Insights From Language‐Trained Apes: Brain Network Plasticity and Communication

ABSTRACT

Language is central to the cognitive and sociocultural traits that distinguish humans, yet the evolutionary emergence of this capacity is far from fully understood. This review explores how the study of the brains of language‐trained apes (LTAs) offers a unique and valuable opportunity to tease apart the relative contribution of evolved species differences, behavior, and environment in the emergence of complex communication abilities. For example, when raised in sociolinguistically rich and interactive environments, LTAs show communicative competencies that parallel aspects of early human language acquisition and exhibit altered neuroanatomy, including increased connectivity and laterization in regions associated with language. Sustained and enriched early exposure to symbolic experience may also alter molecular pathways, including modifications in the expression of genes involved in synaptic plasticity, neural connectivity, and cognitive function, thus critically underpinning speech and language processing. This theoretical synthesis highlights how research on language‐trained apes can inform our understanding of experience‐dependent plasticity in distributed neural networks, providing insights into the evolutionary origins of human communication.

1. Introduction

Language is a natural gift that enables humans to refine and communicate our thoughts, with its capacity for abstraction, precision, and endless recombination serving as a foundation for cumulative culture. It involves a distributed network of brain regions, including Broca's area, Wernicke's area, the motor and auditory cortices, angular and supramarginal gyri, basal ganglia, and cerebellum [1]. Among these, Broca's area has long been known as critical for language production, but is now also recognized for its role in broader cognitive functions, such as syntactic processing, working memory, and action understanding [2]. To investigate the evolutionary and experiential factors that shaped this complex neural architecture, researchers have turned to our closest living relatives, great apes, who, in rare cases, have been raised with exposure to symbolic communication.

Studying the brains of language‐trained apes (LTAs), such as chimpanzees, bonobos, gorillas, and orangutans, offers a rare opportunity to disentangle the contributions of human‐specific brain development and socio‐environmental experience in shaping the neural substrates of language processing. From early life, these individuals were immersed in structured communication involving gestures, signs, or visual symbols. Consequently, LTAs can serve as an unparalleled model for examining how exposure to symbolic communication affects the developing brain in animals that, unlike humans, do not naturally acquire language, yielding comparative insights that cannot be obtained from studies of either standard‐reared apes or only humans.

Over the past several decades, experimental efforts to teach elements of human language to great apes (e.g. [3, 4, 5]) have yielded important insights into the capacities and limitations of nonhuman primates. These studies have explored symbolic representation, semantic learning, vocabulary development, and the comprehension of syntax and grammar. While the language capabilities of apes fall short of those observed in humans, the circumstances of language training, often beginning in infancy and extending across many years, provide a rare window into how early language immersion can shape brain structure and function in nonhuman primates.

Although most of these apes are now deceased, the availability of their postmortem brain tissue opens new opportunities for research. By integrating behavioral data with analyses of brain anatomy, cellular composition, white matter connectivity, and gene expression and regulation we can begin to assess how symbolic communication can sculpt distributed neural networks, broadly defined here as sets of interconnected brain regions that collaboratively contribute to cognitive and behavioral functions across the entire brain rather than within a single localized area or circuit [6]. These networks include regions commonly associated with language processing, such as homologs of Broca's area, Wernicke's area, as well as other cortical and subcortical structures involved in communication [7, 8, 9, 10].

There are inherent challenges when interpreting neural differences in LTAs due to variability in developmental timing, training duration and quality, and species‐specific neural maturation trajectories. Despite these complexities, the diverse conditions under which LTAs have been trained may allow researchers to examine the interaction of experience‐dependent plasticity with species‐typical developmental trajectories, highlighting the importance of considering cross‐species variation in life history and neural maturation in interpreting results.

In this review, we first outline the history of ape language training and compare the communicative competencies of LTAs to those of human children. We then evaluate neuroanatomical, molecular, and epigenetic evidence for how language exposure may alter the structure and function of the brain. Finally, we propose testable hypotheses about the impacts of experience‐dependent structural plasticity in LTAs, highlighting how these findings can inform broader theories of brain evolution and development. This paper synthesizes existing neuroanatomical, genetic, and behavioral research to provide a theoretical framework, demonstrating how examining LTAs offers potential insights into the evolution and developmental plasticity of the neural systems supporting symbolic communication.

2. Language Acquisition in Great Apes: Bridging the Gap

Language is widely viewed as one of the most distinctive traits to emerge in human evolution, fundamentally differentiating our species from other animals [11, 12, 13]. To explore the boundaries of this uniqueness, researchers over the past five decades trained great apes to use augmentative communication systems as a means of assessing their linguistic capabilities [3, 4, 5, 14]. These studies employed a range of methods, including manual sign language (primarily American Sign Language, or ASL), gesture‐based systems, and artificial languages using keyboard‐based visuographic symbols called lexigrams, which represent objects, actions, people, and concepts.

