I agree with the tack you're taking Enrique, certainly we're all related and now that we have DNA analysis, we're going to come closer and closer to being able to determine precisely how closely related we are to each other.
As I thought about my take on this, I came up with this analogy: Think of the human family as not a tree branching out from one pair of individuals but as an intertwining set of vines. Whether that starter population of homo sapiens was one pair or one small set of related family members doesn't really matter. It's a virtual certainty that the original population had to be small - less than 150 members, and given the sociology of primate groups, it's likely those 150 or less were already strongly related to each other. In any case, within a couple of generations, a small population like that will necessarily cross over upon itself with all members marrying their own 1st and 2nd cousins (if not siblings), so any notion of Adam and Eve-like "original" ancestors will get lost in the intertwining vines.
As the group expands with success, that intertwining doesn't stop. Every generation is a product of the one that came before it. The vines go from past to present to future, but they remain intertwined. As populations move and isolate themselves, they become less related to the populations they're isolated from, but wherever there is interchange between populations, those vines connect us.
The Early American colonies served to again focus those vines on a smaller group of people, so it's far more likely that anyone with an ancestor in that group is related to anyone else with an ancestor in that group. Our ancestors also kept fairly decent records and made sure that some of them made their way down to us in present day, so we can document those intertwining vines for ourselves today. Lucky us!
