Populations change with time. Early civilizations had brutal cycles of rising and falling, with entire cities getting wiped out and replaced with new inhabitants. By the time Xenophon walked through Nineveh, the city was nothing but empty ruins getting covered with sand, and the locals didn't even know who they were. When that city was at it's height Egypt had already forgotten how to build pyramids for thousands of years, and Egypt thought the Scythians (Aryans) were a much older civilization than they were. The area inhabitated by the Scythians was wiped out by various tribal coalitions, first (?) by the Huns, later by various tribal coalitions, then by the Mongols and their descendants. Only after the Russians reconquered it was it populated by whites. Another cradle of civilization, Anatolia, has been inhabitated for a thousand years by another invader tribe, the Turks. Egyptian mummies have been analyzed and found to have closer relation to the British isles than to modern Egyptians, who are basically Arabs. Now what's more plausible, some British sailors came down and conquered a genetically Arab Egypt, at a time when Britain was a remote and undeveloped backwater and Egypt was the most fertile civilization in the world? Or, a founding migration of what is today European genetics created all of Egyptian society, but over thousands of years and dozens of major invasions and plagues got wiped out by newcomers? Consider the history of America, and how just 3 decades ago we were 85% white but today we're rapidly approaching 50%, then consider what effect thousands of years of ups and downs could have and you have your answer.