Abstract
Human language likely emerged from pre-symbolic social cognition rather than from a sudden, language-specific innovation. Across social animals and human infants, gaze following, affective attunement, and intention reading point to early forms of mind reading that precede explicit theory of mind and symbolic communication. In development, language and theory of mind appear to support each other: prelinguistic social cognition scaffolds later belief reasoning, while growing linguistic competence enables more flexible representations of others' mental states. In evolution, this trajectory may be explained by a neural threshold hypothesis, according to which quantitative increases in hominin brain size and connectivity yielded qualitatively new computational capacities. On this view, Homo erectus may have marked a critical transition, with expanded working memory, hierarchical integration, and social inference supporting rudimentary symbolic thought and early language. Rather than attributing language to a single mutation or an isolated neural mechanism, this account proposes that it emerged from an interacting trait package, a synthesis shaped by ecology, energetics, development, and social behavior. Modern language is therefore best understood as a culturally elaborated expression of a deeper neurobiological capacity rooted in early hominin evolution.
1 Introduction
The origin of human language remains one of the central questions in the field of psychology, anthropology, and neurobiology. Although many theories have focused on language as a specialized and uniquely human system, increasing attention has been given to the broader cognitive and cortical capacities from which language came about. This shift in perspective encourages a move away from explanations that treat language as the product of a single, possibly minor mutation, an isolated computational mechanism, or a sudden evolutionary event (e.g., ; ; ). Instead, it raises the possibility that language emerged within a larger framework of evolving mental abilities already present in earlier hominins (e.g., ; ; ).
A key issue is how to relate language to capacities that are not exclusively linguistic. Social attention, affective sensitivity, mind reading, working memory, action planning, and symbolic representation are all relevant to the structure and use of language, yet none of them is reducible to language alone. Comparative evidence from non-human animals and developmental evidence from human infants suggest that some linguistic behavior has its roots in social cognition. At the same time, language in modern humans clearly exceeds these precursor capacities in its open-ended combinatorial power, semantic flexibility, and cultural transfer. The challenge, therefore, is to explain how such precursor systems were transformed into a capacity that supports symbolic thought and language.
Here, we approach this issue from the perspective of an evolving mental capacity in the primate lineage. We ask whether quantitative increases in brain size, cortical connectivity, and behavioral flexibility and mobility may have crossed a functional threshold, producing qualitatively new forms of computation. In this context, Homo erectus is of particular interest. This species combines substantial changes in brain size, body form, mobility, tool use, and ecological range, and may therefore represent an important stage in the emergence of a more powerful and integrated mental capacity.
The discussion proceeds in three steps. First, we examine pre-symbolic forms of mind reading in social animals and young children. Second, we consider the idea that hominin brain expansion may have generated a neural threshold for more advanced cognition. Third, we evaluate whether the relevant transition is best explained not by any single cause, but by a package of interacting traits shaped by ecology, energetics, development, and social life. On this view, language becomes understandable as one expression of a broader evolutionary reorganization of the human mind.
2 Mind reading abilities
Many social animals across different lineages including great apes, monkeys, dogs, elephants, dolphins and corvids show pre-symbolic mind sharing abilities on a spectrum. This includes bodily and action-based behaviors such as following another's gaze, responding to emotional displays, coordinated group behavior, anticipate another's next move or noticing whether an individual is attentive or inattentive. For example, a chimpanzee may observe a group member who suddenly freezes and stares upward into a tree. The observing chimpanzee may also look up, become alert, and adjust its behavior. In such a case, attention has been socially transmitted. Social animals can read and be influenced by others' feelings and affective states. This may be described as an “affective theory of mind” (ToM) or as sensitivity to affective mental states (Shamay-Tsoory and Aharon-Peretz, 2007).
A similar affective sensitivity can be observed in human infants before speech onset. They follow adults' gaze, anticipate actions as goal-directed, and become distressed when others are distressed (e.g., Woodward et al., 2009; ). For example, while an infant is in a room with a parent who suddenly looks toward the door with a worried facial expression may also turn toward the door and become uneasy. Through the parent's gaze and expression, their private mental state becomes accessible to the infant. Humans and other social animals particularly express their emotions through facial expressions, including fear, anger, desire, excitement, forms of positive affect, and grief (e.g., ; ). This does not imply that these animals share minds in the full human sense.
