The two main hypotheses are that Proto-Indo-European groups originated in Neolithic Anatolia and the Bronze Age Pontic-Caspian steppes. Archaeologist Colin Renfrew and others argue that the Proto-Indo-European homeland was in Anatolia and that Indo-European languages spread into Europe with the spread of agriculture. The more popular hypothesis states that the Proto-Indo-European homeland was in the Pontic-Caspian steppe, north of the Caspian and Black seas.