Early attempts to teach apes to produce speech sounds [15, 16] failed largely due to the anatomical constraints of their vocal tract and their limited voluntary neural control over vocalizations [15, 17, 18]. However, alternative approaches focused on visual‐gestural modalities proved far more successful. Chimpanzees such as Lana (1970–2016), Austin (1974–1996), Sherman (1973–2018), and Panzee (1985–2014), and bonobos Kanzi (1980–2025) and Panbanisha (1985–2012), were trained with lexigram systems and demonstrated substantial symbol learning and use. In parallel, ASL was introduced to apes in pioneering projects including Project Washoe (begun in 1966), in which the chimpanzee Washoe was taught signs beginning at 11 months of age (Gardner, 1978), and Project Koko (initiated in 1972), which involved the gorilla Koko, who began ASL training at the age of one [4]. Washoe reportedly acquired 132 signs within 4 years of training, while Koko's vocabulary reached approximately 260 signs, although subsequent analyses suggest these figures may be underestimates due to conservative criteria for vocabulary inclusion [19]. Following Washoe and Koko, additional apes, including the gorilla Michael, chimpanzees Moja, Pili, Tatu, and Dar, and the orangutan Chantek, also participated in sign language training studies.

Table 1 presents an overview of the known language‐trained apes (LTAs) across species. Collectively, these projects demonstrate both the cognitive potential of great apes and the environmental and developmental factors that constrain or enable their linguistic performance.

Table 1.

List of language‐trained apes.

Ape name	Species	Sex	Age of first exposure to sign/symbols	Type of language (ASL/Lexigram)	Age at death/current age
Koko [20]	Gorilla	Female	1 year	ASL	Died at age 46 in 2018
Michael [21]	Gorilla	Male	3 years	ASL	Died at age 27 in 2000
Kanzi [22, 23]	Bonobo	Male	Infant	Lexigram	Died at age 44 in 2025
Panbanisha [5]	Bonobo	Female	Infant	Lexigram	Died at age 26 in 2012
Nathan [24]	Bonobo	Male	Infant	Lexigram	Died at age 13 in 2013
Matata [25]	Bonobo	Female	Adult (as caregiver to Kanzi)	Lexigram	Died at age 50 in 2014
Teco [26]	Bonobo	Male	Infant	Lexigram	Born in 2010; currently 14
Mulika [23, 27]	Bonobo	Female	Infant	Lexigram	Died at an early age
Nyota [27, 28]	Bonobo	Male	Early Age	Lexigram	Born in April 4th, 1998; currently 27
Washoe [3, 29]	Chimpanzee	Female	10 months	ASL	Died at age 42 in 2007
Nim Chimpsky [30]	Chimpanzee	Male	2 weeks	ASL	Died at age 26 in 2000
Loulis [29, 31, 32]	Chimpanzee	Male	8 months (learned from Washoe)	ASL	Born in 1978; currently 46
Lana [33]	Chimpanzee	Female	1 year	Lexigram	Died at age 46 in 2016
Panzee [34]	Chimpanzee	Female	Infant	Lexigram	Died at age 29 in 2014
Sherman [23, 35]	Chimpanzee	Male	1 year	Lexigram	Died at age 45 in 2018
Austin [23, 35]	Chimpanzee	Male	1 year	Lexigram	Died at age 22 in 1996
Ericka [36]	Chimpanzee	Female	Infant	Lexigram	Died at age 43 in 2016
Peony [36, 37, 38]	Chimpanzee	Female	Infant	Lexigram	Died at age 23 in 2001
Tatu [29, 32, 39, 40, 41]	Chimpanzee	Female	Infant	ASL	Born in 1975; currently 49
Viki [42]	Chimpanzee	Female	Infant	Spoken English	Died at age 7 in 1952
Moja [29, 40, 41]	Chimpanzee	Female	Infant	ASL	Died at age 29 in 2002
Pili [40, 41, 43]	Chimpanzee	Male	Infant	ASL	Died at age 9 in 1979
Sarah [38, 44]	Chimpanzee	Female	5 years	Plastic symbols	Died at age 59 in 2019
Ai [45]	Chimpanzee	Female	1 year	Lexigram	Born in 1976; currently 48
Dar [29, 40, 41]	Chimpanzee	Male	Infant	ASL	Died at age 24 in 1999
Chantek [46]	Orangutan	Male	9 months	ASL	Died at age 39 in 2017

Comparative studies of LTAs and human children have revealed notable similarities, as well as important differences, in their use and development of symbolic communication. Both groups use symbols to refer to people, objects, and actions; both demonstrate developmental patterns in combining symbols; and both can make comments about past and future events [24]. Some LTAs, such as the bonobos Kanzi and Panbanisha and the chimpanzee Panpanzee (Panzee), exhibited declarative uses of communication, such as requesting, commenting, and interacting, similar to behaviors observed in human toddlers [47].

Additionally, both LTAs and young children show comparable patterns in ordering symbols according to semantic relationships, highlighting shared underlying cognitive strategies [24]. Early in their communicative development, LTAs appear to exhibit limitations in their ability to construct complex symbol sequences, approximating the linguistic stage of human children around 2 years of age [47]. However, the mean length of utterances in LTAs seemed to plateau over time, whereas human children typically progress to more complex syntactic structures [23]. Human children also more frequently employ declarative forms such as showing, offering, and attention‐getting gestures, which may reflect a qualitative divergence in socio‐communicative motivation or cognitive abilities [24].

