The caste systems in Africa have been linked to the a pre-developed trading network, invasions from North Africa and the Middle East after the 7th century, followed by a slavery system that targeted the pagans. According to Susan McIntosh – a professor of Anthropology specializing in African societies, archeological evidence shows that Arabs and Berbers had expanded and established an integrated sub-Saharan trade and transport network with West Africa, building upon the pre-existing trade routes through Western Sudan. This trade by 9th to 10th centuries, states McIntosh, included commodities and slaves. The reach of slave trading had extended into Ghana and the western Atlantic coast by the 11th century, and the slave raiding, capture, holding and trading systems became increasingly sophisticated in 13th and 14th century Mali Empire and 16th century Songhai Empire.