Integration Hypothesis: A Parallel Model of
Language Development in Evolution*
Shigeru Miyagawa
MIT/UTokyo
Abstract
There are generally two views of how language emerged in evolution: emergent andgradual. The emergent view holds that language appeared relatively rapidly within thelast 100,000 years, possibly due to some minor mutation. The gradualist view postulates
stages of "protolanguage" that began as a simple system that progressively developed into
ever-complex systems until language as we know it emerged. The original protolanguage
may have been singing, as Darwin conjectured, or lexical in nature as proposed by anumber of linguists. Human language is enormously rich and complex, which makes it
difficult to imagine that all the components of it emerged somehow out of the blue inrecent evolutionary time, yet there is no evidence for such a system earlier in evolution.
The Integration Hypothesis holds that language is an integration of two independently
occurring systems in nature that underlie communication. One system, exemplified by thealarm calls of primates, is the Lexical system, which is composed of isolated units ofutterance that typically have a specific referent, such as "leopard," "snake," and "eagle"
we see in the calls of Vervet monkeys. The Expression system, associated with birdsong,
creates patterns without use of lexical items. Each system has developed over a long spanof time, millions, and possibly hundreds of millions of years. At some point in recent
evolutionary time, the two systems, L and E, integrated uniquely in humans to give rise tolanguage, which gives the appearance of rapid emergence. I will speculate on how the
integration may have been triggered.
1. Introduction
Language is a defining trait of who we are as human beings. We use it constantly andin many ways it is fundamental to our unique successes as a species. How did humanlanguage emerge in evolution? Our best estimation is that it appeared within the past
100,000 years, around the time that our ancestors were migrating out of Africa into theEurasian continent. Because language doesn't fossilize, we have no direct evidence forwhat precisely happened to give rise to language. One might wonder if it is even worth
asking the question about language in evolution if we cannot come up with direct proof.
Indeed, back in 1866, the Linguistic Society of Paris banned all debate about evolution of_____*I am grateful to Esther Clarke, Takashi Morita, Vitor Nóbrega, Kaz Okanoya, andBridget Samuels for numerous suggestions on an earlier version of this chapter.
1language because of lack of empirical evidence. It isn't suitable for serious study, theysaid. But we are in a very different world today, a world in which we know a great deal
more about our brain and the brain of other animals, primate communication, birdsong,
and, most importantly, human language, some or all of which will form a promisingfoundation for exploring the question of how language emerged in evolution.
The great amount of knowledge we have amassed over the past 150 years could help
to solve many of the puzzles that form the mystery behind the emergence of language.
But that is not the only reason why we should pursue this question. The emergence of
language is one of the great mysteries of evolution and, some say, one of the hardest
problems in science (Christiansen and Kirby 2003). This great mystery about language
and the mind that produces it is all the more perplexing when we note there is anenormous gap between our closest relatives, primates like the chimpanzees and thebonobos, and humans in terms of our cognitive abilities. Our closest relatives don't
produce art, they don't cook with fire, they don't fight for ideals as opposed to justfighting common enemies, and, of course, they don't have language like we do. It is atrait unique to humans and not shared with other branches of the same monophyletic
group (Tallerman and Gibson, 2009). Other members of the group do have modes ofcommunication, but human language possesses richness and complexity far beyond
anything else that we see in the animal kingdom. The mind that makes language possible
is so much more powerful than what we see elsewhere in the animal world that Darwin
himself commented, "the difference between the mind of the lowest man and that of thehighest animal is immense" (Darwin 1871:100). In a similar vein, Alfred Russel Wallace,a leading evolutionary thinker who was a contemporary of Darwin, noted that evolution
should have endowed the humans with a brain a little better than that of an ape, yet what
we ended up with is far more powerful than what a gradual evolutionary adaption would
predict.
The problem of how language emerged in evolution is complex, just as complex as
language itself. Not surprisingly, just as there is debate about every aspect of humanlanguage including its core function — is it primarily for internally representing thought,
or is it for communicating ideas and emotions externally, or is it for some other purpose?
— there is debate about what evolutionary steps contributed to the development of
human language. I will review some of the major proposals from Darwin (1871) to more
recent proposals by linguists, particularly by Bickerton (1990, 2002, 2014) andJackendoff (1999, 2002), and an extension of it by Progovac (2010, 2015). I will argue
that there are promising components to these proposals, but also shortcomings. I will thenpresent the Integration Hypothesis of language evolution (Miyagawa et al. 2013, 2014),
which incorporates Darwin's idea that language began as song much like birdsong, andalso the proposal by linguists that there was a phylogenetic precursor of language,commonly referred to as "protolanguage" (e.g., Hewes 1973). An important question thathas been posed about language is, how quickly did it emerge? Was it gradual, traversing
through possibly many stages of protolanguage before arriving at language as we know ittoday? This is the so-called gradualist view, and it has many prominent proponents (e.g.,
Bickerton, 1990, 1995, 1998, 2000, 2014; Pinker and Bloom, 1990; Newmayer, 1991,
1998; Pinker, 1994; Jackendoff, 1999, 2002; Tallerman, 2007; Hurford, 2012; Progovac,
2006, 2008, 2009, 2010, 2012, 2015; Progovac and Locke, 2009). Or did the essence of
today's language form rapidly? This is the so-called emergent view (Berwick, 1998;
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Hauser et al., 2002; Chomsky, 2005, 2008, 2010, 2012, 2013, 2014; Berwick andChomsky, 2011; Di Sciullo, 2011, 2013, 2014; Bolhuis et al., 2014; Miyagawa et al.
2013, 2014; Nóbrega and Miyagawa 2015). The rapid-development hypothesis comes in
two versions, the better known being that something happened, possibly a mutation, that
led to a rapid emergence of language where nothing like it existed before (e.g., Berwick
and Chomsky 2011). The other view is that language arose rapidly from integrating preadapted systems that existed independently in nature (Miyagawa et al. 2013, 2014). The
latter is the Integration Hypothesis, which I will take up to show its advantages and also ashortcoming, a shortcoming shared with the gradualist view proposed by Bickerton,
Jackendoff, and Progovac. I will speculate on a possible solution to the problem, which
will allow us to utilize Darwin's "song" idea and combine it with the proposals by
linguists that are based on a sophisticated knowledge of human language.
2. Did protolanguage exist?
Homo sapiens emerged in Africa some 200,000 years ago, and their brain continued
to evolve. It was perhaps some 60,000 - 100,000 years ago that a number of things
appeared that demonstrate enhanced cognitive capability, including refined tools, carved
and painted art, and sophisticated weapons. It isn't clear whether language emerged
concurrent with these other achievements of high-order cognitive capability, but certainly
language requires immense cognitive capability that is consistent with these otherachievements of the Homo sapiens. Another piece of evidence for when language
emerged is the discovery of so-called ochres in the Blombos Caves in South Africa.
Ochre is an iron-rich mineral, and more than 8,000 pieces of ochre-like material havebeen found in the Blombos Caves, and they have been dated back to 75,000 - 100,000
years ago. Some, like this one, have engravings and incisions.
Figure 1. Ochres from the Blombos Caves (Evolutionary Studies Institute
University of the Witwatersrand)Though some have questioned it, Tattersall (2009) and others have argued that thesepatterns represent early abstract or symbolic depiction, possibly similar to language. It is
certainly within the approximate time span when it is believed that human language
arose.
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Although there is disagreement on just about every aspect of human language, one
point that would be difficult to deny is that it is an enormously complex systemcomposed of a countless number of elements — sounds, words and subwords, syntax,
and meaning. Given this complexity, it would be natural to posit that human language
must have developed gradually from an earlier, simpler protolanguage system. I will
briefly review two proposals for protolanguage, the song protolanguage of Darwin (1859,
1871) and the lexical protolanguage of Bickerton, extended by Jackendoff. For otherproposals, see Fitch (2010), who gives an excellent summary and critique of the major
protolanguage proposals. For the development of cognition in Homo sapiens, see, forexample, Mithen (2014).
2.1 Song Protolanguage
Darwin (1859, 1871) was the first to note that there may be a connection between
birdsong and language; he described it as the "nearest analogy to language." In the same
way that humans have an instinct to speak, songbirds have an instinct to sing, and just as
language is learned, so are birdsongs learned, in both cases the learning process opening
up the possibility of variety, as in dialects in both human language and birdsong. In bothlanguage and birdsong, there is a stage of acquisition that precedes mastery; in humans
this stage is infant babbling and in juvenile birds it is the singing of "subsongs." On thelast point of acquisition, recent studies show a surprising parallel between language andbirdsong beyond simply sharing a pre-mastery stage. A birdsong, such as that of zebra
finch, is composed of notes that are combined to form syllables, and syllables combined
to form motifs, which are then combined to form a complete song. Liu, Gardner, andNottebohm (2004) identified two learning styles, the “serial repetition” strategy and the“motif" strategy. For a juvenile bird that adopts the serial repetition strategy, anapproximation to one syllable of the model is repeated many times, and because the unitof repetition is small, each syllable is clearly articulated. In the motif strategy, thejuvenile bird adopts a global imitation of its father's song (only males sing), and because
the unit of repetition is larger, the articulation is noisy and imprecise. The following
spectrograms show the striking difference in these two strategies among two groups of
41-day old zebra finch juvenile males.
Figure 2 Serial repetition and motif strategies of song learning (Liu, Gardner,
Nottenbohm 2004)