The interpretation of many of these observations is challenging due to communication modality differences, specifically, the use of lexigrams by some LTAs versus spoken language by children. To address this, Bonvillian [19] and colleagues compared the sign language development of gorillas Koko and Michael to that of 22 deaf children of deaf parents acquiring ASL from birth. The study found notable developmental parallels in grammatical category acquisition, vocabulary size, and sign iconicity. However, human children learned signs at a significantly faster rate and used a more diverse vocabulary, as reflected by a higher type‐token ratio (a measure in linguistics to assess the diversity of vocabulary, calculated by dividing the number of unique words by the total number of words). Iconic signs, those that visually resemble their referents, were more frequent in the initial vocabularies of gorillas than in those of children, suggesting differences in reliance on visual‐motor representations. Importantly, using a shared language modality, such as ASL, allowed researchers to minimize some of the confounding factors inherent in cross‐modality comparisons and reframe the differences between LTAs and hearing children [48]. These findings suggest that when using comparable communication systems, some apparent limitations in LTA language capacities may be less stark, although key distinctions remain.

Other factors further complicate direct comparisons. LTAs generally began language training later than human children, who are immersed in linguistic environments from birth, and even prenatally [49]. In addition, LTAs were raised and trained under highly variable conditions, with differences in instructional frequency, social enrichment, and the nature of ape‐caregiver relationships, all of which influence motivation and learning outcomes. Methodological differences in age at training initiation, duration and intensity of training, and individual rearing conditions must be considered when evaluating the cognitive‐linguistic capacities of LTAs, recognizing that these factors significantly complicate direct cross‐species comparisons and interpretations of neuroplasticity findings.

Despite these caveats, the existing evidence indicates that LTAs, particularly those raised in socially enriched, enculturated environments, can achieve communicative and cognitive competencies that far exceed those of standard‐reared apes. Table 2 summarizes the reported linguistic and cognitive abilities of LTAs and underscores how enculturation and symbolic exposure drive enhanced performance.

Table 2.

Language and cognitive capacities of LTAs reported in the literature.

Trait	Reference
Language comprehension (spoken English, lexigrams)	[5, 23, 50, 51]
Inventing new vocabulary items uninstructed and making creative vocabulary combinations (e.g., labeling duck “water bird”, radish “cry hurt food”, or brazil nuts “rock berry”)	[3, 46, 52, 53, 54, 55, 56]
Combining signs into meaningful statements	[3, 21, 46, 47, 53, 54, 57, 58]
Initiating communicative interactions using the language system (ie. ASL or lexigram)	[3, 5, 29, 46, 47, 53, 55, 56, 57, 59, 60]
Signing to conspecifics (e.g. chimpanzee‐to‐chimpanzee)	[3, 29, 56, 59, 60]
Self‐directed signing when alone without obvious external stimulus (in solitary plays etc.)	[21, 29, 46, 61, 62]
Discriminating humor from deceptive behaviors based on context	[29, 52]
Spontaneous utterances about surroundings without prompting, and noticing responses	[5, 21, 46, 52, 53, 58, 59, 61]
Correcting human partner during communication	[52, 53, 58]
Understanding and replying to wh‐questions	[3, 24, 55, 59, 62]
Asking yes/no and wh‐questions in sign language, although rare	[4, 52]
Subject‐object discrimination, indicating syntactic comprehension (although the word order issue is known to be complicated in ASL)	[5, 21, 23, 47, 53, 61]
Early generalization and over‐generalization of the words (e.g., using “open”, “drink”, or “straw”, “green onions” for similar objects)	[3, 4, 21, 23, 46, 53, 55, 59]
Communicating among LTAs without human prompting or intervention	[29, 60, 61]
Competencies in conversational turn‐taking, negotiation, and argumentation	[23, 52, 54, 58, 60, 61, 63]
Using symbolic communication as a social influence or persuasion	[29, 52, 58]
Evidence of self‐awareness from conversations via symbolic play and mirror recognition (Using mirrors to inspect otherwise unseen body parts; such as inspecting teeth, nose‐picking, inspecting eye regions, etc.)	[58, 64, 65]
Spontaneous acquisition of symbols through observation of other LTAs or humans, without explicit human instruction/Cultural transmission and social learning	[23, 52, 55, 57, 58, 60]
Naming out‐of‐sight but desired objects/Spontaneous referential symbol use for absent objects	[3, 46, 53, 58, 59, 60, 66]
Use of function words (prepositions, conjunctions, determiners), where permitted by ASL or the lexigram system	[3, 4, 46, 53, 55, 56]
Long‐term retention of symbolic vocabulary	[51]
Expressing opinions and emotions	[29, 58]
Understanding emotional states of others/Empathy	[52, 54, 61]
Expressing creativity through fantasy play	[29, 58]
Remembering and referring to past events	[5, 24, 29, 58, 62]
Pointing explicitly to direct human attention (joint attention)	[4, 24, 52]

