You might want to turn to the final chapter, entitled "How Africa Became Black," to find the answer to this question. At the end of this fascinating discussion of Africa's history and how varying tribes and cultures rose to prominence, Diamond serves to restate the central thesis of this entire impressive book regarding world history and the vexing question of why it was that some cultures and peoples did better than others. He restates his central thesis in three sentences:
In short, Europe's colonisation of Africa had nothign to do with differences between European and African peoples themselves, as white racists assume. Rather, it was due to accidents of geography and biogeography--in particular, to the contintents' different areas, axes, and suites of wild plant and animal species. That is, the different historical trajectories of Africa and Europe stem ultimately from differences in real estate.
Again and again, throughout this excellent trawl through world history, Diamond proves his point that different people groups gained power and dominance over others not because of any racial differences, but because of their environment and the different advantages and disadvantages that their environment gave them. That, in a nutshell, is the thesis of this book in one sentence.