4

In the upper half, we see three spectrograms of juveniles that adopt the serial repetition
strategy, with each syllable clearly sung. In the bottom half are spectrograms of juveniles
with motif strategy; given the larger unit being sung, the overall song lacks the precisionof the other strategy. Regardless of the strategy that a juvenile adopts, by the time it
matures, it is able to sing the song perfectly. As it turns out, the two strategies foracquisition are also found in human language acquisition. O'Grady (2005) (see also
Lipkind et al. 2013) points out that a human infant may adopt one of two styles oflearning: the “analytic” style, which focuses on breaking speech into its smallest
component parts to produce short, clearly articulated, one-word utterances in the earlystages; and the “gestalt” style, in which the infant memorizes and produces relatively
large chunks of speech that are often poorly articulated that correspond to entire
sequences of words in the adult language. Just as with songbirds, the human infant,regardless of the style of acquisition it adopts, ultimately acquires the language perfectly.
2.2 Lexical Protolanguage
One idea that a number of linguists have suggested is that before human language
became the full-fledged system that we are familiar with, it went through a simpler
linguistic stage, what Fitch (2010) calls "lexical" protolanguage.1 According to Bickerton(1990, 2002, 2014), this earlier stage was composed typically of a simple utterance that
had no structure. One way to think about this, as Jackendoff (1999) suggests, is that
protolanguage was today's language minus syntax, a point also suggested by Bickerton(1990). So, it is just made up of words only. In other words, for linguists such as
Bickerton and Jackendoff, protolanguage was the capacity to use unstructured symbolic
units. How did such a system arise, if, indeed, it did exist? Bickerton (2014) suggests thatone possibility for the emergence of such a system of communication is to convey thelocation of a dead prey in scavenging. This was a time when our ancestors were huntergatherers. Why would our scavenging ancestors require such a system? It is surmised that
1"Lexical protolanguage" is called by some as "compositional protolanguage" (e.g.,

