Among Pirahã's many peculiarities is an almost complete lack of numeracy, an extremely rare linguistic trait of which there are only a few documented cases. The language contains no words at all for discrete numbers and only three that approximate some notion of quantity—hói, a "small size or amount," hoí, a "somewhat larger size or amount," and baágiso, which can mean either to "cause to come together" or "a bunch."
I've written about the implications such a language has for certain theories of Chomsky and other linguist/philosophers here:
...it challenges Chomsky's ideas of a "Universal Grammar" and a "Language Organ". Essentially, in this idea the Language Organ is seen as an inference machine, a computer that is loaded with some version of First Order Logic elaborate enough to give you simple mathematics (as, for example, in Russel's Principia Mathematica). Thus, "number" should be present in all human languages.
And yet Piraha lacks this supposedly "universal design feature" (as I believe Everett calls it). Hence no Universal Grammar. And since a UG and, according to some (Fodor at one time), a universal lexicon are supposedly our heritage as a species, perhaps no Human Nature either.