There was no central authority. Each of these cities was autonomously ruled, usually, although not always by a king. But there were three things that linked the city-states such that we consider them a common culture. Language, trade, and religion. The Swahili language is part of a language group called Bantu, and its original speakers were from West Africa. Their migration to East Africa changed not only the linguistic traditions of Africa, but everything else, because they brought with them ironwork and agriculture. Until then, most of the people living in the east had been hunter-gatherers or herders, but once introduced, agriculture took hold, as it almost always does.