The earliest form of Indo-European that has been reconstructed by comparative linguistics must have differentiated from a parent language when it developed features not shared by the parent. What these features were is unknown as is the date of the separation.

Features of the landscape, climate, fauna and flora of the region in which the earliest form of the language was spoken have been deduced from the vocabularies of the Indo-European languages still in use and those known from texts. Where exactly or even approximately this region was, is still being debated.

Colin Renfrew has postulated that Indo-European speakers introduced agriculture during the Neolithic and that they spread east and west from a centre in Anatolia (or possibly from several centers). With the introduction of agriculture went a new Indo-European speech. Many linguists feel that the diffusion of agriculture was too early for the diffusion of the Indo-European languages.

More recently bearers of the Kurgan culture, spreading from the region north of the Crimea into Bulgaria and Romania, have been proposed as carriers of Indo-European.

This suggestion has wide support but the transmission route of the Indo-Iranian group in the east is unclear. The oldest Indo-European languages known are Hittite and its close relative Luvian. Assyrian records date them to 1900 BCE on the Cappadocian plateau. It is assumed that they did not develop there.

Placing the origins of Hittite and Luvian in the third millennium BCE calls for care when using the term "pre Indo-European" in a Western European context.Many other Indo-European languages will have been used without ever reaching the state of literacy and consequently, having a chance of preservation.