The april 16 issue of The New Yorker magazine has an engaging essay (titled "The Interpreter") by John Colapinto describing the work of Dan Everett and others with the Piraha people of the Amazon. Since they were found in the 1700s, they have rejected everything from outside their world. They use one of the simplest language sound systems known. There are just eight consonants and three vowels, yet it possesses such a complex array of tones, stresses, and syllable lengths that its speakers can dispense with their vowels and consonants altogether and sing, hum, or whistle conversations (using what the linguists call "prosody"). The Piraha have no numbers, no fixed color terms, no perfect tense, no deep memory, no tradition of art of drawing, and no words for "all," "each," "every,""most," or "few" which some linguists take to be among the common building blocks of human cognition. They have a "one,""two," and "many" counting system and concerted teaching efforts fail to teach them to count to higher numbers. Everett thinks that the tribe embodies a living-in-the-present ethos so powerful that if affects every aspect of their lives. Committed to an existence in which only observable experience is real, the Priaha do not think, or speak, in abstractions - and thus do not use color terms, quantifiers, numbers, or myths.
Everett claims that their language lacks any evidence of recursion, which Hauser, Chomsky, and Fitch declared, in an influential 2002 paper in Science, to be the distinctive feature of the human faculty of language. He argues that recursion (embedding entities within entities) is primarily a cognitive, not a linguistic, trait. Many complex structures (like Microsoft Word) are organized into tree structures. Piraha appears to be a language that has phonology, morphology, syntax, and sentences, but no recursion.
Colapinto's article describes Fitch's visit with Evertt to the Priaha to perform tests trying to find any evidence for their recursive abilities. His results were largely inconclusive.