I'm not sure that the Roman senatorial elite is properly described as an 'aristocracy', since ancestry had become less and less relevant wrt. social status even as early as the late Republic. Wealth and social connections became a lot more important at some point, but people from different backgrounds could acquire those with comparable ease. Of course citizenship status still mattered, but that was gradually extended over time as well. Ancient Rome seems to have been a remarkably open society by the standards of their time.