  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.
