Let's next examine whether this scheme, derived from the collision of Europeans with Native Americans, helps us understand the broadest pattern of African history, which I'll summarize in five minutes. I'll concentrate on the history of sub-Saharan Africa, because it was much more isolated from Eurasia by distance and climate than was North Africa, whose history is closely linked to Eurasia's history. Here we go again:

Just as we asked why Cort�s invaded Mexico before Montezuma could invade Europe, we can similarly ask why Europeans colonized sub-Saharan Africa before sub-Saharans could colonize Europe. The proximate factors were the same familiar ones of guns, steel, oceangoing ships, political organization, and writing. But again, we can ask why guns and ships and so on ended up being developed in Europe rather than in sub-Saharan Africa. To the student of human evolution, that question is particularly puzzling, because humans have been evolving for millions of years longer in Africa than in Europe, and even anatomically modern Homo sapiens may have reached Europe from Africa only within the last 50,000 years. If time were a critical factor in the development of human societies, Africa should have enjoyed an enormous head start and advantage over Europe.

Again, that outcome largely reflects biogeographic differences in the availability of domesticable wild animal and plant species. Taking first domestic animals, it's striking that the sole animal domesticated within sub-Saharan Africa was [you guess] a bird, the Guinea fowl. All of Africa's mammalian domesticates � cattle, sheep, goats, horses, even dogs � entered sub-Saharan Africa from the north, from Eurasia or North Africa. At first that sounds astonishing, since we now think of Africa as the continent of big wild mammals. In fact, none of those famous big wild mammal species of Africa proved domesticable. They were all disqualified by one or another problem such as: unsuitable social organization; intractable behavior; slow growth rate, and so on. Just think what the course of world history might have been like if Africa's rhinos and hippos had lent themselves to domestication! If that had been possible, African cavalry mounted on rhinos or hippos would have made mincemeat of European cavalry mounted on horses. But it couldn't happen.

Instead, as I mentioned, the livestock adopted in Africa were Eurasian species that came in from the north. Africa's long axis, like that of the Americas, is north/south rather than east/west. Those Eurasian domestic mammals spread southward very slowly in Africa, because they had to adapt to different climate zones and different animal diseases.

The difficulties posed by a north/south axis to the spread of domesticated species are even more striking for African crops than they are for livestock. Remember that the food staples of ancient Egypt were Fertile Crescent and Mediterranean crops like wheat and barley, which require winter rains and seasonal variation in day length for their germination. Those crops couldn't spread south in Africa beyond Ethiopia, beyond which the rains come in the summer and there's little or no seasonal variation in day length. Instead, the development of agriculture in the sub-Sahara had to await the domestication of native African plant species like sorghum and millet, adapted to Central Africa's summer rains and relatively constant day length.

Ironically, those crops of Central Africa were for the same reason then unable to spread south to the Mediterranean zone of South Africa, where once again winter rains and big seasonal variations in day length prevailed. The southward advance of native African farmers with Central African crops halted in Natal, beyond which Central African crops couldn't grow � with enormous consequences for the recent history of South Africa.

In short, a north/south axis, and a paucity of wild plant and animal species suitable for domestication, were decisive in African history, just as they were in Native American history. Although native Africans domesticated some plants in the Sahel and in Ethiopia and in tropical West Africa, they acquired valuable domestic animals only later, from the north. The resulting advantages of Europeans in guns, ships, political organization, and writing permitted Europeans to colonize Africa, rather than Africans to colonize Europe.