Scholars who believe that Proto-Italo-Celtic was an identifiable historical language estimate that it was spoken in the 3rd or 2nd millennium BCE somewhere in South-Central Europe.
[10] More recently, Schrijver (2016) has argued that Celtic arose in or close to northern Italy as the first branch of Italo-Celtic to split off, with areal affinities to Venetic and Sabellian, and identified Proto-Celtic archaeologically with the Canegrate culture of the Late Bronze Age of Italy (c. 1300–1100 BC).
[11] The most common alternative interpretation is that the proximity of Proto-Celtic and Proto-Italic over a long period could have encouraged the parallel development of what were already quite separate languages, as areal features within a Sprachbund.
As Watkins (1966) puts it, "the community of -ī in Italic and Celtic is attributable to early contact, rather than to an original unity".
[14] The r-passive (mediopassive voice) was initially thought to be an innovation restricted to Italo-Celtic until it was found to be a retained archaism shared with Hittite, Tocharian, and possibly the Phrygian language.