During the 3rd millennium BC, an intimate cultural symbiosis developed between the Sumerians and the Akkadians, which included widespread bilingualism. Akkadian gradually replaced Sumerian as the spoken language of Mesopotamia somewhere around the turn of the 3rd and the 2nd millennium BC (the exact dating being a matter of debate), but Sumerian continued to be used as a sacred, ceremonial, literary and scientific language in Mesopotamia until the 1st century AD.