Entire ethnic groups have arisen in the last 7,000 years. Norwegians, Greeks, Latvians and Bengalis show "major differences" and they all came from the same Indo-European ancestry. How do we know that the Africans did not change? All we have from Africans 70,000 years ago (you got your dates wrong) are skeletons. We don't really know what they looked like beyond that. Skin color is one of the most ephemeral traits. There have been some arguments posted against that statement on SciForums but I find them inconsistent with the evidence and unpersuasive. Bengalis and Lithuanians are separated by no more than 4,000 years of migration. The entire aboriginal population of the Western Hemisphere with its great diversity (except for the Pacific regions of Canada and the USA and the arctic) is descended from one wave of migration generally dated around 13,000BCE but unlikely to be further back than 25,000BCE.