Africa in World History stresses Africa's interrelatedness to other regions and cultures, from early trade routes, the arrival of Christianity and Islam, and the ramifications of colonialism to contemporary issues such as HIV/AIDS and apartheid that have thwarted Africa's efforts to establish unity. Africa stretches across more than 11 million square miles, from the Sahara and Sahel in the north to the mineral-resource-rich south, the endangered rain forests of the west, and the Serengeti savannas of the east. Fossils from Ethiopia tell us that the human species originated in Africa, and scholars have different theories about the journey out of Africa made by Homo sapiens some 60,000 years ago. Today, Africa is home to over 1 billion people speaking more than a thousand different languages.