According to Guns, Germs and Steel, how did Africa become black?

When Diamond asks how Africa became black, he is asking
how and why the Bantu peoples in Sub-Saharan Africa came to dominate and largely
displace the Khoisan and Pygmy peoples.  His argument in Chapter 19 about this mirrors
his argument in the book as a whole.  What he is saying is that the Bantu people were
luckier in terms of where they arose.  Because of their geographical luck, they had
agriculture and technology and were able to displace the other ethnic groups which had
none.

Basically, Diamond is saying that the Bantu's
ancestral homeland was in a place where there were crops that could be domesticated.  In
addition, it was near enough to North Africa that it was able to get domesticable
animals through diffusion from the Middle East.  Because of these advantages, the Bantu
were able to have larger and sedentary populations and could create and borrow
technology.  The Khoisan and pygmies, whose homelands did not have the advantages of the
Bantu's homelands, were not able to have these things and they, therefore, got
displaced.  In this way, Africa "became black."