Thomas and Kirby (2018) argued that once language structure is understood as the product of cultural evolution, the biological question becomes how two precursor traits evolved: transmission of a communication system through learning, and the ability to infer the communicative intent behind a signal. Domestication independently produces each trait in other species — the domesticated Bengalese finch shows expanded vocal learning, and the domestic dog shows heightened sensitivity to human communicative intent — yet no non-human domesticate combines them, and only humans converted the combination into a structure-creating process of cultural transmission via self-domestication. The source identifies five constituent open issues: the unique human combination of precursor traits; the interplay of relaxed and sexual selection in generating learned song complexity; the untested temperament-unleashes-latent-cooperative-communication hypothesis; the contested neural-crest-cell link to human brain globularity and language readiness; and the still-speculative role of selection on temperament in human evolution. Later work has deepened, complicated, and operationalized these questions — new putative self-domesticated vocal learners (marmosets, elephants), mechanistic proposals for domestication-driven song complexity, partial replication failures of tolerance-cooperation links, and an active controversy over the neural-crest hypothesis — but none of the five issues is resolved, and the central question of why the full precursor combination arose only in humans remains without an empirically grounded answer.

Background

A long tradition in language evolution asks what biological evolution had to contribute to language. One influential body of work shows that linguistic structure — phonological and syntactic regularities — can emerge through cultural evolution: when a communication system is repeatedly learned and used across generations (laboratory 'iterated learning' experiments, reviewed in Smith 2020), the transmission bottleneck itself favours learnable, compositional, structured systems. If cultural evolution can account for language structure, then biological evolution need not encode structure in the genome; instead, it must explain the origin of the traits that made structure-creating cultural transmission possible. Thomas and Kirby (2018) identify two such precursor traits: (1) the central role of learning in transmitting the communication system, and (2) the capacity to infer the communicative intent of a signal or action (recognizing that a signal is ostensive, i.e. deliberately produced to inform, rather than a mere cue).

Domestication — the evolutionary process by which species adapt to a human-controlled, selection-buffered environment — is a natural laboratory for both traits. The domestication syndrome is a recurring suite of traits in domesticated mammals: reduced defensive and reactive aggression (impulsive, fear- or anger-triggered aggression), smaller brains, floppy ears, depigmentation, and craniofacial changes. The Bengalese finch, a domesticated strain of the white-rumped munia, develops a larger song repertoire and more complex, learned song syntax than its wild ancestor, plausibly because natural selection for simple, canalized song was relaxed while female preference for complex song persisted; the precise interplay of these two forces was already unclear when Thomas and Kirby wrote. The domestic dog, in contrast, shows little vocal production learning (the ability to modify vocal output through experience) but remarkable sensitivity to human communicative intent: dogs spontaneously use human pointing, gaze, and ostensive cues in object-choice tasks (a standard paradigm in which a subject must use an experimenter's cue to locate hidden food), outperforming even chimpanzees. The Siberian farm-fox experiment — decades of selective breeding of silver foxes solely for tameness — showed that selection acting on temperament and aggression can produce the whole domestication syndrome as a correlated by-product, indicating that dog-like cooperative-communicative capacities can emerge without direct selection for communication.

Humans exhibit hallmarks of a domesticated species (reduced reactive aggression, craniofacial gracilization, depigmentation, extended juvenile period), which grounds the human self-domestication hypothesis: an earlier process of selection against aggression in our own lineage set the stage for the cultural evolution of language. A mechanistic candidate is the neural crest cell hypothesis (Wilkins, Wrangham and Fitch 2014): neural crest cells are a migratory embryonic cell population that gives rise to, among other things, adrenal medulla cells, craniofacial cartilage and bone, and pigment cells, so mild deficits in neural crest development could jointly produce tameness and the morphological domestication syndrome; some authors extend this to links between neural crest regulation, the distinctive globular shape of the human brain, and neural connectivity relevant to language. Against this background, Thomas and Kirby left a precise set of open issues, of which the largest is the assembly problem: domestication reliably produces each language precursor in isolation — expanded learning in a songbird, communicative-intent reading in a canid — so why is a language-like communication system not simply part of the standard domestic phenotype, and what made the human case, where primate social cognition was apparently combined with the capacity to learn new signals, unique? Candidate explanations they raised — differences in pre-domestication evolutionary history, in the domestication pathway followed (commensal, prey, or self-domestication), and in the effects of self-domestication on brain size and cognition — were explicitly speculative.

Problem Statement

As stated by the source (Thomas & Kirby 2018): if domestication sets the stage for the cultural evolution of language, why are language-like communication systems not part of the domestic phenotype generally — why does the Bengalese finch exhibit only expanded vocal learning (learned transmission of its song) and the domestic dog only enhanced communicative sensitivity (a heightened ability to infer human communicative intent), while humans alone exhibit both precursor traits together in a way that enabled full language? The problem is to provide an empirically grounded account of the distribution and assembly of domestication-linked communication precursor traits across species, adjudicating the candidate explanations the source proposed — differences in pre-domestication evolutionary history, in the domestication pathway followed (commensal, prey, or self-domestication), and in self-domestication's distinctive effects on brain size and on combining primate social cognition with the capacity to learn new signals — which remain speculative. The source also identified constituent empirical links bearing on this account: (i) the precise interplay between relaxed selection and sexual selection in generating learned complexity in the Bengalese finch; (ii) whether changes in temperament can unleash latent cooperative-communication capacities, as suggested by chimpanzees whose tolerance constraints on cooperation (Melis et al. 2006) might be remediable through temperament change; (iii) the status of the neural crest cell hypothesis linking the domestic phenotype to human brain globularity and language readiness; and (iv) the relative roles of selection on temperament versus adaptation to a human-made environment in human evolution, which requires archaeological and paleontological evidence. The scientific question is why domestication produces each precursor trait in isolation in other species while their combination — the enabling condition for structure-creating cultural transmission — arose only in the human lineage, and it demands a causal, testable account grounded in the cross-species distribution and assembly of the precursor traits.

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