Africa is the ancient stage where humankind is believed to have evolved millions of years ago. Second only to Asia in size, much of Africa is rocky plateau covered by grasslands, with huge interior lakes feeding rivers such as the Nile, the world's longest. The first great civilization in Africa arose 6,000 years ago on the banks of the lower Nile. The search for maritime trade routes opened Africa to Europe in the 15th and 16th century. Slavery and colonialism followed. Although illiteracy, poverty, disease, and civil strife plague much of the continent, Africa's substantial natural resources offer hope.