lack of navigable waters in Africa as the decisive factor,

The problem with such theories, even if one accepts them, is that they prove too much. If lack of navigable rivers explains why Africans did not advance, it also explains why nature did not select for the intellectual attributes best adapted to a place with navigable rivers. Thus, the river thing explains not only the lack of advancement but also the lack of ability.

The problem you face is what decisions you will make - as opposed to what opinions you may hold - as a consequence of your studies. You say you won't be happy if your daughter brings home a black man. But do you not trust her ability to find an outlier? What will you do when this dinner happens?