Carl Zimmer, whose work I've enjoyed for years, recently had an article on how we're all (those of us with any European ancestry at all, which is more than you might think) related to Charlemagne (Carolus Magnus, Very great grandpa Chuck) and each other. I contributed a few comments, and the scientist author of the article 'cousin' Carl was writing about entered the discussion. I took the chance to grill, er, ask a couple of questions.
I've read many a book on historical geography -- the activity of trying to construct what the boundaries of the Roman Empire, Holy Roman Empire, Burgundian States, Tang Dynasty, Inca culture, and so forth through many an example through all of time. One of the few conclusions I can reach (and related to the above research) is that anyone who talks of being 'pure' anything, is ferociously ignorant of history.
My English ancestors probably include some Vikings, Normans, and others in their history. The Germanics probably had, well, pretty much anything from Spain through Russia, including the Balkans. The Romanians probably included anything west to Spain, and east to Mongolia. The Jewish ancestors probably tie me to anything from the Middle East through to the parts of the Holy Roman Empire that they emigrated to the US from. And that's all just within the last 1000 years, from the parts I know about. As I know less than 40% of my ancestry, I lay claim to the rest of the world from the 60% I don't know.
In any case, take a look at the Charlemagne article for some insight to why it is we're probably all related to each other from not more than about 3400 years back, and far less than that if you have some European ancestors.