3. Importance of Rearing Environment

Language development depends not only on innate biological potential but also on the quality and richness of environmental input. This principle holds true not just for humans [67, 68], but also for great apes [69]. A growing body of evidence suggests that enculturated apes, those raised in human‐like, socially enriched environments from an early age, consistently outperform standard‐reared conspecifics across a variety of cognitive tasks [70, 71, 72, 73, 74]. Here, “standard‑reared” denotes apes raised in typical zoo or laboratory settings (either mother‑reared or nursery‑reared in conspecific peer groups) whose human contact is largely limited to routine husbandry (and, depending on facility, structured testing) rather than intensive socio‑communicative enculturation (e.g. [69, 75, 76]). The most well‐known LTAs, including Kanzi, Koko, and Washoe, achieved their symbolic communication abilities in contexts that provided not only structured language input but also ongoing social interaction, emotional engagement, and opportunities for spontaneous communicative exchange (Figure 1).

Figure 1.

Relationship between rearing histories and special experiences in great apes. The three large circles summarize broad rearing contexts: mother‑raised in captivity (pink; reared primarily by the biological mother or foster mother within a conspecific group), wild‑raised by the mother (green; early infancy spent with the mother in the wild before capture/rescue), and hand‑raised by humans (blue; includes nursery‑rearing or intensive human caregiving). Overlaps indicate mixed histories (e.g., a wild‑born infant later separated and hand‑raised; or a wild‑born infant subsequently placed with a foster mother in captivity). The small central intersection represents rare cases that experienced all three contexts at different early ages; it is included for completeness rather than to imply frequency.

In contrast, apes introduced to training later in life or raised in restricted, stimulus‐poor settings showed limited success in acquiring symbolic communication. These differences underscore the importance of immersive early exposure to socio‐communicative environments in shaping language‐related cognition. Indeed, enculturation appears to enhance not only language‐like skills but also broader cognitive functions, particularly those associated with social understanding and shared intentionality. For instance, enculturated apes consistently outperform standard‐reared apes in tasks involving declarative pointing, a behavior once thought to be uniquely human [75, 76, 77, 78], as well as in theory‐of‐mind assessments. The classic work of Premack and Woodruff [79] showed that chimpanzees could infer human intentions from observed behavior; later research refined this finding by demonstrating that such abilities are highly dependent on the quality of early rearing and communicative exposure [75, 80]. Buttelmann et al. [81] extended this study by showing that enculturated chimpanzees, like human infants, engage in rational imitation, modulating their behavioral copying based on the perceived intentionality behind actions, a skill thought to underpin key aspects of language acquisition and communicative learning. These results suggest that many of the presumed cognitive gaps between humans and nonhuman apes may be attributable, at least in part, to differences in developmental environments rather than fixed species‐level limitations.

Neurobiological findings provide further support for this interpretation. Cytoarchitectural analysis of Nathan, a bonobo raised in a symbol‐rich, enculturated setting, revealed a greater leftward asymmetry in the planum temporale than other great apes, which is a region encompassing Wernicke's area and long associated with language comprehension in humans [69].

Together, these behavioral and neuroanatomical findings underscore the profound influence of rearing environment on brain development and cognitive performance in apes. They support the hypothesis that some neural substrates thought to be uniquely human may instead reflect the cumulative effects of experience‐dependent plasticity in the context of socially enriched communication. In this light, LTAs serve as a powerful model for examining how early symbolic environments interact with biological predispositions to shape the neural architecture of language.

4. Comparative Neuroanatomy of Language‐Associated Areas in Humans and Great Apes

Language processing involves extensive, distributed networks spanning multiple cortical and subcortical areas, each contributing critical integrative functions [7, 8]. Complex language processing abilities arise from dynamic interactions within these networks rather than individual localized brain regions or singular genetic factors [7, 8].

Neuroimaging studies have revealed overlapping motor and linguistic activations within the inferior frontal cortex, underscoring its important role in linking perception and action with symbolic communication within broader neural networks [82, 83, 84]. Historically, Broca's area, located in the inferior frontal gyrus, was associated with expressive language and speech production following Paul Broca's landmark observations in the 19th century [85]. However, contemporary research has expanded our understanding of this region to include broader functions such as language comprehension, syntactic processing, working memory, action observation and execution, gesture recognition, calculation, and even music processing [86, 87, 88, 89, 90, 91].

Cytoarchitecturally, Broca's area can be divided into Brodmann's areas 44 (BA44) and 45 (BA45). In the human brain, these subregions are asymmetrical and typically show leftward lateralization, a feature that is evident as early as 1 year of age and becomes more pronounced with development [92, 93]. Notably, adult‐like asymmetry in the density of neuronal cell bodies is attained by age 5 in BA45 and age 11 in BA44, suggesting that early life experiences, such as exposure to language, may contribute to shaping this lateralization [93]. Interestingly, asymmetry continues to increase with age in BA45, while BA44 appears to stabilize earlier. These findings suggest a developmental window during which Broca's area is particularly sensitive to experience‐driven plasticity, highlighting the significance of the starting age of language training on communicative abilities and cognitive performance.

