If you are looking for one overall argument that summarizes the whole chapter, it would be that geography and agriculture caused Sub-Saharan Africa’s population to look like it did before Europeans came and that those same factors caused Europeans to be able to dominate when they did come.
Diamond argues that Sub-Saharan Africa is “black” because of geography and agriculture. He argues that the Bantu people had agriculture while other Africans, such as the Khoisan and the Pygmies, did not. This allowed the Bantu to sweep across Southern Africa and dominate the other ethnic groups in most areas. However, geography made it so that their crops would not work in certain areas and the original ethnic groups continued to live in those areas.
Diamond then argues that geography and agriculture allowed the Europeans to dominate the Africans when they arrived. He argues that there were four reasons for this. First, Europe had more domesticable animals. Second, it had more domesticable plants. Third, Sub-Saharan Africa is not very big and therefore did not have as many people as Eurasia. Finally, Africa is oriented on a north-south axis, thus making it hard for crops and culture to spread.
Thus, the main argument of this chapter is that geography and agriculture shaped the destiny of Sub-Saharan Africa.