And the "waxing and waning" was not due to genes but shifting social, political and institutional conditions - culture and society in short.
Note also that when the Hittites, ancient Egyptians, Harappans, Akkadians and even the Zhou dynasty China existed, at that time Europeans were primitive hunter-gatherers or just starting agriculture - did that make them 'genetically backward'?? NO
read Conrad's 'the heart of Darkness' - the first chapter to see how civilizational achievements aren't genetic related.
In the first chapter of Heart of Darkness, Joseph Conrad reflects on the Roman conquest of Britain. He describes the ancestors of the modern 'Anglo-American' population—the people you seem to think are the only ones capable of civilization—as 'savages' living in a 'dark place of the earth,' prone to the very 'dysfunctional' behaviors and lack of technological development that you currently attribute to Sub-Saharan Africa.
If your theory of 'fixed, innate racial traits' were correct, the Roman Empire should have seen those ancestors as a permanently inferior, unassimilable 'problem group' and walked away, or predicted that they would never build a global civilization. Instead, those people were subjected to the 'institutional framework' of Rome—its laws, its infrastructure, and its systems. They 'waxed' from that primitive state to become the architects of the modern world.
Conrad’s point, and the historical reality, is that the 'darkness' is not a feature of the people; it is a feature of the lack of institutional development. When you concede that China, India, and Persia 'waxed and waned,' you are admitting that human potential is tethered to the system, not the blood.
read Conrad's 'the heart of Darkness'
While I admire Joseph Conrad as a writer (amazing stylist - I think Lord Jim and Nostromo are his great masterpieces - I'd also add Under Western Eyes), he was hardly an expert on questions of human heredity or cognitive testing. The bottom line is that IQ is 70-80% heritable (narrow-sense) and that a group of people with an average IQ of 80 aren't going to collectively achieve what a group of people with an average IQ of 100 can achieve.
Conrad assiduously elides the draconian measures which the Romans had to implement to do that.
I don't think that the modern U.S. would be keen on, say, the mass-crucifixion of intransigent newcomers slow in adopting our cultural values.
Regards,