S1, E2: The second hour of “Africa’s Great Civilizations” charts the emergence of two powerful forces of global change, Islam and Christianity. Viewers will learn how pervasively “the Cross and the Crescent” reshaped the landscape and people of Africa between the first and 12th centuries A.D. — and for centuries to come. Setting the stage, host Henry Louis Gates, Jr. takes viewers to the horn of Africa, the meeting place of the Red and Arabian Seas and a trade corridor between Africa, the Middle East and Europe for thousands of years. A book written in Greek in first century A.D., The Periplus of the Eythrean Sea, speaks of the Red Sea’s illustrious port Adulis, the gateway to Aksum.