The answer to this can be found mainly in Chapter 19. There, Diamond tells us that agriculture first arose in North Africa and extended as far as the highlands of Ethiopia. However, we should note that agriculture did not develop on its own here but, rather, diffused from the Fertile Crescent.
North Africa has a climate similar to that of the Fertile Crescent. Therefore, it was possible for crops that were domesticated in the Fertile Crescent to spread to Africa. This was, in fact, what happened. The crops spread from the Fertile Crescent to North Africa by at least 6000 BC, making the ancient Egyptian civilization possible.
In Table 5.1, you can see that agriculture may have arisen independently in the Sahel, West Africa, and Ethiopia. However, this is not completely clear and it certainly happened (if it did) after agriculture arose in North Africa.