In contrast, chimpanzees, our closest living relatives, do not appear to exhibit population‐level asymmetries in these same regions, based on analyses of cortical region volume, neuronal density, and neuropil fraction [94, 95]. Recent work by Gallardo et al. [96] (2023) aligned population‐based cytoarchitectural maps of human and chimpanzee brains using advanced cortical registration techniques. Their analysis revealed that chimpanzee BA44 overlaps only with the posterior portion of human BA44 in the left hemisphere, an area implicated in action‐related functions, while showing no overlap with the more anterior subregions of BA44 that in humans support syntactic processing. Moreover, while human brains are approximately 3.6 times larger overall than chimpanzee brains, the left BA44 region is 6.6 times larger than its chimpanzee counterpart. This disproportionate expansion suggests a significant evolutionary elaboration of the anterior BA44 region in humans, possibly to support complex syntactic abilities. Conversely, BA45 appears to have undergone a more modest expansion and is primarily involved in semantic and lexical retrieval processes [97, 98, 99, 100]. Beyond Broca's area, the temporal cortex, particularly the superior temporal gyrus (STG) including Wernicke's area (primarily Brodmann's area 22), plays a central role in auditory processing and language comprehension [1, 7, 8]. Human Wernicke's area is prominently lateralized to the left hemisphere in the majority of individuals and extensively interconnected with frontal regions via the arcuate fasciculus, facilitating linguistic integration and speech production [7]. Comparative neuroanatomical studies indicate similar population‐level leftward asymmetry of the planum temporale (which includes part of Wernicke's area) in great apes, but differences in connectivity with humans, suggesting human‐specific modifications in STG connectivity patterns may underpin advanced linguistic capabilities unique to our species [101, 102].

Adjacent to temporal areas, the inferior parietal lobule (IPL), comprising the angular (BA 39) and supramarginal gyri (BA 40), acts as a crucial multimodal integration hub supporting semantic integration, reading, and the convergence of visual, auditory, and somatosensory information during linguistic tasks [103, 104, 105]. Comparative analyses highlight a substantial expansion of human IPL relative to nonhuman primates, particularly linked to language and tool‐use capabilities, suggesting evolutionary coupling between linguistic and sensorimotor cognitive functions [104, 106]. Neuroimaging studies further demonstrate IPL involvement during symbolic gesture comprehension and production, reinforcing its pivotal role in multimodal symbolic communication networks, relevant also to symbolic‐trained apes [107].

Subcortically, the basal ganglia significantly contribute to language through procedural learning, motor planning, and cognitive sequencing [108, 109]. The basal ganglia circuits interconnect cortical regions via striatal, pallidal, and thalamic relays, playing a key role in speech production, syntactic processing, and language acquisition. Lesions in these subcortical structures result in linguistic deficits such as aphasia and speech‐motor disturbances, emphasizing their integrative functions in language systems [110]. Comparative neuroanatomy reveals homologies across primates, yet uniquely enhanced connectivity in human basal ganglia‐cortical networks, particularly involving dopamine systems, may underlie sophisticated linguistic capabilities exclusive to humans [111, 112].

Additionally, the cerebellum has increasingly been recognized for its extensive linguistic and cognitive contributions beyond classical motor functions. Cerebellar regions, particularly lateral hemispheres and the dentate nucleus, maintain robust connections with prefrontal, parietal, and temporal cortical areas, forming cerebro‐cerebellar loops vital for precise temporal sequencing of linguistic tasks [113, 114]. Functional neuroimaging consistently demonstrates cerebellar activation during phonological processing, verbal working memory, and syntactic processing tasks. Correspondingly, cerebellar lesions are frequently associated with linguistic impairments, including dysarthria and agrammatism [113, 115]. Comparative anatomical studies highlight considerable evolutionary enlargement and specialization of the lateral cerebellar hemispheres in humans, aligning with the cognitive and linguistic specializations of our species [116, 117].

Given the notable neuroanatomical differences described, LTAs represent an essential model to investigate experience‐dependent plasticity within these neural regions. If they exhibit structural shifts toward human‐like organization, such as increased volume, enhanced connectivity, or emerging asymmetry, selectively in areas like Broca's region, STG, IPL, basal ganglia, or lateral cerebellum compared to standard‐reared apes, this would strongly indicate plasticity driven by symbolic communication. Such evidence would support the hypothesis that human language networks are dynamically shaped through developmental exposure rather than strictly genetically predetermined. Determining whether LTAs display neural reorganization remains an open empirical question. Addressing this through comparative analyses provides crucial insights into the evolutionary trajectories and cognitive capacities influenced by symbolic communication and neuroplasticity.

5. Connectivity Differences and Experience‐Dependent Plasticity

Two major white matter pathways, the dorsal and ventral language streams, anchor connectivity among cortical areas involved in language networks (Figure 2). These streams comprise several tracts that link Broca's area to regions involved in phonological processing, semantics, multimodal integration, and motor planning.

Figure 2.

