The internal reconstruction of the Indo-European proto-language done by Émile Benveniste and Winfred P. Lehmann has set Proto-Indo-European (PIE) typologically quite apart from its daughters.
In 1960, Aert Kuipers noticed the parallels between a Northwest Caucasian language, Kabardian, and PIE.
It was Paul Friedrich in 1964, however, who first suggested that PIE might be phylogenetically related to Proto-Caucasian.
In 1981, John Colarusso examined typological parallels involving consonantism, focusing on the so-called laryngeals of PIE and in 1989, he published his reconstruction of Proto-Northwest Caucasian (PNWC).
Eight years later, the first results of his comparative work on PNWC and PIE were published in his article Proto-Pontic: Phyletic Links Between Proto-Indo-European and Proto-Northwest Caucasian, an event which may be considered the actual beginning of the hypothesis.