These peoples then migrated back to Asia and Europe after the last ice age -- the Akkadians moved into Sumeria around 3000 BC and Greek mythology tales talk of how they got civilisation from the Egyptians and the Phoenicians)
The "black" race of people arose in west africa and since the time of Christ they moved south and pushed out the other two races in africa -- the pygmies who became slaves to the Bantu speakers and the Khoi-San who were killed off (germs and lands taken from these hunter gatherers)
On the whole the Ice Age was getting colder and colder, and it was probably relatively easy to walk back to North Africa since sea level was about 400 feet lower. So if I were in freezing Europe, I might have wanted to migrate back to Africa.