1.  The cognitive categories needed for keeping track of "Who owes what to whom" (evolved for minimizing the freeloader problem in reciprocal altruism) are a nice setup for the agent-theme-goal role tags so useful for quickly communicating "Who did what to whom."

2.  The prefrontal-premotor circuitry needed for planning novel throwing and hammering movements looks a lot like the binary trees of phrase structure.

The repeated payoffs of even more altruism, or even more accurate throwing, seem capable of adapting brain circuitry that could be occasionally borrowed for structured language.

3.  The cortical "plainchant choirs" needed for pre­programming accurate throws can, when extensive enough, make the long corticocortical paths (corpus callosum and such) temporarily coherent, overcoming the usual blur and jumble.  Coherence would permit the various phrases and clauses to be located at diverse cortical sites, yet still "sing as a unified choir" as alternative sentence interpretations compete with one another.

 The threshold for coherence provides a "capstone" candidate for fluent structured thought and talk -- something that might have triggered the flowering of art and technology seen late in hominid evolution, after brain size itself had stopped growing.

            Since corticocortical coherence is the one that builds atop the other two, and is the one that might have had a sudden emergence (the others look like gradual carryover so far, perhaps for millions of years), let me start by briefly explaining the first two pre-syntax "stones" of this arch so that the "capstone" role in enabling fluent syntax can be appreciated.

Antecedents of Argument Structure.  Bickerton's preadaptation is from reciprocal altruism.  What pays for the improvements in abstract cognitive categories for actor, recipient, and action are that they help keep track of who owes what to whom.  This is not only handy for minimizing the cheater problem, telling the individuals possessing such cognitive traits when they ought to find a partner more likely to reciprocate occasionally, but it aids the furtherance of coalitions between nonrelatives.

Bickerton's idea is that, once you have the cognitive categories for keeping track of "Who owes what to whom," you can start communicating "Who did what to whom" -- and have a syntax-ready recipient with the preformed mental categories needed for disambiguating the longer utterances.  You get the essentials of argument structure (categories for agent, goal, and theme) from the payoffs of ever more cooperation.  You may further improve the underlying neuroanatomy with new payoffs from language itself, but the foundations came "free," paid for via the prior cooperation/altruism use. 

Multifunctional Cortex.  Conversions of function are a familiar story in evolutionary theory (Darwin spoke of them before introducing his example of the fish swim bladder converting into an amphibian lung).  And any conversion goes through an intermediate phase of multifunctionality.

Despite our mapmaking tendencies that lead us into erroneously assuming one region, one function (it's another instance of the reification fallacy), cortical regions are notoriously multifunctional.  While a region may have a specialization that is essential (you can't do the function without it, as seen in the stroke and cortical surface stimulation results), the region may participate in other functions (as seen via its increase in blood flow when performing nonspecialist tasks).  Even the common strokes illustrate the multifunctionality: most aphasic patients also suffer from hand-arm apraxia, suggesting a core of neural machinery for novel sequencing that is shared by novel hand-arm and oral-facial movements. 

Antecedents of Phrase Structure.  The other common way of disambig­uat­ing a long string of words is what, in pre-minimalist days, was called phrase structure.  The nested embedding of phrases and clauses, one within another, is strongly reminiscent of the structured planning needed for planning multi-joint ballistic movements during "get set."  The hand movement is embedded in the elbow rotation which is embedded in the shoulder rotation which is embedded in the body's forward motion.  Planning involves getting the whole thing right; there may be dozens of ways to hit the target, but they're hidden in a sea of millions of wrong solutions, ones that would cause dinner to run away.

What pays for planning improvements is aimed throwing for hunting and precision hammering for toolmaking; when novel sequences of hand-arm movement are not being planned, such neural machinery may be available for structured planning of other things, such as long utterances or agendas.  Again, once structured language pays for further improvements, it may in passing improve aimed throwing or precision hammering, to the extent that they continue to share neural machinery.

The important feature of ballistic planning is its use of cortex for novel sequences (i.e., not standardized target distances as in darts and basketball free-throws, where subcortical circuits probably take over) with demanding requirements for timing precision (small targets have very brief "launch windows" and so timing jitter must be minimized). 

Corticocortical Coherence for Fluent Structured Thought.  I recently refined Hebb's 1949 notion of a cell-assembly^[2] whose spatiotemporal firing pattern (think of a short song) represents a concept, relationship, action -- or a phrase or a clause.  Even the complete sentence should have one, if it is to compete with alternatives.  This firing pattern would be a "code" for the concept, but one would expect the code for, say, comb to be different in visual association cortex than near auditory or motor areas.

A uniform-across-the-cortex code would be powerful but, for the same reasons as it took Europeans so long to invent the Euro, it isn't a default solution.  The corticocortical connection from concept-laden temporal lobe to movement-schema-laden frontal lobe via the arcuate fasciculus (second only the corpus callosum in size) is surely jumbled (neighboring axons may not remain neighbors at the other end) and the fanout of connections guarantees blur.  So a different code arrives, equally good in most respects (just like changing money when border crossing) but when it gets sent back to the originating cortex, it is doubly distorted.  For frequently used concepts, of course, an identity relation can soon be learned.

But when novel messages are being sent around ("'A square green tomato' -- anyone recognize this phrase?"), it runs into the same problem as trying to get moneychangers to recognize a wooden nickel.  You can't get the virtues of a universal code for novel on-the-fly concepts without the equivalent of a coherent corticocortical connection.

What I demonstrated^[3] in Chapter 7 of The Cerebral Code was that enough clones of the code in the sending cortex (think of a plainchant choir recruiting additional members from neighboring cortex, all singing the same song) would suffice to create a small choir singing the same song at the destination.  (Such choirs are needed for reducing jitter and are also an outcome of a Darwinian copying competition for achieving quality on-the-fly.)

This allows novel codes to be passed back and forth without slowly learned identities.  While handy for association tasks in general, the role of corticocortical coherence in nested embedding is where the common code really shines.  It seems capable of making syntax an everyday, subconscious task that operates in seconds.

The "meaning of the sentence" is, in this model, an abstract cerebral code which competes for territory with codes for alternative interpretations, often in the manner of a Darwinian cloning competition.  Phrases and clauses require coherent corticocortical links to contributing territories, having their own competitions and tendencies to die out if not reinforced by backprojecting codes.

It starts to look like a choral work of many voices, each singing a different tune but with the requirement that it mesh well with all the others.  Indeed, the symphonic metaphor might be appropriate for the more complex sentences that we can generate and understand.  Certainly the reverse-order analogy to Benjamin Britten's Young Person's Guide to the Orchestra, the all-together version being succeeded by the various voices playing separately, is the best metaphor I know for the read-out process that converts the parallel-structured plan into serial-ordered speech.