If you consider inbreeding between humans it is clear that to avoid it your ancestors would need to be unrelated. But in this case your ancestors simply get more and more numerous the further and further back you go.
'Everybody has two parents, four grandparents, eight great grandparents, and so on back through time, with the number of ancestors doubling in each generation. Go back 30 generations and the number of ancestors tops one billion. Eventually we arrive at a time when we have more ancestors than there could have been people in the world. How can this be?'
Fortunately we can trust Cecil Adams to give us the Straight Dope on this subject. Now that you appreciate the issue you can appreciate this table which shows some stunning amounts of pedigree collapse in European Royal families. At some level of course we are ALL inbred - geneticists estimate that everyone on earth is at least a 50th cousin to everyone else. (Although we are all much more closely related to Kevin Bacon....)
There's another interesting article on the subject here.
"In a population of between three and five hundred people, after six generations or so there are only third cousins or closer to marry. During most of human history, people have lived in small, isolated communities of about that size, and have in fact probably been closer to the genetic equivalent of first cousins, because of their multiple consanguinity. In nineteenth-century rural England, for instance, the radius of the average isolate, or pool of potential spouses, was about five miles, which was the distance a man could comfortably walk twice on his day off, when he went courting- his roaming area by daylight. Parish registers bear this out. Then the bicycle extended the radius to twentyfive miles. This was a big shakeup."
I never really thought of the bicycle as a major advancement in dating technology before.