Most ancient "Empires" were not monolithic gene pools, they were generally confederations (a base gene pool with admixtures from their conquered people). The male yDNA haplotypes (R,E,J,I etc) are more or less snapshots of male genetics 10-20,000 years ago (the female genetics show different patterns, due to conquest geographics). Someday, genetics will probably show exactly who the root conquerors were, and who were the conquered.