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. They didn't believe that Africans were sophisticated enough to found these great cities. Now scholars recognize that all the major Swahili cities were founded well before Islam arrived in the region, and that in fact trade had been going on since the first century CE. But Swahili civilization didn't begin its rapid development until the 8th century, when Arab traders arrived seeking goods that they could trade on the vast Indian Ocean network, the Silk Road of the Sea.