the external, phonetic expression of feelings in the form of sound (i.e. language). Intelligent animals such as dolphins, Eurasian magpies, and chimpanzees live in communities, wherein they assign themselves roles for group survival and show emotions such as sympathy.<ref>{{Cite journal |last=Gallup |first=G. G. Jr. |year=1970 |title=Chimpanzees: Self recognition |journal=Science |volume=167 |issue=3914 |pages=86–87 |bibcode=1970Sci...167...86G |doi=10.1126/science.167.3914.86 |pmid=4982211 |s2cid=145295899}}</ref> When such animals view their reflection ([[mirror test]]), they recognize themselves and exhibit [[self-consciousness]].<ref>{{Cite journal |last=Mitchell |first=R. W. |year=1995 |title=Evidence of dolphin self-recognition and the difficulties of interpretation |journal=Consciousness and Cognition |volume=4 |issue=2 |pages=229–234 |doi=10.1006/ccog.1995.1029 |pmid=8521261 |s2cid=45507064}}</ref> Notably, humans evolved in a quite different environment than that of these animals. Human survival became easier with the development of tools, shelter, and fire, thus facilitating further advancement of social interaction, self-expression, and tool-making, as for hunting and gathering.<ref>{{Cite journal |last=Ko |first=Kwang Hyun |year=2016 |title=Origins of human intelligence: The chain of tool-making and brain evolution |url=http://www.drustvo-antropologov.si/AN/PDF/2016_1/Anthropological_Notebooks_XXII_1_Ko.pdf |journal=Anthropological Notebooks |volume=22 |issue=1 |pages=5–22}}</ref> The increasing brain size allowed advanced provisioning and tools and the technological advances during the Palaeolithic era that built upon the previous evolutionary innovations of bipedalism and hand versatility allowed the development of human language.{{Citation needed|date=May 2018}}

=== Self-domesticated ape theory ===
According to a study investigating the song differences between [[white-rumped munia]]s and its domesticated counterpart ([[Society finch|Bengalese finch]]), the wild munias use a highly stereotyped song sequence, whereas the domesticated ones sing a highly unconstrained song. In wild finches, song syntax is subject to female preference—[[sexual selection]]—and remains relatively fixed. However, in the Bengalese finch, natural selection is replaced by breeding, in this case for colorful plumage, and thus, decoupled from selective pressures, stereotyped song syntax is allowed to drift. It is replaced, supposedly within 1000 generations, by a variable and learned sequence. Wild finches, moreover, are thought incapable of learning song sequences from other finches.<ref name="Soma2009">{{Cite journal |last1=Soma |first1=M. |last2=Hiraiwa-Hasegawa |first2=M. |last3=Okanoya |first3=K. |year=2009 |title=Early ontogenetic effects on song quality in the Bengalese finch (''Lonchura striata var. domestica''): laying order, sibling competition and song syntax |url=https://ir.soken.ac.jp/?action=repository_uri&item_id=3818 |journal=Behavioral Ecology and Sociobiology |volume=63 |issue=3 |pages=363–370 |doi=10.1007/s00265-008-0670-9 |bibcode=2009BEcoS..63..363S |s2cid=23137306}}</ref> In the field of [[bird vocalization]], brains capable of producing only an innate song have very simple neural pathways: the primary forebrain motor centre, called the robust nucleus of [[arcopallium]], connects to midbrain vocal outputs, which in turn project to brainstem motor nuclei. By contrast, in brains capable of learning songs, the arcopallium receives input from numerous additional forebrain regions, including those involved in learning and social experience. Control over song generation has become less constrained, more distributed, and more flexible.<ref name="Soma2009" />

One way to think about human evolution is that humans are [[Self-domestication#In humans|self-domesticated apes]]. Just as domestication relaxed selection for stereotypic songs in the finches—mate choice was supplanted by choices made by the aesthetic sensibilities of bird breeders and their customers—so might human cultural domestication have relaxed selection on many of their primate behavioural traits, allowing old pathways to degenerate and reconfigure. Given the highly indeterminate way that mammalian brains develop—they basically construct themselves "bottom up", with one set of neuronal interactions preparing for the next round of interactions—degraded pathways would tend to seek out and find new opportunities for synaptic hookups. Such inherited de-differentiations of brain pathways might have contributed to the functional complexity that characterises human language. And, as exemplified by the finches, such de-differentiations can occur in very rapid time-frames.<ref>{{Cite journal |last1=Ritchie |first1=Graham |last2=Kirby |first2=Simon |year=2005 |title=Selection, domestication, and the emergence of learned communication systems |url=http://homepages.inf.ed.ac.uk/s0237680/pubs/ritchie_05_selection.pdf |url-status=dead |journal=Second International Symposium on the Emergence and Evolution of Linguistic Communication |archive-url=https://web.archive.org/web/20120121153322/http://homepages.inf.ed.ac.uk/s0237680/pubs/ritchie_05_selection.pdf |archive-date=21 January 2012}}</ref>