AFRICA IS IMMENSELY OLD. ITS CORE IS AN ELEVATED PLATEAU OF ROCKS formed between 3,600 million and 500 million years ago, rich in minerals but poor in soils. Unlike other continents, Africa's rocks have experienced little folding into mountain chains that might affect climate. Lateral bands of temperature, rainfall, and vegetation therefore stretch out regularly northwards and southwards from the equator, with rainforest giving way to savanna and then to desert before entering the belts of winter rainfall and Mediterranean climate on the continent's northern and southern fringes. The great exception is in the east, where faulting and volcanic activity between about 23 million and 5 million years ago created rift valleys and highlands that disrupt the lateral climatic belts.
This contrast between western and eastern Africa has shaped African history to the present day. At early periods, the extreme variations of height around the East African Rift Valley provided a range of environments in which living creatures could survive the climatic fluctuations associated with the ice ages in other continents. Moreover, volcanic activity and the subsequent erosion of soft new rocks in the Rift Valley region have helped the discovery and dating of prehistoric remains. Yet this may have given a false impression that humans evolved only in eastern Africa. In reality, western Africa has provided the earliest evidence of human evolution, a story still being pieced together from surviving skeletal material and the genetic composition of living populations. The story begins some 6 to 8 million years ago with the separation of the hominins (ancestral to human beings) from their closest animal relatives, the ancestors of the chimpanzees, perhaps when a cooler and drier climate privileged walking over climbing. The skull of the earliest candidate for hominin status, Sahelanthropus tchadensis, was discovered in 2001 by an African student examining the shores of an ancient Lake Chad. Apparently some 6–7 million years old, its small brain – no bigger than a chimpanzee's – and disputed upright stature have left its hominin status uncertain.1 Similar doubts surround the earliest fossils found in the East African Rift Valley, but there is wide agreement that some of the Australopithecines, who appeared there about 4.4 million years ago, were human ancestors.