I just read Don't Sleep, There Are Snakes, a book by Daniel L. Everett about his time with the Pirahã (pronounced pee-da-HAN) tribe in the Brazilian Amazon. The Pirahãs have no grammatical terms for single or plural, and no words for numbers. According to Everett they cannot even be taught to count or do simple arithmetic. Their language also lacks relative clauses. Everett argues from this evidence that some aspects of grammar that Chomskyans believe to be innate actually aren't.
Everett also says the Pirahã language won't permit you to talk about events you haven't witnessed directly or heard about from a direct witness. This makes it impossible to convert them to Christanity -- Everett's original intention. Instead, his time among the Pirahã converted Everett to a nontheistic worldview. I am not the first commmentator to observe that this sounds like a Borges story.
The Pirahã have no creation myths. Generally, they don't do abstraction. In this 2002 paper, Marc Hauser, Noam Chomsky, and Tecumseh Fitch argued that “recursion” - the ability to include units of meaning inside other units – is “the only uniquely human component of the faculty of language:” Everett denies that the Pirahã language has recursion, and has a new book coming out soon arguing that language is not innate and that there is no language instinct.
The idea of a missionary in the Amazon refuting the thought of the most prominent theoretical linguist of the last-half century has great journalistic appeal -- which should be cause for suspicion. The Edge Foundation website provides some arguments from Stephen Pinker and others against the “strong version” of Everett's claims, along with Everett's counter-responses.
The most poignant detail for me is that Everett's ex-wife Keren is still a missionary to the Pirahã. John Colapinto's “New Yorker” article about them from a few years back quotes Keren on Pirahã -- “This language uses prosody much more than any other language I know of” -- and conveys her belief that, when her understanding of their language is deep enough, she will finally succeed in cconverting the Pirahã to Christianity.
Mark Liberman compares Pirahã syntax to Elmore Leonard dialogue -- I'm going to stop now, because it's starting to sound even to me as if I'm making this up...