Tallerman 2012).
5

this system allowed the hunter-gatherers to communicate the location of dead prey at a
distance. To be successful in claiming the dead prey from competitors, these pre-humans
had to recruit members of their community in large number and communicate to thesemembers the location of the prey and some idea of its size and the desirability of being
able to claim it. This requires displacement, which is the ability to communicate about
things that are not in the immediate vicinity of the speaker-hearer. Human language is
adept at displacement; so are the systems employed by honeybees and ants for similar
resource-locating purposes. The idea is that, in humans, protolanguage emerged to makedisplacement possible. This is an intriguing idea. Did language actually go through such a
protolanguage stage? It is hard to say, of course, because it happened so long ago. We
will return to both the song and lexical protolanguages later in the chapter.
3. Both Gradual and Rapid
The gradualist viewpoint based on protolanguage makes sense if one were only
focused on language and how it may have evolved over time. Given the complexity ofhuman language, it would be reasonable to assume that this complex system began as asimpler mechanism that over a span of evolutionary time came to take on the kind ofcomplexity and richness we are familiar with. But there is a crucial point here: evolution
is about living organisms, but language is not a living organism. From this perspective,
languages may change, but they don't evolve (Chomsky 2014).2 But clearly something
happened in evolution to give rise to language. Was it an event, or series of events, thattook place gradually over a long span of evolutionary time, or was it fairly rapid innature? My suggestion is that it was both. I begin with the gradualist part of my idea,
leaving the rapid part until the next section.
The time span I have in mind for the gradual development needed for the eventual
emergence of language is much longer than the evolutionary time implied in the various
models of protolanguage. In fact, it is millions of years, and in one case, hundreds ofmillions of years. It is our brain that provides the cognitive capacity to produce language,
and the brain we have inherited has been growing ever larger long before Homo sapiens
appeared in Africa 200,000 years ago. Homo habilis, which appeared on the scene some
two million years ago, started to show an increase in brain size (Deacon 1997). Why isthis? Some attribute the growth in brain size to eating habits. The two systems, or organs,that consume the most energy in our body are the digestive system and the brain. As ourancestors became adept at preparing food, including grinding it, and most importantly,
heating it with fire to soften it, less energy was needed for digestion, and the brain
received the benefit of the excess energy and grew larger (Wranghan et al. 1999,
Fonseca-Azevedo and Herculano-Houzel 2012).
The Homo sapien brain is not just larger relative to the body size, but well developed
in regions that contribute to higher order cognitive functions such as the prefrontal cortex

2 Chomsky makes the same point earlier, in a 2012 interview:

http://www.phenomenologyandmind.eu/wpcontent/uploads/2012/12/16_Intervista-CHOMSKY.pdf
6which is responsible for high-level cognitive activities.3 Language requires a number of
special components in the brain and in our physiology for speech. Within the brain, twoof several important centers for language are Broca's area and Wernicke's area. Both arelocated almost always on the left hemisphere. Broca's area, located in the frontal lobe,
supports speech production, while Wernicke's area, in the superior temporal gyrus,
supports speech perception. Both are needed for language. So the question is, when didthese regions of the brain develop? Did they develop around the time that language arosein evolution, 100, 000 years or so ago? There is evidence from study of skulls of Homo
habilus and Homo erectus that these centers may have been around in the brains of ourancestors over a million years ago, long before language actually emerged (Tobias 1998,
Wynn 1998). In addition, there is evidence that homologues of these structures arepresent in some great ape brains today (Gannon et al. 1998; Cantalupo and Hopkins
2001) suggesting they may have developed over 9 million years ago.
This idea that some of the key centers of the brain that support language have existed
for several million years, maybe more, is part of a picture that is emerging recently about
who we are. Although it was only recently in evolutionary time that we began to see
evidence of high cognitive achievements, such as painted art, sophisticated weapons,
refined tools, and language, the brain circuits that make them possible apparently havebeen present much longer. In recent articles, it is pointed out that the human cortex,
where higher-order cognitive computation is carried out, has similar cell types, patternsof wiring, and gene expression as other mammals (Calabrese and Woolley 2015, Harris2015). Furthermore, the computations that these brain circuits make possible also occur
in birds. It is as if there are only a limited number of brain microcircuit designs thatnature came up with and these circuits have been repurposed to make the higher ordercognitive achievements possible in humans.
Among the recent studies, one in particular has direct implication for Darwin's songprotolanguage in which he surmised that pre-humans sang and this ultimately led tolanguage. Pfenning et al. (2014) look at vocal learning as reflected in regions of the brainof songbirds (zebra finch) and humans, and also birds that don't sing (dove, quail), and a
primate that doesn't have language (macaque). Taking a computational approach, theyscreened gene expression databases from humans and vocal-learning birds as well as thenon-vocal-learning birds and primate. Their study indicates molecularly analogous
regions that are homologous for song and speech for vocal-learning birds and humans.
They also demonstrate that non-vocal learning birds and macaques do not share in any
significant way these traits. The idea that humans and songbirds have homologous
regions that are not found in non-singing birds and also in macaques raises an interestingquestion. Are these shared regions due to convergent evolution or to a common
progenitor? Convergent evolutions are sometimes known to occur among unrelated living
beings, as similar solutions may arise for similar problems (Gould 1976), as in theexample of the emergence of the eye in unrelated organisms (Ogura et al. 2004, Fernald
2006). On the convergent evolution view, the shared regions are analogies of each other.
The other possibility is that the regions with similar functions in vocal-learning birds and3 Recent studies point more on expansion of the temporo-parietal junction. (e.g, Bruner, E., &Iriki, A. (2016). Extending mind, visuospatial integration, and the evolution of the parietal lobes
in the human genus. Quaternary International, 405, 98-110.