The classic hidden-chocolate experiment illustrates that preschool children by the age of four or five become increasingly aware of the minds of others, including the possibility of false-beliefs (e.g., Wimmer and Perner, 1983). Here, the child has understood something profound. Another person's belief about the world may different both from reality and from the child's own knowledge. The child begins to understand that other people inhabit their own private mental world. Around the same time, the child can produce longer, more grammatical sentences, understands more complex spoken language, talks about past and future events, and uses language pragmatically by requesting, explaining, asking, imagining and interacting socially with others. The gradually developing explicit ToM does not completely depend entirely on language because infants show evidence in non-verbal false-belief paradigms before fluent speech (e.g., ).
Language nevertheless appears to play a major role in supporting the development of an explicit and flexible ToM, while itself relying on prelinguistic social cognition such as joint attention, intention reading, and affective attunement. Conversely, language abilities appear to scaffold more explicit forms, especially belief reasoning. A 5-year-old may use a single-embedded clause, as in She thinks (that X), or sometimes even a recursively embedded clause as in I believe [that she thinks (that X)]. The child gradually acquires the tools needed to model someone else's (possibly mistaken) perspective. Initially, children they show sensitivity to attention, goals, and knowledge access before the ToM, including false-belief understanding, develops in the context of increasing language and conversational experience ().
In evolution, ToM may have emerged without a full language. Complex human facial expressions, for example, are unlikely to be simply the outcome of language, but they were later elaborated through the coevolution of vocal, gestural, and coordinated social behaviors during early stages in the emergence of symbolic minds (e.g., ). In this vein, recursive sentence structures do not in general require a ToM. A sentence such as X [that chased Y (that stole Z)] is recursive but it does not include a mental-state clause.
The human mind supports the use of complex symbols in an open-ended combinatorial system through cumulative cultural transmission across multiple domains. This includes language, music, architecture, and tool production, although some biological and behavioral precursors of these capacities can also be found in other animals. Thus, the evolution of language has its roots in the mind's broader capacity (e.g., ; ; ; ). It is not the outcome of a single mutation or an evolutionarily encapsulated neural process but rather of a general cognitive capacity that emerged along with the evolution of the primate brain. It is evident that, in the primate lineage, a range of mental abilities and computational capacities became increasingly diversified and complex, with each capacity feeding back into the others. Early forms of mind sharing supported cooperation, which in turn enhanced mind-reading abilities. Improved mind reading then facilitated the development of explicit ToM, which further strengthened symbolic thought and language.
3 Mental capacity as a neural threshold
Human mental capacity is unique among extant species in its complexity and computational power. We cannot exclude the possibility that closely related extinct ancestors, in particular H. erectus, possessed human-like mental capacities. Some of the oldest archaic and modern Homo sapiens fossils were found in Morocco. The earliest modern human fossils are the Jebel Irhoud remains, dated to about 300–315 ka (; ), whereas archaic human fossils dated to about 773 ka were discovered at Thomas Quarry I/Grotte à Hominidés in Casablanca (). The oldest fossils currently assigned to our genus Homo date to about 2.8 Ma and are associated with the Ledi-Geraru jaw (LD 350-1) from Afar, Ethiopia (Villmoare et al., 2015). The emergence of H. erectus around 1.9–1.8 Ma is evidenced by the skull D3444 from Dmanisi, Georgia (). The Kenyan occipital fragment KNM-ER 2598 also marks a substantial increase in brain size relative to earlier hominins ().
Evidence from hominin brain evolution indicates quantitative neural change. However, such quantitative scaling may have shifted the brain into a new functional regime. A larger and more highly connected brain may not merely perform the same functions more efficiently but may also enable new kinds of computation. In this vein, it has been argued that the human brain is, in cellular terms, broadly a scaled-up primate brain (; ). Here, we argue that accumulated neurobiological and behavioral changes reached a neural threshold and brought about domain-independent mental capacities such as reasoning, planning, social cognition, symbolic thought or language ().
Scaling up a neural network can produce new functional properties when neuron number, conduction delays, recurrent loops, and cross-areal interactions support a non-linear transition that reaches a cutoff-like effect. More neurons increase representational capacity and support more stable neural circuits. Moreover, primate brains did not enlarge uniformly, but evolved additional cortical areas, especially the granular prefrontal cortex, which supports working memory and cognitive top-down control, and the parieto-temporal association cortex, which supports multisensory integration and memory-guided attentional behavior, including sentence planning and comprehension. The evolutionary expansion and prolonged maturation of white-matter pathways, especially within frontoparietal networks, support a distributed network that is critical for sentence generation involving speech planning and executive control (e.g., Saur et al., 2008; ; Sierpowska et al., 2022; ).
