Well, first and foremost there is no such thing as a 'white' or 'black; race, there are only the broad races of Caucasoid, Negroid and Mongoloid. They are broad and there is a lot of mingling. why are they different? BEcause at the end of the last ICE age people were separated by insurmountable regions -- Negroids were separated from Caucasoids by the SAhara, The Mongoloid were separated by Siberia and the Gobi desert and the Himalayan mountains.
All three races have a common ancestor or the Afroid is the ancestrial race to the other two.

The Sumerians and Indus valley were NOT Afroid, they came too late for that. They were Caucasiod. It is possible for Caucasoids to live in hot climates -- India, Arabia, the Mediterrranean etc. The people of Egypt were a mix between the Afoid Ethiopian/nubians or Upper Egypt and the Semitic peoples of Lower Egypt.