7

in humans descended from a common ancestor. Given that the ancestors of present-day
birds and mammals split 300 million years ago (Benton 1990), this suggests that, on thecommon progenitor view, the genetic resources for singing in primates existed over 300
million years ago. Darwin probably did not imagine that the song protolanguage stage
lasted that long, and our ancestors may not have sung for all of that time, but just as we
saw for Broca's and Wernicke's areas, what we see is that the genetic resources may havebeen present long before language emerged. On the convergent view, we are looking at atime depth of at least around nine million years if the existence of regions similar to the
Broca's and Wernicke's areas in the brain of some great apes is an indication of the age ofthese regions. On the common progenitor view, the brain centers for songbirds imply amuch older evolutionary history. But what does singing have to do with human language?
While birds still sing today, we see on the primate side only four species that sing: titi
monkeys, indris, tarsiers and gibbons. Of these four, gibbons are the only apes
(Hylobatidae) and thus the most closely related to humans. Their songs have been studied
for decades (Marshall and Marshall 1976, Haimoff 1984) but have only recently been
compared to human language (Clarke et al. 2006). I will present a hypothesis for how
language emerged in evolution that considers the system underlying singing to play a
critical role, thus inheriting Darwin's idea of a singing pre-human, and linking it to theregions of the brain for singing that may have existed for million years and perhaps forover 300 million years.4
4. Basics of Human Language
According to the UNESCO report, Endangered Languages, there are approximately
6,000 languages in the world. Some may question this number as an underestimation —for example, just in Papua New Guinea we find 820 languages, and there are many otherregions of the world that hold a large number of languages. The precise number is hard topin down because sometimes it is difficult distinguish between dialects and languages.
However many languages there are, one thing is certain: every language has the function,
as Darwin described it, to connect “definite sounds with definite ideas” (Darwin 1871:
54). How does language do this? In the linguistics literature, the proposal is made thateach sentence is composed of two layers of meaning, called “duality of semantics”(Chomsky 2005). We can use the following example to demonstrate this duality.
(1) Did you eat bread?
This sentence contains three content words, you, eat, and bread. These words may occur
in a variety of contexts with a fixed meaning, such as I want you to eat bread, you
shouldn’t eat bread, You seem to eat bread everyday. This is the lexical layer, containing
the meaning of individual words of a sentence. The sentence also contains the word did, afunction word that has not just one, but two functions. By inflecting for tense, it indicatesthat the event represented in the sentence took place in the past, and by occurring at thehead of the sentence, it signals that the sentence is interrogative in form. Tense and
4Samuels (2015) argues that songbirds have capacity to generate sound structure

analogous to human language phonology.
8interrogative are two elements in the other layer of meaning, which I call the expression
layer (Miyagawa 2010). The expression layer constitutes a chunk of the expression,
typically a sentence, and its purpose is to give shape to the expression, such as
interrogative, and any meaning that is associated with the entire expression, such as tense.
This is in sharp contrast to the lexical layer, in which the meaning is represented atstrictly the local level of each individual word. The two layers of meaning differ in twoother significant ways. The lexical layer is composed of content words, and an adultEnglish speaker knows around 60,000 such words. In contrast, the expression layer iscomposed of function words, and these number in the single digits to less than fifteen
depending on the theory one adopts. Also, the words of the lexical layer connects tospecific meaning, often, though not always, referring to an entity in the real word, such astable, school, horse. The expression layer provides the shape of the utterance, and it
commonly communicates the intention of the speaker, such as the intention to ask aquestion, make a statement, issue a command, and so forth (Austin 1962). The two layersof meaning are represented as follows for the earlier example.
(2) Duality of Semantics (Chomsky 2005, Miyagawa 2010)
Expression structure
QUESTION
TENSE
Lexical structureyou

