plosone | In contrast with animal communication systems, diversity is
characteristic of almost every aspect of human language. Languages
variously employ tones, clicks, or manual signs to signal differences in
meaning; some languages lack the noun-verb distinction (e.g., Straits
Salish), whereas others have a proliferation of fine-grained syntactic
categories (e.g., Tzeltal); and some languages do without morphology
(e.g., Mandarin), while others pack a whole sentence into a single word
(e.g., Cayuga). A challenge for evolutionary biology is to reconcile the
diversity of languages with the high degree of biological uniformity of
their speakers. Here, we model processes of language change and
geographical dispersion and find a consistent pressure for flexible
learning, irrespective of the language being spoken. This pressure
arises because flexible learners can best cope with the observed high
rates of linguistic change associated with divergent cultural evolution
following human migration. Thus, rather than genetic adaptations for
specific aspects of language, such as recursion, the coevolution of
genes and fast-changing linguistic structure provides the biological
basis for linguistic diversity. Only biological adaptations for flexible
learning combined with cultural evolution can explain how each child
has the potential to learn any human language.
Linguistic diversity and the biological basis of language have
traditionally been treated separately, with the nature and origin of the
latter being the focus of much debate. One influential proposal argues
in favour of a special-purpose biological language system by analogy to
the visual system [10]–[13].
Just as vision is crucial in navigating the physical environment,
language is fundamental to navigating our social environment. Other
scientists have proposed that language instead relies on domain-general
neural mechanisms evolved for other purposes [14]–[16]. Just as reading relies on neural mechanisms that pre-date the emergence of writing [17],
so perhaps language has evolved to rely on pre-existing brain systems.
However, there is more agreement about the origin of linguistic
diversity, which is typically attributed to divergent cultural evolution
following human migration [9]. As small groups of hunter-gatherers dispersed geographically, first within and later beyond Africa [18], their languages also diverged [19].
Here,
we present a theoretical model of the relationship between linguistic
diversity and the biological basis for language. Importantly, the model
assigns an important role to linguistic change, which has been
extraordinarily rapid during historical times; e.g., the entire
Indo-European language group diverged from a common source in less than
10,000 years [20].
Through numerical simulations, we determine the circumstances under
which the diversity of human language can be reconciled with a largely
uniform biological basis that enables each child to learn any language.
First, we explore the consequences of an initially stable population
splitting into two geographically separate groups. Second, we look at
the scenario in which such groups are not fully separated, but continue
to interact to varying degrees. Third, we consider the possibility that
linguistic principles are not entirely unconstrained, but are partly
determined by pre-existing genetic biases. Fourth, we investigate the
possibility of a linguistic “snowball effect,” whereby linguistic change
was originally slow–allowing for the evolution of a genetically
specified protolanguage–but gradually increased across generations. In
each of these scenarios, we find that the evolution of a genetic
predisposition to accommodate rapid cultural evolution of linguistic
structure is key to reconciling the diversity of human language with a
largely uniform biological basis for learning language.
