The historical record opens in Africa around 3300 B.C. with the rise of literacy in Ancient Egypt, an empire which continued, with varying levels of influence over other areas, until 343 B.C. Prominent African civilizations at different times included Carthage, the Kingdom of Aksum, the Nubian kingdoms, the empires of the Sahel, Great Zimbabwe, and the Congo. Ancient Africa possessed as many as 10,000 different states and polities characterized by different systems of political organization and rule. In 1418, a Chinese expedition led by Admiral Zheng He reached Africa's east coast; the Chinese traveled at least as far as Malindi in Kenya. In 1482, they were followed by the Portuguese, who established the first of many trading stations along the coast of Ghana at Elmina. The chief commodities dealt in were slaves, gold, ivory, and spices. The slave trade continued until its gradual decline in the 1820s was prompted by a lack of demand for slaves in the New World and increasing anti-slavery legislation in Europe and America. This caused dramatic shifts in local polities, obliging African states to implement new economies. The largest powers of West Africa adopted different ways of adapting to the shift. The Asante Confederacy and the Kingdom of Dahomey concentrated on the development of legitimate commerce in the form of palm oil, cocoa, timber, and gold, forming the bedrock of West Africa's modern export trade, while some, like the Oyo Empire, were unable to adapt, and collapsed into civil wars. In the mid-nineteenth century, European explorers became interested in opening the heart of the continent for trade, mining, and other commercial exploitation. Central Africa was still largely unknown to Europeans at this time. Missionary David Livingstone explored the continent between 1852 and his death in 1873; he was the first European to see Victoria Falls. A prime goal for explorers, one shared by Livingstone, was to locate the source of the River Nile. Subsequent expeditions by explorers located Lake Tanganyika and Lake Victoria; the latter was eventually proven as the Nile's main source. By the end of the century, much of Africa was well explored; this paved the way for the colonization which followed. In the late nineteenth century, the European imperial powers engaged in a territorial scramble, occupying most of Africa and creating many colonial states. Only Liberia and Ethiopia remained independent nations. Colonial rule by Europeans continued until the conclusion of World War II, when all these states gradually obtained formal independence. Today, Africa contains 53 independent countries, which mostly retain the borders drawn during the era of European rule. Since colonialism, African states have frequently been hampered by instability, corruption, and violence. Few African nations have been able to sustain democratic governments; many have instead cycled through a series of coups, producing military dictatorships. Great instability was mainly the result of the marginalization of other ethnic groups by military generals who were ignorant on matters of governance. For political gain, many leaders fanned ethnic conflicts that had been exacerbated by colonial rule. In many countries, the military was perceived as the only group that could effectively maintain order, and it ruled many nations in Africa during the 1970s and early 1980s. Territorial disputes were common, with the European-imposed borders of many nations being widely contested through armed conflicts. Cold War conflicts between the United States and the Soviet Union also played a role in instability. When a country became independent for the first time, it was often expected to align with one of the two superpowers. Many countries in Northern Africa received Soviet military aid, while many in Central and Southern Africa were supported by the United States, France, or both. Some countries were ruled by communist parties that sought to impose Soviet policies resulting in atrocities such as the Ethiopian famine of 1985-89. AIDS has also remained a prevalent issue in post-colonial Africa.