eat bread

QUESTION indicates that the entire expression has the form of interrogative, andrepresents the speaker’s intention to ask a question, while TENSE contributes to theoverall expression by indicating the tense of the utterance. Each individual word in thelexical layer is represented in the lower structure.
In Miyagawa et al. (2013), I argued, based on the work of a number of linguists, that
although the expression and lexical layers are intricately intertwined in human language,
they can be viewed as separate components each with its own specific properties, as wesaw. The two components are in fact so fundamentally different that it appears that they
just happen to occur together in language, but they could just as well function as separate
systems. In Miyagawa et. al (2013, 2014), I in fact propose that the two layers correspondto two distinct systems that occur independently in nature. I call it the Integration
Hypothesis of language evolution because the two layers happened to get integrated
uniquely in humans and gave rise to language as we know it today. By separating
language into these two components, we are able to entertain a range of possibilities thatare difficult to consider in the other proposals for language in evolution. Given therichness and complexity of human language, it is certainly challenging to imagine that it
developed rapidly within the last 100,000 years, yet there is no evidence of language9prior to that time, as far as we can see (e.g., Tattersall 2016). How do we make sense ofthis enormously complex system that just “popped up” 100,000 years ago? By the
Integration Hypothesis, the process of integration of the two independent systems
occurred in recent evolutionary time. But the two systems themselves are older, mucholder, their progenitors being millions, or in one case, possibly hundreds of million years
old. Below, I turn to the Integration Hypothesis.
5. Lexical and Expression layers: Separate systems in nature
If the two layers of language, lexical and expression, have correlates in independent
systems in nature, what are these systems? The calls of monkeys and apes are a natural
candidate for the lexical system. There is a large body of literature on this topic (Seed andTomasello 2010), an earlier work of which is on the Kenyan Vervet monkeys (Seyfarth etal. 1980), which possess alarm calls for pythons, eagles, and leopards. Sticking to these
simplest lexically based systems, what we see is an uttered object that correlates with aparticular real-world state of affairs. An important point about this system is that eachuttered object is co-extensive with a real-world state (“holistic” in the sense of Wray
1998). A simple, and possibly a simple-minded, way to view this system is that it is acollection of vocal gestures that have a specific referent in the real world (Seyfarth andCheney 1986, Miyagawa et al. 2013). It is a “pure” L(exical) system in that each unit is
an isolated entity with a definite “meaning.” The lexical protolanguage view points to thisL system in our ancestors as forming the progenitors to language.5
Miyagawa et al. (2013, 2014) links the E(xpression) layer of human language to thesystem underlying birdsong.6 As Darwin first noted, there is a list of striking
resemblances between birdsong and language, a list to which we can also add: a need forexternal input; sensitive developmental periods ending at sexual maturity; hemisphericlateralization, and motor-auditory rehearsal systems (Bolhuis et al. 2010). There is one
striking difference: lexical items in the sense of human language are absent from every
variety of birdsong that has been studied, so that a birdsong appears to be a phonologicalsyntax without a lexicon (Marler 2000). Every birdsong is a system that generates
particular patterns, as in the case of the well-studied song of the zebra finch, which has ahighly restricted set of “notes” that combine to form syllables, and syllables into motifs,
and motifs into a complete song “bouts” (Berwick et al. 2011).
Figure 3 The song of zebra finch (Berwick et al. 2011)

5Although I consider the L system to consist of independent vocal gestures, there are studies thatsuggest that primates can use multiple calls to construe novel meaning (Dessalles 2007, Arnold
and Züberbuhler 2006, Quattara, Lemasson, and Züberbuhler 2009).
6 See Okanoya (2002) for an earlier proposal that links birdsong syntax and human language.

10

The zebra finch song is one of the simplest; other vocal learning bird species possess
more complex patterns with branches, loops, and repetitions (Berwick et. al 2011).
Figure 4 Bengalese finch (Okanoya 2004)

A unique property of human language is that a regular grammar (type 3) isinadequate for modeling it (Chomsky 1956). But this is not the case for the two proposed
antecedents of human language. The lexical systems that employ isolated uttered units
that correlate with real-world references, such as the alarm calls of Vervet monkeys, areclearly couched in a simple regular grammar. The other layer, the expression layer, finds
its antecedent in the kind of system underlying birdsong. Birdsongs have specificpatterns, and these patterns can be complex, as in the example of the Bengalese finch,
which loops back to various positions in the song and lead to considerable variation.
Nevertheless, all known birdsongs can be described as a k-reversible finite state
automaton, a form of regular grammar (Berwick et al. 2011); for example, we do not see
center-embedding, which was an example Chomsky used to prove the inadequacy ofregular grammar for describing human language. While suggestions have been made thatcertain bird species can acquire recursive syntactic structures reminiscent of humanlanguage either through conditioning (Gentner et al. 2006) or spontaneously (Abe and
Watanabe 2011), these results remain controversial (Beckers et al. 2012) andunconfirmed.
The Integration Hypothesis proposes that these two major systems in nature that
underlie communication, L(exical) and E(xpression), integrated uniquely in humans togive rise to language. One challenge to the Integration Hypothesis is that, as noted above,contemporary languages are known to be based on a system more powerful than a regular
grammar. As evidence for the Integration Hypothesis, in Miyagawa et al. (2013, 2014),
we demonstrate that when we consider the two components of contemporary languages
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separately, L and E, each component may be characterized by a regular grammar, thus
reflecting their antecedents in nature. It is only when the two systems integrated that gave
rise to a system more powerful than what we see elsewhere in the animal kingdom.
Let us begin with the L layer. A trait of L units is that they don’t combine directly,
just as the discrete units of the Vervet alarm calls do not combine to form new calls(Miyagawa et al. 2013).
(3) Impossible lexical structures