The dorsal and ventral connectivity pathways in humans (A) and nonhuman primates (B). The main difference between humans and nonhuman primates refers to the increase in size of the dorsal stream. The white matter tracts are represented in different colors: The AF, arcuate fasciculus (dark blue); the SLF, superior longitudinal fasciculus (light blue arrows parallel to AF); the MLF, middle longitudinal fasciculus (orange); the EC, extreme capsule (light green); the UF, uncinate fasciculus (pink); the ILF, inferior longitudinal fasciculus (dark green); the IFOF, inferior fronto‐orbital fasciculus (yellow).

The dorsal stream, which includes the arcuate fasciculus (AF) and the superior longitudinal fasciculus (SLF), connects Broca's area to posterior temporal, parietal, and premotor areas. It plays a central role in speech production, phonological processing, and the learning of both spoken and signed languages [7, 118, 119, 120, 121, 122]. While homologous pathways are present in macaques and chimpanzees, human dorsal tracts, particularly the AF, exhibit more extensive projections into the temporal lobe, reaching well beyond the posterior STG into the middle and inferior temporal regions. This elaboration, particularly into the middle temporal gyrus (MTG) and superior temporal sulcus (STS), has been proposed as a key neuroanatomical feature supporting the uniquely complex syntactic and semantic capabilities of human language [101, 106, 123, 124, 125].

Humans also display a distinct leftward asymmetry in the AF dorsal tract, which is substantially reduced or absent in other primates. This pronounced asymmetry is thought to enhance cognitive efficiency through hemispheric specialization [126, 127, 128, 129]. This asymmetry is linked to more efficient phonological processing and may underlie the high‐speed demands of spoken language. Interestingly, in chimpanzees, variability in AF volume has been associated with individual differences in the use of communicative gestures and vocalizations [129], suggesting that these tracts may be sensitive to developmental experience.

Importantly, preliminary diffusion tensor imaging (DTI) studies of LTAs such as Panzee and Lana [69] reveal more human‐like patterns of asymmetry in the SLF, indicating that symbolic communication training may drive plastic changes in language‐relevant white matter tracts. Whether this plasticity also extends to SLF III, which connects Broca's area to the anterior inferior parietal lobe (aIPL) and is involved in integrating somatosensory information related to mouth and hand actions, remains an open question. SLF III exhibits increased size and connectivity in humans compared to chimpanzees and does not reach BA45 in macaques [130, 131, 132, 133, 134]. Its expansion and lateralization in humans are thought to support the integration of motor and conceptual information, an essential feature of spoken and signed language processing.

The ventral stream complements the dorsal stream by integrating auditory and visual sensory input with semantic knowledge. It includes several tracts, namely the inferior longitudinal fasciculus (ILF), inferior fronto‐occipital fasciculus (IFOF), uncinate fasciculus (UF), middle longitudinal fasciculus (MdLF), and the extreme capsule (EC), that collectively support semantic representation and higher‐order language comprehension [135, 136, 137, 138, 139, 140]. These tracts converge to form the extreme capsule fiber system (ECFS), which connects Broca's area to the temporal lobe, including Wernicke's area, and enables bidirectional communication between speech comprehension and production centers [9, 104, 141, 142].

Although the ECFS is conserved across primates [132, 143, 144, 145, 146], humans show a distinctive connectivity in the anterior temporal lobe, including expansions in the UF, ILF, and MdLF, particularly connecting to the middle temporal gyrus (MTG) and anterior temporal lobe (ATL) [125, 147, 148, 149]. These enhancements are believed to underlie the advanced semantic processing capabilities of our species. Whether LTAs show partial extension of these ventral connections as a result of symbolic training has yet to be tested, but represents an exciting avenue for future research.

A particularly notable organizational feature is the distinct termination patterns of the dorsal and ventral streams in Broca's area. The dorsal stream projects predominantly to BA44, supporting syntactic and phonological processing, while the ventral stream terminates more strongly in BA45 and BA47, areas implicated in semantic integration [132, 150, 151, 152, 153]. Notably, in humans, dorsal projections via SLF III extend more anteriorly into BA45 and BA47/12 than in chimpanzees, potentially reflecting an evolutionary elaboration relevant to linguistic processing [130]. This spatial segregation and its anterior extension may help explain how multiple linguistic processes operate in parallel within the inferior frontal cortex and provides a framework for testing how experience‐dependent changes might reshape Broca's connectivity in LTAs.

In summary, both dorsal and ventral language pathways exhibit species‐specific differences in size, lateralization, and target specificity, differences that correlate with humans' unique language abilities. The study of LTAs offers a unique opportunity to investigate whether symbolic communication experience can shift white matter architecture in apes toward a more human‐like configuration. If LTAs demonstrate increased tract volume, altered termination zones, or greater lateralization in either pathway, it would offer compelling evidence that the neural circuitry supporting language is not fixed but is, in part, sculpted by communicative experience.

