The speakers of the Proto-Indo-European language are usually believed to have originated to the North of the Black Sea (today Eastern Ukraine and Southern Russia), and from there they gradually migrated into, and spread their language by cultural diffusion to, Anatolia, Europe, and Central Asia Iran and South Asia starting from around the end of the Neolithic period (see Kurgan hypothesis). Other theories, such as that of Colin Renfrew, posit their development much earlier, in Anatolia, and claim that Indo-European languages and culture spread as a result of the agricultural revolution in the early Neolithic.