To make these combinations possible, something from the E layer must intervene: theD(eterminer) represented by apostrophe ‘s (John’s book), the copula that carries tense, an
E layer phenomenon (book is long), and to, which helps to form the E layer of a clause (Iwant to eat pizza). Later, we will look at compounds, which ostensibly pose a challenge
to this view that L units do not directly combine.
To see the regular-grammar status of the E layer of human language, let us begin
with a unique feature of human language not seen in other systems in the animal world,
the feature commonly called “labeling” (Chomsky 1995). Given a word, its lexical
category (Noun, Verb, etc.) forms the “label” of the larger phrase that contains it. For
example, for the pair eat and bread, the verb eat labels the larger phrase, eat bread, as aV(erb), forming a Verb Phrase.
(4) LabelingThis property of labeling gives human language its unique ability to form hierarchicalstructures (Chomsky 1995, 2008; Hornstein 2009) and gives human syntactic structure
the property of “discrete infinity” (Chomsky 2000) through recursively merging andlabeling structures. However, as noted in Miyagawa et al. (2013), there is a severe
limitation on the depth of the hierarchy when we isolate structures in the E layer. As wesaw earlier, the E layer can contain Tense; there is a second item, conventionally labeled“C(omplementizer)” that contains a range of expressive phrases such as Q(uestion),F(ocus), and so forth.
(5) Expression structure

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C and T are the two most frequently cited labels within the E layer. Strikingly, theselabels cannot occur as hierarchical structures of arbitrary depth; rather the CP-TP
structure can only be one layer deep, a limitation consistent with the E layer being
characterizable as a regular grammar.
(6) An impossible E-layer structure

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The limitation is that the E-layer is restricted to a depth-one hierarchical structure, similar
to the systems we see in the Bengalese finch and nightingale songs.7 This suggests that
the E layer of human language closely reflects the birdsong structure, a system of regular
grammar. While there are theories of linguistics that posit multi-layer within E (Rizzi
1997), there are alternatives that do not assume such a multi-layer (e.g., Miyagawa
2010).8
6. Progenitors of human language
The previous proposals about how human language developed all have some“beginning” that is a simpler stage of today’s language, and through a progression ofstages, each more complex than the earlier one, we arrive at today’s human language. For
example, and as already mentioned, lexical protolanguage, proposed by linguists (e.g.,
Bickerton 1990, 2000, 2014; Jackendoff 1999, 2002), views human language as having
undergone a sequence of stages, from a one-word stage to a more complex system of
expression; in the case of Jackendoff, and more recently Progovac (Progovac and Locke
2009, Progovac 2012, 2015), this second stage is characterized by the combination of
single words into compounds through a rudimentary recursive n-ary operation thatgenerates flat structures (Progovac and Locke 2009, Progovac 2012, 2015). Onceequipped with this proto-language that can form a two-word sequence, a primate's brain
becomes ready at some point to create structures that ultimately lead to sentence
formation. Why did syntax emerge? If it is the case that in protolanguage, any two words
7Arsenijevic and Hinzen (2012) also notice this limitation in the E layer and attribute thislimitation to meaning. We believe that characterizing this limitation in terms of finite state
grammar captures the restriction, which we believe holds of all kinds of E layers and not just the
CP-TP structure, which is what Arsenijevic and Hinzen account for.
8 Tallerman (2016) criticizes the idea that the expressive layer of human language parallels the
structure of birdsong. She points out that while the elements of birdsongs always occur in anordered sequence, in human language elements may undergo permutations — movement as
called in linguistics — that alter the ordering sequence of items. This criticism is based on afundamental misunderstanding of the expressive layer. As noted in Miyagawa et al. (2013, 2014),
an operation such as movement is only possible after the two layers, E and L, integrated. The
point is quite simple: what is moved are words in phrases, hence L as well as E must be involved.
Tallerman incorrectly notes that movement is strictly handled by the E component. Rather, a nonfinite state operation such as movement does not occur in E layer in isolation, but is the outcomeof integration (Miyagawa et al. 2013, 2014).

14

could be strung together, there would be no structure to define the relationship between
the two words, so that there is a great deal of burden on the context to give meaning to
the combination. For example, if one combines child and picture, as child-picture, wecannot tell if this means a picture drawn by a child, a picture that depicts a child, a picture
for children, and so forth. One way to think about the reason why the protolanguage
developed structure is to clearly mark the relationship between the combined items(Jackendoff 1999). This reduces burden on the brain to compute the meaning. From this,the system could have developed structures that define such relationships as modifiermodified as in blue sky, verb-complement, such as eat bread, and an agent of an act, suchas horse-rider. Linguists such as Jackendoff and Progovac point to the existence of
certain compounds (daredevil) in contemporary languages as living fossils of an earlier,non-syntax stage of language.
Let us consider the lexical protolanguage proposal by looking at the most recent
work on the topic — Progovac (2015). This work, by a scholar well-versed in modern
linguistic theory and with the major literature on evolution, has amassed together
considerable data from contemporary languages to argue that from its initial one-wordstage, language evolved through progressively complex stages. A unique feature of thiswork is that Progovac specifically argues that each of these stages reflects the syntactic
structure postulated in the modern linguistic theory called Minimalism (e.g., Chomsky
1995). Thus, from a one-word stage, language evolved into a two-word stage, andeventually it evolved to stages that progressively took on layers of functional structures,as in vP > TP > CP. Her interesting claim is that we find each of these stages as living
fossils or in some related form in contemporary languages. Whether one agrees with herconclusion or not, the extensive data she presents is fascinating in and of themselves; I
found particularly interesting the numerous "exocentric" compounds in Chapter 6 from
Serbian and other languages. Independent of issues of language in evolution, these data,and other data in other chapters, present a body of empirical material for carrying out
research on topics such as compounding and phrase structure. Progovac also presents thecore of the recent linguistic theory (e.g., Chomsky 1995) in a way that is comprehensible
to non-linguists, which is a valuable service to the field.
There are some issues with her work. Progovac considers the emergence of language
as an evolutionary process, but as already noted, living entities evolve, and language is
not a biological entity. It is telling that all the examples Progovac gives as examples of
evolutionary change are biological in nature except language — lungfish, body hair, eye.
So, if she is correct in her description of how language developed, one must wonder
precisely what this development is telling us about the organism that produced each ofthese stages.
Furthermore, if we consider "one-word" and "two-word" systems, they are
fundamentally different. In "one-word" systems, such as the Vervet alarm calls and theemotional interjections in contemporary human languages (ouch, wow), the units do not
combine with other units to form a new unit. This is because each unit of utterance is coextensive with the referent or the emotion it represents; since they don't have any lexical
category, these single units are better called "roots" (Nóbrega and Miyagawa 2015). It issimply not possible to combine the alarm calls such as leopard and eagle because eachhas an independent reference and there is no sense in which one unit can combine withanother unit to form a third unit with some other reference, unless these units undergo15

