Via Language Log/John Cowan: Dan Everett's finally gotten around to publishing a few more examples of his claims about Piraha - notably, that they have no recursion, and in particular no subordinate clauses Even quoted speech and conditionals, he claims, are not embedded. Here it is: Pirahã culture and grammar: A response to some criticisms.
Now, recursion means being able to embed a given kind of phrase within another example of the same kind of phrase, as many times as you want. In "the door of the house", one noun phrase ("the door") is embedded within another one ("the door of the house"); in "I will visit you when it stops raining", a clause "it stops raining" is embedded within a larger one ("I will visit you when it stops raining"). You can also keep doing this ("the edge of the handle of the door of the house", "I will visit you when I know whether Khaled said that James is right about the forecast that it will rain tomorrow.") In Piraha, Everett reports that for noun phrases you can only do this once (no more than one possessor), and for clauses that you can't do it at all (he insists that all the examples that look like subordinate or adverbial clauses are actually separate sentences whose linkage is left for the listener to interpret, and in this paper presents some arguments for this.)
The thing is, a language with such properties has obvious potential to be expanded into a language like English or Arabic. For possessors, all it would take is a little analogical expansion - that's what allows us to interpret a phrase like "my brother's wife's cousin's friend's cat's teeth" as grammatical, even though you may well never have heard a noun phrase with six possessors before. For subordinate clauses, all it would take is grammaticalising some kind of erstwhile adverb or intonation pattern or quotative marker into a signal that these two clauses are more closely bound than others; such changes occur all the time in languages that already have subordinate clauses (eg "with what" > "in order to" in Algerian Arabic.) If the Piraha haven't done this, then why not? If they used to speak a language with multiple possessors and subordinate clauses in the past, why and how did they abandon these features - and if they never have, then why have most languages gained these features? In short, what motivates the expansion of grammar, and how does it happen?
One place (doubtless not the only one) where I think you can see expansion of grammar in action is technical terminology; consider mathematics. "The set of all p/q such that q!=0 and p, q are integers" is perfectly clear mathematical English, but is rather unlikely to be heard in everyday English (? "the set of all couples such that the husband is not an accountant and both the husband and wife are from Belgium"). The needs of mathematical communication have motivated the use of a kind of relative clause, with a complementiser and neither a gap nor a resumptive pronoun nor a relative pronoun, which is at best marginal in normal English; if enough people were trained as mathematicians, it might get used more widely. Maybe multiple possessors and subordinate clauses are technical features to cope with the demands of socialising with large numbers of people. Or maybe Piraha has a little more embedding than Everett reports. Speculation is fun, but a nice big, searchable, publicly available corpus would be a lot more convincing.