Such a neural threshold effect for complex mental computation may have been reached with the appearance of H. erectus around 2 Ma (, ,). At that point, the brain may have entered a new functional regime supporting more stable and enhanced working memory functions, which in turn enabled multi-step planning, hierarchical integration, cross-domain integration, richer social inference, increased theory of mind, and pre-symbolic or symbolic thought. This laid the groundwork for the mental capacity that enables computation in domains such as language, music, arithmetic, strategic planning, and toolmaking.
The expansion of the hominin brain occurred along with neural reorganization at the cellular and circuit levels. Bipedalism was a precondition for later cortical growth. Clear evidence of upright posture and gait is associated with Australopithecus afarensis (about 3.9–2.9 Ma), which had a relatively small brain of about 375–550 cc, although fragmentary fossil remains of earlier hominins already suggest gradual adaptation toward upright posture (). Genetic and developmental changes affecting hominin brain size and organization likely interacted with ecologically driven behavioral adaptations to increasingly open and variable habitats (e.g., , ; ; Will et al., 2021).
The neural threshold hypothesis, however, is supported only indirectly through the analysis of hominin fossil remains and the interpretation of behavioral traces. Published sources suggest approximate cranial capacities of about 600 cc for Homo habilis, broad overall variation in H. erectus around 727–1,251 cc, with earlier African individuals tending to be smaller than many later representatives, and about 1,350 cc for modern H. sapiens (e.g., ; ). These values indicate a broadly continuous expansion of cranial capacity across the Homo lineage, although brain size alone does not reveal differences in cortical organization, connectivity, or computational capacity ().
The neural threshold hypothesis is further supported by several major behavioral changes relative to earlier hominins such as H. habilis. H. erectus used more advanced Acheulean tools, including handaxes and cleavers, for more systematic carcass processing and meat acquisition. It also had a more human-like body anatomy that supported greater long-distance mobility, as indicated by its dispersal from Africa into Eurasia. Environmental change in turn appears to have promoted broader ecological flexibility. Evidence for habitual fire use and more regular hunting is plausible, though still debated (e.g., ). There is also some evidence that H. erectus engaged in pre-symbolic behavior. For example, the engraved Trinil shell and the deliberate symmetry of Acheulean bifaces are often discussed as possible evidence for pre-symbolic abilities (). Taken together, this cumulative behavioral evidence supports the view that H. erectus possessed a markedly advanced mental capacity. The associated neurobiological and behavioral changes suggest that the mind-reading abilities of modern humans, including language and ToM, evolved from a gradually scaled mental capacity.
4 The effects of an adaptive trait package
In exploring the successive expansion of neural structures and mental computing power in the hominin lineage, various anatomical, cognitive, and behavioral traits may have played a major role. Accordingly, various hypotheses have been proposed to model how the human mental capacity may have evolved. Here, we assume that the neural threshold for complex capacities emerged from the interaction of social, energetic, and ecological factors, each contributing to varying degrees to these significant neurobiological changes that enhanced mental computations. As part of an adaptive trait package, two factors appear to have played a particularly important role.
The “expensive tissue hypothesis” is one plausible approach to primate evolution. It states that, with a diet that is easier to digest and of higher energetic quality, energy can be shifted away from a large digestive tract and toward a larger brain (e.g., ). Non-human great apes have a gut in which the large intestine makes up a much greater proportion of total gut volume, whereas in modern humans more than half of total gut volume lies in the small intestine. Relative to body mass, humans also have a smaller total gut volume than non-human great apes (Watkins et al., 2010). However, some studies indicate that brain expansion likely reflects multiple energetic and life-history trade-offs rather than gut reduction alone. Large brains are metabolically expensive, and the trade-off may involve reduced non-fat tissue such as muscle mass and fat reserves (e.g., ).
Because soft tissues do not fossilize, any assumptions about the relation between internal organs and brain size in hominins must remain inferential. It is noteworthy that H. habilis still lived largely in woodland habitats, whereas H. erectus moved from woodland into savannah environments. Various behavioral changes are associated with this more challenging habitat around 2.0–1.8 Ma. H. erectus had a meat-rich diet and may have controlled fire and understood cooking. It is therefore plausible to infer that it had a relatively smaller gut than earlier hominins or chimpanzees ().
A different and important view is the “social brain hypothesis.” It states that across primates there is a strong correlation between neocortical ratio and mean group size. This correlation is assumed to reflect the cognitive demands of social life, including monitoring relationships, deception, cooperation, and intentions. In this view, the relation between neocortex size and mean group size can even be used to estimate a “natural” human group size (e.g., , ). The model refers mainly to living primates, but it has later been extended to fossil hominins. It addresses an important factor in cortical advancement, since mind reading and the management of social roles are central to primate cognition. The main limitation of this hypothesis is that it relies on proxy measures: group size does not by itself capture social complexity, and correlations between neocortex size and group size do not establish causation. More generally, cognition depends on distributed cortical and subcortical networks rather than on the neocortex alone (e.g., ; Shi, 2025).