fundamental change. Same goes for human language interjections: there is no sense inwhich ouch and wow could combine to give a third emotional interjection simply because
each interjection is a fully independent and isolated unit of utterance that expresses a
specific emotional state. In contemporary languages, combining words into compounds
or sentences minimally requires that each word ("root" to be more precise) be assigned acategory (noun, verb, etc.), which automatically gives the word more structure than an
unstructured "root" form of alarm calls or interjections; thus a word in a compound iscomplex (see Di Sciullo 2012, 2013, 2014).9 As noted in Nóbrega and Miyagawa (2015),
if one looks at compounds beyond those used as evidence for "living fossils," one finds
structure that is just as complex as a full sentence, thereby questioning the idea that there
was an unstructured two-word protolanguage stage. From this viewpoint, the four stages
that Progovac suggests should be questioned (she refers to her proposal as a three-stage
model, but I am including the "zero" stage as the first stage).
(7) Four stages of language evolution (Progovac 2015: 15)(o) One word stage;(i) Paratactic stage: conjoin two words without creating hierarchy or headedness;(ii) Proto-coordination stage: emergence of "conjunction/linker" that provide"segmental glue to hold the utterance together" (p. 13), presumably making
multiple-word utterances possible;(iii) Specific-functional category stage: "specific functional categories become
available" (p. 13). "it is only at this stage that hierarchical structure, Move, andrecursion become available." (p. 14)
Based on what we have said, there are really only two crucial stages, the one-word stage,
which is made up of isolated roots, and a stage that Progovac calls the "specificfunctional category stage," where "functional categories" — what I am calling the E
system — becomes available. Once the E layer becomes associated with the isolated unitsof the L layer, combining the L units under Merge is possible, with labeling of the newly
created structure. From this perspective, there would be no need to postulate theparatactic and proto-coordination stages. That also makes the picture simpler for creating
multi-word expressions. Instead of having to postulate "proto-Merge" which presumablyjoined roots, and which arguably does not occur in contemporary languages (Di Sciullo
2013, 2014),10 we would simply need to postulate the same Merge that exits today thattakes words associated with lexical category and other E properties.
Despite these issues, there are a number of points I agree with, including Progovac's
suggestion that the progenitors of human language may have been around millions ofyears ago. I will return to this point below.
Turning to Darwin, a crucial feature of his view of how language emerged is that heseparates cognitive abilities from the initial emergence of language-like behavior: "Themental powers in some early progenitor of man must have been more highly developed
than in any existing ape, before even the most imperfect form of speech could have come
9This is similar, and possibly the same, as the notion of "lexical envelop" that allows roots tomerge with other "enveloped" units (Spelke 2003, Ott 2009, Boeckx 2011).
10See Boeckx (2015) for an alternate view.

16into use" (Darwin 1871: 57). The protolanguage Darwin conjectured is musical in nature,and involved singing with the intention, same as songbirds and gibbons, of courtship andterritoriality. This eventually developed into a full-fledge language, according to him,through imitations aided by signs and gestures that became associated with definite
meaning.11 Darwin goes on to add that the emergence of language aided in the further
development of the mind, very much in the spirit of the German philosopher and linguist
Wilhelm von Humboldt, whose work Darwin studied closely.
The idea that pre-humans sang appears to be particularly plausible given the recent
genetic finding that humans with their speech, and birds with their song, share regionsthat are homologous for speech and song (Pfenning 2014). Where Darwin's conjecture
becomes questionable is, how could a musical system, which lacks lexical units, come totake on such units built into the song?
The Integration Hypothesis differs from all other hypotheses of language
development in that it does not postulate a singular "proto" system that developed
progressively into ever-complex systems that ultimately ended up as language as weknow it today. Any "proto" system runs into the kinds of problems I noted above. If one
begins with a word (or more accurately, root, which has no structure), how does syntax
develop out of it to make it possible to combine these words into two-word utterancesand beyond, to sentences? If one starts with a musical form of protolanguage, how dowords get introduced into this system? What the Integration Hypothesis suggests is thatthe sources for words and syntax existed independently as the L system underlying alarm
calls and similar modes of communication, and the E system underlying birdsong (andpossibly gibbon song). It is reasonable to assume that these two systems have beenaround for a long time, millions of years or even longer. At some point in recent
evolutionary time, the two systems integrated uniquely in humans to give rise to a systemthat generates patterns as an E system, and contains lexical items as an L system. Why
this happened is not clear, but we could imagine some things that had to have happened
to make this integration possible. As noted earlier, there is a fundamental difference
between one-word and two-word systems. While a one-word utterance can be coextensive with the referent or an emotion, a two-word (or more) combination requires
each word to be non-referential -- underspecified for meaning, in other words -- so thatthe two can combine to form a novel utterance that has its own unique reference. Thismeans that each word must have undergone some process of abstraction. Thus, in childpicture, child does not refer to a definite entity, but it is a kind of an entity that sharesessential properties (human, youth, etc.). The same goes for picture. What could haveshifted the isolated utterances of the L system into these abstract entities that can be
combined into patterns furnished by the E system? It is not clear, but again, we can see
what must have occurred: each item, as a process of abstraction, took on category, suchas noun, verb, and so forth. As a noun, for example, it no longer referred to a specificentity, but stood for a kind of an entity (bread). Furthermore, by taking on categorical
identity, it is able to merge with another unit, and, crucially, to label the newly formed
unit (V label for {eat bread}).
What about the E system? Birdsong is a pure E system that generates finite-statepatterns without any specific reference to entities in the real world. It is, as already noted,
11See Jesperson (1922) for a different model of how song developed into language.

