There's an interesting article I found via the browser that discusses recent research on demographic collapse. It makes the case that everyone currently living shares an ancestor 2,000-3,000 years ago. Moreover:
It doesn’t get any less weird when you look at it from the other angle: While you more than likely have four distinct grandparents and eight distinct great-grandparents, past a certain number of generations back, your number of ancestors stops growing exponentially, because they start being the same people. By the time a couple who married in 1450 in Holland, has had a few hundred descendants over the span of several generations, those people are distantly related enough that some of them start marrying (and, yes, reproducing with) each other.
I have found one instance of third cousins marrying in my family tree.