Here, we are particularly interested in the cranial-capacity changes leading to early and late H. erectus (see Table 1). First, there is direct evidence that H. erectus moved in groups that included multiple adults. The Ileret footprint assemblage from Kenya (about 1.5 Ma) records 20 or more individuals of the species. This may indicate social dynamics involving increased coordination, possibly including sexually differentiated foraging (). However, coordinated movement does not by itself reveal tribe size. There are no independent methods for directly tracing changes in hominin social-group or tribe size over time (Shultz et al., 2012). Model-based estimates place the median tribe size for early Homo species and H. erectus broadly in the range of 50–100 individuals. This overlaps with actual chimpanzee community size. Even so, H. erectus presumably had a more human-like social structure in terms of cooperation and role allocation ().
Table 1
| H. habilis | Early H. erectus | Late H. erectus | H. sapiens | |
|---|---|---|---|---|
| Period (Ma) | 2.4–1.4 (1.0) | 1.9–1.5 (0.4) | 1.0–0.1 (0.9) | 3.0-present |
| MBR (cc) | 500–700 | 800-950 | 1,100–1,250 | 1,350–1,400 |
| ΔMBR (cc; mid.) | 100–450 (275) | 150–450 (300) | 100–300 (200) |
Continuous brain mass (MBR) expansion in the Homo lineage based on fossil records.
The brain-size differences between species are broadly comparable, typically ranging from about 200 to 300 cc.
As noted above, H. erectus faced more varied and harsher ecological conditions in East Africa and Eurasia than its primarily forest-adapted ancestors. This may have triggered shifts in foraging, energetics, and developmental plasticity. There is some evidence that trait correlations can arise during development itself (). Highly adaptive traits do not always evolve because natural selection directly favored each of them individually. Instead, development may channel variation in certain directions, and selection may then act on those pathways (see Figure 1).
Figure 1
From this perspective, an enhanced mental capacity, including memory functions, social planning, symbolic or multilevel processing, may not have resulted from one-by-one selection. Challenging conditions, together with an increasingly rich culture, may have shaped development in ways that produced a cluster of co-occurring traits—an adaptive trait package. In this sense, “language,” understood as a mental capacity and in its earliest rudimentary form, may plausibly be associated with H. erectus. Modern language has its roots in neurobiological traits, and subsequent cultural accumulation and transfer may be understood as further beneficial elaborations in response to an increasingly complex human world.
5 Conclusions
Human language appears to have evolved from pre-symbolic social cognition rather than from an isolated, language-specific innovation. Early affective and attentional mind reading abilities provided the basis on which more explicit cognitive theory of mind and symbolic language could gradually co-develop. The neural threshold hypothesis suggests that the human mental capacity emerged gradually through brain expansion and reorganization. On this view, H. erectus may already have approached a functional threshold that laid the groundwork for ToM, abstract thought, and language.
Thus, the emergence of the mental capacity for language was likely not driven by any single factor, such as the expansion of the dorsal frontotemporal pathway, but by a package of interacting traits shaped by ecology, development, energetics, and social behavior. On this view, combined anatomical, cognitive, and behavioral changes began to support more advanced forms of planning, cooperation, mind reading, and (pre-)symbolic systems. Like any mental ability, modern language, including complex semantics, syntax, and phonology, is therefore best understood not as a sudden innovation, but as the outcome of domain-specific cultural accumulation built on a deeper, domain-independent neurobiological capacity that had already begun to take shape in early hominin evolution.
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Author contributions
DH: Writing – review & editing, Writing – original draft.
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We sincerely thank the editor and the anonymous reviewer for their careful evaluation of the manuscript and for their helpful comments.
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Summary
Keywords
Homo erectus, language evolution, neural threshold hypothesis, social cognition, theory of mind, trait package
Citation
Hillert D (2026) Adaptive trait package for mind reading and language. Front. Lang. Sci. 5:1851229. doi: 10.3389/flang.2026.1851229
Received
09 April 2026
Revised
30 April 2026
Accepted
07 May 2026
Published
25 May 2026
Volume
5 - 2026
Edited by
George R. Mangun, University of California, Davis, CA, United States
Reviewed by
Edward Ruoyang Shi, Tianjin University of Finance and Economics, China
Updates
Copyright
© 2026 Hillert.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Dieter Hillert, dhillert@sdsu.edu
Disclaimer
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