MLU: You make a good point. Funny that in the Chomsky quote you mentioned, he acknowledges Pirahã children as at least being capable of creating a "normal" language. But what constitutes a "normal" language? Surely the Pirahã regard their language as being the epitome of normal!
Chomsky doesn't seem to understand (or at least acknowledge) that although you have found no recursion in Pirahã grammar, recursion can be found outside of grammar, in normal human discourse. You say in your book that "…when we examine the stories that the Pirahã tell, we find recursion, not in individual sentences, but in the fact that ideas are built inside of other ideas--some parts of the story are subordinate to other parts of the story. Such recursion is not part of the syntax proper, but it is part of the way that they tell their stories."
Are you saying that recursion may indeed be essential to human thought, but it isn't a crucial component of language and so it need not be expressed in human grammar?
DE: The role of recursion in human cognition has been muddled ever since the publication of Hauser, Chomsky, and Fitch (2002)'s paper in Science. They correctly recognize that there is something special about recursion. But they never define it. In more recent work Chomsky and many of his followers have said that what he means by recursion is the very theory-internal operation of Merge, the way his current 'Minimalist Program' builds grammatical structures. There is speculation among some that the reason that humans are so much smarter than other creatures is that the recursion we need for language has jumped into general reasoning. That is one possibility -- begin in the computation underlying grammar and then be adopted by the rest of cognition. Others suggest that each mental module (vision, hearing, language, etc.) has its own recursion. But I don't really find this type of speculation particularly parsimonious. Still, it is an empirical issue.
So what we have with Pirahã is an interesting example, if I am correct, of a people who show recursion in their thought and stories but not in their grammar. That is unforeseen by Chomsky (one of the many reasons there is so much opposition to my work). The reason that this is a problem is that it suggests that recursion is always available but that some languages might not use it. Why? Cultural reasons.
Some might ask why Chomsky cannot simply say that discourse is language so if they have it in their discourse, there is no big deal. But all of Chomsky's work, including his most recent, is based on the foundation of the sentence as the highest level of grammar, not discourse. Not one of his recent theoretical proposals could be adopted to discourse without severely undermining his theory, because discourse is not the 'perfect' computational system that Chomsky claims the grammar of sentences is.
MLU: You mention in your book that there is a growing consensus among linguists that grammars without recursion precede grammars with recursion evolutionarily. That seems plausible to me. As with biological evolution, complex structures tend to evolve from simpler ones. Since the Pirahã resist change on many levels, I wonder if their language is older than most others--held fast by their unwillingness to change. Does their nonrecursive language function as a defense against seepage from other cultures or languages? Is that its "purpose?"
DE: There is nothing in what I say about Piraha that 'requires' that it be older, but that is certainly possible--we know almost nothing about the source of the Piraha language genetically (in the linguistic sense of its source language) or how long it has been separate from other languages and cultures. The lack of recursion, resulting, if I am correct, from cultural values, would indeed, along with those values, serve as a buffer against change and 'seepage'. This could be part of its purpose, but secondarily, as a way of supporting another cultural value of extreme inertia (not a cognitive limitation, mind you, a cultural value).