3. Africa
Modern humans evolved in Africa and south of the Sahara this antiquity carries with it a much greater genetic diversity than is found among the human populations of the rest of the world. This diversity includes the most drastic, or at least the most conspicuous, physical adaptations of any human populations to climatic conditions: the short stature of the Pygmies in the hot and humid rainforest, and the tall, thin body build of a scattering of peoples of the hot and dry savannas. Yet in Africa, as elsewhere, the prime factors explaining the diverse trajectories of human societies are more directly environmental. What sort of an environment does Africa provide for humans?
It makes some sense to think of Africa as a southern continent. Its current position on the globe is to the south of western Eurasia, and in origin it is the largest fragment of the old southern supercontinent of Gondwana. Yet Africa is not as southern as we tend to think. It actually extends slightly farther to the north of the equator than it does to the south, and it is about twice as wide in the north.
This location gives Africa a climatic symmetry comparable to that of the Americas, but considerably more limited in scope. As in the Americas, there is a substantial tropical belt around the equator characterized by rainforest, though the amount of it is far less generous. To the north and south of this band there is open country, which may be grassland where the summer rainfall is adequate or desert where it is not. On the northern side the grassland forms a belt immediately to the north of the rainforest; still farther north lies the Sahara, the world's largest and hottest desert, stretching continuously across the continent at its widest (though this region was significantly more hospitable several thousand years ago, when the Holocene climate was warmer than it is now, and consequently wetter). On the southern side of the rainforest the desert areas are in the south and west. Beyond the deserts lie the two extremities of the continent that enjoy a Mediterranean climate, with its winter rainfall. To the north Africa ends in a long coastline; while about half of this is desert, the other half has enough rainfall to share the climate of the Mediterranean at large. To the south a small region at the bottom of Africa has a climate of the same Mediterranean type.
As with the Americas, this picture of climatic bands is confused by the presence of mountains. But there the resemblance ceases. Africa is a relatively undisturbed piece of continental crust, remote from subduction zones except on the north, so it is generally flat. Such mountains as it has are concentrated in two regions. One is the northwest, where the Atlas Mountains are of the same recent vintage as the Alps, and result from Africa's collision course with Europe. The other, much larger region is East Africa, where the formation of mountains is associated with the splitting of the earth's crust that has given rise to .the rift valley. The rifting runs from north to south, and is one branch of a massive system of faults; the two most conspicuous branches of the system are the Gulf of Aden and the Red Sea. So whereas the Americas have their mountains in the west, Africa's are mostly in the east; and their size does not bear comparison with the massive ranges of the New World.
A final major feature of African geography runs from south to north: the Nile. Unlike such rivers as the Amazon or the Mississippi, the Nile brings water to a part of the world that desperately needs it. The Mediterranean generates little rainfall along the eastern half of the northern coast of Africa; Egypt, if left to the mercy of the Mediterranean, would be an unrelieved desert. What the Nile does is to give Egypt a transfusion of East African rainfall that has its origin in evaporation off the Indian Ocean.
Several features of this layout are crucial to the place of Africa in history. One is the proximity of the northern part of the continent to the western half of Eurasia. Africa is joined to it at the Sinai Peninsula, and comes close to it at the Straits of Gibraltar and the mouth of the Red Sea. Moreover, what separates northern Africa from western Eurasia is mostly the Mediterranean, which has been a zone of high maritime interaction over the last few thousand years. This means that the northern end of Africa is far from isolated, in sharp contrast to Australia or the Americas. Not so the southern end. This too shares a climate with other regions in its hemisphere—central Chile and western Australia—but it is utterly remote from them. Until the development of navigation on the oceans, any contacts between the southern fringe of Africa and a wider world had to cross the climatic bands of the continent. Here the open grasslands are good for interaction, but the deserts and the dense rainforest are not. We can add to this that the mountains of East Africa are not a corridor comparable to the Andes. In short, we can expect the history of the continent to be marked by a steep cultural gradient, with the advantage going to the north. And this is indeed what we find.