Lets just say that Graham Hancocks (and the other guys) theory about the Younger Dryas Event holds true. In that a great catastrophe swept across Northern Africa, through the Middle East and through the lower elevations of Southern Europe. With that, we concluded that there was a civilization in the area - but we barely have any true remnants of it simply because if it was as destructive as they say it was - we would be hard pressed to find a present day civilization if it happened now. Especially if the technology was in fact, archaic.
If you believe all of that, then its easy to believe that Africa hosted a large population of people at one point with a completely different climate. A people that, at the time, were a seafaring people that could sail the world. Then look at the current day cultures surrounding this area, from mud people to arab princes - theyre all incredibly violent cultures that have little to no cultural shame. With a large history of slavery in and around them and their ancestors.
I believe that the majority of the world has went through a colonization event similar to what the Europeans did at least one other time. But this time, the Africans conquered and outright obliterated other cultures around the world. This would include your theory about the other genetic variants of the human species. Imagine Neanderthals, dumb enough not to be technologically advanced, but incredibly strong and easily manipulated. They'd be shipped off to the nearest quarry in mass. Where as your smarter races of people were likely wiped completely the fk out. Or, in the example of White People - they stayed far away from the coast and hid in the mountains.
I think the human race is much older than we have any clue, and that our genetics are only linked to Africa simply because of this event. Similar to Genghis Khan siring thousands of children across the entire continent. Instead, the world had African Colonizers spread their genes across huge areas. (Not all humans carry the genetic markers that come from Africa, which makes that theory fall apart even further and in fact, lends credence to my theory.)