6. Molecular Mechanisms Linking Language and Environment

Environmental input during development can have lasting effects on brain function by altering gene expression through epigenetic mechanisms. These mechanisms, such as DNA methylation, histone modification, and the activity of noncoding RNAs, allow experience to influence biological systems without changing the underlying genetic code [154]. These mechanisms operate across distributed neural networks rather than isolated brain regions or single genetic pathways. In the context of human language acquisition, it has been shown that epigenetic regulation plays a crucial role in modifying the expression of key genes involved in synaptic plasticity, neural connectivity, and cognitive function [155].

Twin studies have further supported the role of experience in shaping language‐related brain activity, indicating that while genetics account for about half of the variability in cerebral oscillatory patterns in the left frontal cortex, the remaining variance is attributable to environmental factors, such as social enrichment and early language exposure [156]. Sriganesh et al. [157] similarly found that neuroepigenetic regulation helps explain individual differences in language learning and may be particularly important in early developmental windows.

One compelling example is CNTNAP2, a gene that encodes CASPR2, a neurexin protein crucial for brain development and function [158, 159, 160]. Studies have shown that humans exhibit reduced DNA methylation and increased expression of CNTNAP2 compared to chimpanzees in prefrontal cortex (frontal pole; Brodmann's area 10) [161]. This hypomethylation pattern is thought to facilitate greater expression of CNTNAP2 in humans, potentially enhancing neural connectivity and plasticity, suggesting that epigenetic regulation may also contribute to enhanced expression of CNTNAP2 in regions like Broca's area. Furthermore, evidence from zebra finches show that CNTNAP2 expression is regulated by FOXP family proteins and is responsive to auditory experience, demonstrating a clear link between environmental stimuli and gene expression relevant to communication [162]. Future work should also consider broader neuroanatomical networks and multifactorial genetic interactions that underlie language capabilities.

Variants of CNTNAP2 have been implicated in speech delays, stuttering, and autism spectrum disorders [163, 164, 165]. In humans, CNTNAP2 exhibits a remarkable anterior‐enriched expression during developmental periods, a pattern not observed in mice or rats [166], with particularly higher levels in Broca's area compared to the premotor cortex during adulthood [167]. Functional imaging studies have shown that common variants of the CNTNAP2 gene modulate neural responses to syntactic violations, indicating its role in shaping individual differences in language processing [160]. Nevertheless, the broader implication is that multiple distributed neural circuits likely work in concert, emphasizing the need to expand beyond single‐region or single‐gene analyses.

Moreover, comparative genomic analyses have identified human‐specific regulatory variants in CNTNAP2, suggesting that evolutionary changes in this gene's expression could be tied to our species' unique capacity for language [168, 169]. While compelling, these genetic insights should be integrated into a broader framework that accounts for cross‐species developmental and life‐history trajectories, and the complexity of socially and symbolically enriched environments.

Among the genes most strongly implicated in language development and evolution is FOXP2, a transcription factor initially identified in a family with a hereditary speech disorder involving verbal and orofacial dyspraxia [170, 171]. FOXP2 regulates a network of downstream genes involved in synaptogenesis, neural development, and plasticity, including CNTNAP2, SRPX2, EGR1, PLAUR, DISC1, GAD1, and DLX5 [159, 172, 173]. Gene ontology analyses group these FOXP2 targets into functional categories related to learning and memory, synaptic signaling, and the organization of neural circuits.

Comparative studies have found that FOXP2 and its targets are differentially expressed across species. For instance, Konopka et al. [173] demonstrated that the human version of FOXP2 drives distinct gene expression profiles in the frontal pole, caudate nucleus, and hippocampus compared to the chimpanzee ortholog. Although research investigating FOXP2 expression, specifically in mature human or great ape Broca's area is limited, these findings suggest that differences in FOXP2 regulation could contribute to the unique linguistic capabilities of our species, and encourage further investigations regarding this region.

Recent advances in single‐nuclei RNA sequencing and chromatin accessibility analyses have revealed human‐specific upregulation of FOXP2 in neuronal subtypes in regions such as the posterior cingulate cortex. These findings open avenues for understanding distributed neural responses to symbolic communication beyond the localized emphasis on Broca's area. LTAs, whose brains are shaped by symbolic communication experience, present a unique opportunity to determine whether environmental exposure alone can modulate FOXP2 expression or its downstream targets across disparate brain regions.

In sum, LTAs provide a rare experimental window into how language‐like environments may regulate gene networks in the brains of nonhuman primates. By applying transcriptomic and epigenetic analyses to LTA tissue representing a broad sample of brain regions, we can test long‐standing hypotheses about the experience‐dependence of language‐related genes and better understand the evolutionary and developmental dynamics that gave rise to human linguistic abilities. This integrative theoretical framework leverages existing molecular and epigenetic research findings to emphasize the distributed and emergent properties of neural plasticity underpinning symbolic communication.

