These once-isolated people, a tiny group, have no system of numbers; their sentences cannot accommodate subordinate clauses or other forms of recursion (embedding phrases), and they are not impressed by the Gospel of St Mark in Pirahã, not least because it is a story composed by someone they do not know, about someone they have never heard of, in a time and place that has no meaning for them. The Pirahã people tend to confine their discourse to things they know about, and their verb forms can be suffixed to distinguish between hearsay, inference and observation. They have no perfect tense.
On the other hand, they can also sing, hum, yell and whistle information to one another. So they have four additional speech forms as well as a very precise vocabulary for their environment and everything in it that matters to them. If there is some deep structure that underpins all 7,000 human languages – a universal grammar or language acquisition device or language instinct, already hard-wired in the human brain at birth – Pirahã seems to be an exception.