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 hemispherecentral Chile and
western Australiabut 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.