7. Cellular and Metabolic Correlates of Symbolic Communication Experience

The human brain, despite representing only about 2% of total body weight, accounts for approximately 20% of basal energy consumption, driven by the energetic demands of synaptic communication in a relatively large brain [174]. Mitochondria supply the metabolic energy required for synaptic plasticity, learning, and sustained neural activity. Mitochondrial function has been shown to vary across brain regions and hemispheres [175], although direct evidence for whether there are specializations selective to brain regions in the language network is currently lacking. An intriguing hypothesis is that the symbolic environments experienced by LTAs might drive energetic adaptations in ways that parallel human‐like processing demands. For instance, increased mitochondrial density or activity in the left hemisphere of LTAs might align with the lateralized structure and function of human language circuits. While most existing data from nonhuman primates on mitochondrial variation come from studies of the dorsolateral prefrontal cortex in cercopithecoid monkeys [176], future comparative work on LTAs could reveal whether symbolic enrichment produces parallel shifts in the metabolic architecture of brain regions involved in language.

More broadly, metabolic and cellular adaptations are underpinned by gene expression changes associated with learning and neuroplasticity. Comparative transcriptomic studies have shown that humans display distinct neuronal gene expression profiles related to synapse organization, dendritic branching, and cognitive flexibility [177, 178]. In addition, it has been shown that neurons in the human cortex have greater dendritic complexity than those of other primates [179, 180], and interindividual variation in dendritic morphology has been linked to higher cognitive performance [181]. Importantly, educational and cognitive training are known to enhance dendritic complexity and synaptic connectivity in humans [182, 183], raising the possibility that symbolic communication experience could produce similar effects in LTAs.

Astrocytes are a type of glial cell critical for synaptic regulation, neurotransmitter uptake, and metabolic support. Notably, humans exhibit both relatively higher astrocyte density and larger astrocyte soma volumes in the prefrontal cortex (BA9) compared to other primates such as chimpanzees, baboons, and macaques [184]. These features may be related to the metabolic demands of human cognition and would be predicted to differ in language‐associated regions of the brains of LTAs as a reflection of adaptive neurobiological changes in response to heightened communicative demands.

If LTAs exhibit increased dendritic branching, upregulation of plasticity‐related genes, or region‐specific metabolic enhancements in areas such as Broca's area, this would support the idea that developmental experience with symbolic communication can induce neurobiological changes reminiscent of those seen in humans. Such features, ranging from altered astrocyte morphology to enhanced mitochondrial function, may reflect convergent responses to shared communicative demands, even in the absence of the types of human‐specific genomic changes that have received so much attention. By situating these findings within broader patterns of neural plasticity and energy allocation, this synthesis underscores the possibility that symbolic communication experience in great apes can model key aspects of the evolutionary path toward the human language‐ready brain [174, 184].

8. Future Directions and Implications

The study of LTAs can offer valuable insights into identifying which aspects of brain language‐associated structures are evolutionarily conserved, which demonstrate experience‐dependent plasticity, and which may be uniquely human (Figure 3). Such insights have important implications for understanding neurodevelopmental disorders, given that genes such as CNTNAP2 and FOXP2 are linked to conditions like autism, stuttering, and schizophrenia, all of which involve disruptions in language‐related neural circuits [158, 165, 171]. Investigating experience‐dependent gene expression patterns in LTAs could thus help identify protective or risk‐related molecular factors associated with these disorders.

Figure 3.

Summary of future prospects of LTAs.

Currently available postmortem brain tissues from LTAs already enable comprehensive multimodal analyses, presenting immediate opportunities to substantially refine theoretical models concerning the evolutionary and developmental trajectories of human language. Linking postmortem data with developmental dynamics will further enable valuable information. For that, it seems plausible to (1) link archived longitudinal behavioral records (e.g., training intensity, degree of enrichment, results of cognitive tests, age of first symbolic exposure, levels of acquired symbol vocabulary, mean length of utterance, gesture inventories, etc.) to region‑specific neuroanatomy, white‑matter metrics, and multi‐omics; (2) apply single‑nucleus RNA‑seq, chromatin accessibility, and methylation profiling to quantify cell‑type composition and identify enduring gene‑regulatory signatures associated with symbolic experience; (3) where available, align postmortem findings with non invasive MRI/DTI from living apes engaged in cognitive communicative studies, alongside standard reared conspecifics (e.g., recent chimpanzee AF work [129]); and (4) test a priori predictions derived from ethical human observational cohorts that vary in age of first language access (e.g., early native signers vs. those with delayed access), which consistently implicate experience‑dependent tuning of dorsal/ventral language pathways [118, 120, 129, 138]. Despite methodological complexities, the rarity and depth of longitudinal behavioral data make LTAs an invaluable resource for understanding language‐driven neural plasticity in comparative context.

This integrative perspective synthesizes established neuroanatomical, molecular, and behavioral data from LTAs to highlight how comparative approaches have the potential to significantly enhance theoretical models of language evolution and neuroplasticity. By leveraging existing evidence, this synthesis provides a robust framework for understanding the evolutionary and developmental mechanisms underlying symbolic communication [69, 73, 74].

Erbaba B., Sinha M., Guevara E. E., Hecht E. E., Hopkins W. D., and Sherwood C. C., “Insights From Language‐Trained Apes: Brain Network Plasticity and Communication,” Evolutionary Anthropology: Issues, News, and Reviews 34 (2025): 1‐15, 10.1002/evan.70018.

Data Availability Statement

The authors have nothing to report.