"The children don't know anything about Africa because those who knew about Africa have died," Hassan says.
Many of them were brought as slaves, but some were merchants, travellers and sailors.
Wiki mentions the following regarding the history of slavery. .
The Arab or Middle Eastern slave trade is thought to have originated with trans-Saharan slavery, though it soon became centered around settlements and ports in East Africa. It is one of the oldest slave trades, predating the European transatlantic slave trade by hundreds of years. Male slaves were employed as servants, soldiers, or laborers by their owners, while female slaves, mostly from Africa, were long traded to Middle Eastern countries and kingdoms by Arab, Indian, or Oriental traders, some as female servants, others as sexual slaves. Arab, Indian, and Oriental traders were involved in the capture and transport of slaves northward across the Sahara desert and the Indian Ocean region into Arabia and the Middle East, Persia, and the Indian subcontinent. As many African slaves may have crossed the Sahara Desert, the Red Sea, and the Indian Ocean as crossed the Atlantic, perhaps more. Some sources estimate that between 11 and 17 million slaves crossed the Red Sea, Indian Ocean, and Sahara Desert from 650 to 1900, compared to 11.6 million across the Atlantic from 1500 to the late 1860s. The Arab or Middle Eastern slave trade continued into the early 1900s.