Kaʼapor Sign Language is a village sign language — sometimes called a shared sign language — and the category matters. It did not develop in a school for deaf children, and it was not imported. It emerged inside one community and stayed there, which is why it is classified as an isolate with no demonstrated genetic ties to any other language in the world. The Kaʼapor people themselves speak a Tupí-Guaraní language, and the two systems coexist rather than compete. What makes the signed language unusual by global standards is who uses it. In most of the world, a signed language is the property of a deaf minority and a small circle of interpreters. Here, hearing Kaʼapor sign too, in varying degrees, and most hearing children grow up in both languages at once.

The first substantial description came from the linguist Jim Kakumasu, whose 1968 study remains the foundational account. He recorded a word order of object–subject–verb, a pattern rare enough among the world's languages to be worth noting on its own. He documented name signs, the personal labels that identify individuals — a feature linguists have since confirmed across unrelated signed languages worldwide. Conditional and imperative moods, he found, are carried not by the hands but by the face: eyes widening, facial muscles tensing. Questions are framed by a distinct question sign that can precede or follow the clause, which Kakumasu described as "a motion of the index finger towards the referent (addressee) with a slight wrist twist." None of this is improvisation or pantomime. It is grammar, with rules a speaker can break and be understood to have broken.

Kakumasu counted seven deaf people in a community of roughly 500 — about one in seventy-five, several times the global average. That ratio is the usual explanation for why the whole village signs: at that density, deafness stops being an individual circumstance and becomes a shared condition of community life, and the practical answer is for everyone to learn. Recent estimates put the number of deaf Kaʼapor who use the language as their first and only language at somewhere between seven and thirteen, within a Kaʼapor population now numbering in the low thousands across villages in the Alto Turiaçu Indigenous Territory. Figures vary between sources and between decades, and should be read as approximations rather than a census. What has not varied is the arrangement itself: deaf Kaʼapor are neighbors, relatives, and conversational partners, not a population to be managed.

The language has been catalogued under several names — Urubu Sign Language, Urubu–Kaʼapor Sign Language — and both carry an exonym the Kaʼapor did not choose and consider pejorative. The scholarly record has been slow to drop it. The anthropologist Darcy Ribeiro noted signing deaf people in this region as early as the 1940s, but decades passed with little systematic work. That changed with a documentation project led by the anthropologist Gustavo Godoy and the linguist Vitória Aniz, working with the Museu do Índio and funded through the Endangered Languages Documentation Programme. The resulting archive holds roughly thirty hours of video, ten of audio, some 150 photographs, and a small dictionary with transcriptions and translations. Crucially, part of the material was recorded by Kaʼapor signers themselves — which is how a language that was described about for half a century began to be documented by the people who speak it.