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... it has been difficult to identify nonlanguage predecessors of
structured language. Most books on language evolution seem to dissolve
into a series of vague generalities when it comes to deal with specific
issues, such as how language acquired its universal characteristics, or
what novel processes in the brain made language possible. The linguists
haven't known the relevant neurology or evolutionary theory, and the brain
researchers haven't known enough linguistics to realize what features
required a detailed explanation. And there is, in addition, the requirement
of continuity: How do your proposed mechanisms create an easy series of
improvements or conversions of function, stages that span the whole
distance, starting with chimpanzeelike utterances, working up to words and
protolanguage, and finally to the phrases and clauses that enable you to
say "I think I saw him leave to go home" with its nested embedding?
We do it here with about two-and-a-half preadaptations, yielding both
argument structure and phrase structure or, if you prefer, the newer
minimalist grammar. The coin in which these improvements were purchased was
that of reciprocal altruism, cognitive categories handy for detecting
cheaters, and of ballistic movement planning, handy for toolmaking and
hunting.
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http://weber.u.washington.edu/~wcalvin/1990s/1998LinguaExMachinaPrecis.htm