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. Unless, wait for it, you're the Mongols. Modern-day Swahili, by the way, is still a Bantu-based language, although it's been heavily influenced by Arabic. On that topic, for a long time historians believed that these East African cities were all started by Arab or Persian traders, which was basically just racist.