17

a phonological syntax without a lexicon (Marler 2000). Just as some change must havetaken place to the L system, from its "one-word" units that are co-extensive with themeaning they embody, the E system must have undergone some change to accommodate
the members of the L system in order for integration to have taken place. What could thatchange be? One possibility is found in Darwin's proposal of musical protolanguage. He
conjectures that for the singing of pre-humans to have transitioned to articulate language,
they began to incorporate imitations of natural sounds that represented some entity, aided
by gestures (Fitch 2010: 472), something also suggested by Farrar (1870) and Müller
(1861). One way to think about this change is that the E system changed from a system of
nonreferential patterns to one in which the patterns contained placeholders for items thatrefer to the real world. These placeholders are represented by things like imitation
sounds, and imitation can represent an entity or an event. It then becomes natural to start
inserting the members of the L system, which themselves have undergone change to beunderspecified for meaning. If an underspecified root with the core meaning of dog isinserted into an "entity" (N) placeholder, it gets interpreted as DOG, but if it is inserted
into the placeholder for an event (V), it is interpreted as HARASS.12 As these membersof the L system began to combine within the E system, they start to build structure in theway that lexical protolanguage proponents such as Jackendoff have conjectured. One
crucial difference is that the building blocks of the structure are furnished by the E
system, which comes with a system to link nodes, hence a potential to build structure, as
opposed to the proposal of the protolanguage proponents, who conjecture that thestructure somehow arose directly from L combinations. For that to happen, something(structure) must emerge literally from nothing, something that isn't out of the question inprinciple, but highly unlikely.
Once the integrated system achieves this state of having underspecified L units thatcan be inserted into the E system to take on category, we have the essence of the humanlanguage at work. The idea that human language emerged within the past 100,000 years
does not mean that all the pieces of language developed within this time frame. All theessential pieces have existed for millions of years and developed gradually over a longevolutionary time span, and what happened to give rise to language was that these
independently developed pieces integrated uniquely in humans to give rise to the kind ofrich and complex system we know to be language. This gives the appearance of language
having rapidly developed in recent evolutionary time, and while it is true that there is noevidence for language earlier in evolutionary history, we have ample evidence for the
existence of L and E systems long before language emerged. It is the adventitious
integration of these two highly-developed, pre-adapted systems that gave rise to theremarkable system of human language.
Among all the necessary conditions for integration, it appears that abstraction (orunderspecification) of the L units is one crucial missing link. Other pieces were
essentially there as independent systems: the units of L, and the pattern-generating
capacity of E. For the units of L to combine by tapping the pattern-generating E system,
the L units must undergo abstraction, possibly by being assigned a category such as nounand verb. It is not clear how this happened. One possibility, and I only mention it as aconjecture, is the notion of displacement that Bickerton incorporates into his proposal of
12 Thanks to Vitor Nóbrega for these examples and the general line of thinking outlined.

18

protolanguage. Note that displacement requires abstraction. A unit of L that refers, forexample, to leopard, can do so if the referent is present in the physical proximity. For an
organism to convey the existence of an entity in the absence of this entity, the organism
must convey the idea of the entity. By necessity, this involves some ability toconceptualize the kind of the entity involved, hence abstraction, which is required for
integration. Even if this turns out to be true, how this abstraction leads to the uttered unit
becoming associated with a lexical category remains a mystery.
7. Concluding remarks
Did language emerge rapidly or gradually? Given the rich and complex nature oflanguage, it surely makes sense that the key components of language took a longevolutionary time to emerge. Yet, there is no evidence for such gradual development, inwhich language first appeared, say, as a one-word system followed by a two-word systemand so forth. The Integration Hypothesis suggests a parallel development of independent
systems in nature that underlie communication, the E and L systems, which allows us tocapture the essence of both the emergent and gradualist views of language in evolution.
Each system developed over a long span of time, possibly as long as three hundred
million years. At some point in recent evolutionary history, the two systems integrated
uniquely in Homo sapiens to give rise to the kind of system we see today as language.
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