The development of man and civilization in Africa was not limited merely to the area in the Northeast. There is much evidence of cultural contact between people in all parts of the continent. When the Sahara began to dry out about 2000 B.C., the population was pushed out from there in all directions, thereby forcing the spread of both people and cultures. Even then, the Sahara did not become a block to communication as has been thought. There is clear evidence that trade routes continued to be used even after the Sahara became a desert. Scholars also have found that, shortly after the Iron Age reached North Africa, iron tools began to appear throughout the entire continent, and, within few centuries, iron production was being carried on at a number of different locations. At about the same time, sailors from the far East brought the yam and the banana to the shores of Africa. These fruits spread rapidly from the east coast across most of the continent, becoming basic staples in the African diet. New tools and new crops rapidly expanded the food supply and thereby provided a better way of life. West African Empires Although West Africa had been inhabited since the earliest times, about two thousand years ago several events occurred which injected new vigor into the area. The first event had been the drying of the Sahara, which had driven new immigrants into West Africa and, from the admixture of these new people with the previous inhabitants, a new vitality developed. Then, the introduction of the yam and the banana, as previously noted, significantly increased the food supply. Finally, the developments of iron tools and of iron work further increased the food supply and also provided better weapons. This permitted increased military power and political expansion. These were the necessary ingredients that led to the building of three large and powerful empires: Ghana, Mali and Songhay. Commerce was another factor which contributed to their development. Governmental control of a thriving trade in both gold and salt provided the wealth and power necessary for establishing these large empires. Unfortunately, our knowledge about West Africa's early history is severely limited by the lack of written records from that period. In recent years, archaeologists have been unearthing increasing amounts of material which contribute to our knowledge of early Africa. West Africans tended to build their cities from nondurable materials such as wood, mud, and grass. The area does have a rich oral tradition, including special groups of trained men dedicated to its development and maintenance. As oral history is always open to modification and embellishment, with no means available for checking the original version, this material must be used cautiously. Nevertheless, when employed in conjunction with other sources, it does provide a rich source of information.