Semi-nomadic Indo-European peoples migrated from the steppes north of the Caucasus mountains between 1700-1200 BC, spreading across Europe, India, and Southwest Asia. They interacted and sometimes conquered the settled civilizations they encountered. These migrations helped spread Indo-European languages like English, Spanish, Persian, Hindi and German, which are now spoken by half the world's population. Groups like the Hittites in Anatolia and Aryans in the Indus River valley established new societies and social hierarchies, such as the caste system developed by the Aryans.