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Watching the video... he has a very subtle point.

let me try to explain it mathematically.

Suppose there are what, 4 billion base pairs in the human DNA.

You inherit half of them from your mother and half from your father. They inherit half from their parents, so you get 1/4 from your grandparents. And 1/8 from your great-grandparents and so on.

4 billions is about 2^32, so after 32 generations (roughly 800 years more or less) you are only inheriting 1 gene from your ancestors. Anyone beyond that, there is a good chance, 50% or more, that you inherit NONE of their base-pairs. Let me repeat that: Pick one your ancestors from 33 generations onward, and there's a good chance you share NONE of their DNA with them.

Obviously, this isn't how DNA really works. It isn't just base pairs being shared randomly, but sequences, and those sequences are much smaller in number than 4 billion. Let's say the average length of a sequence is 128 base pairs, or 2^7. That means that after 25 generations or 625 years your ancestors only contribute 1 sequence. Anyone beyond that is likely to contribute nothing to your DNA.

When you look at things this way, you can pretty much draw a line to around 500 years ago and say "No one born before this time matters to my DNA." and you'd be right. So people living before 1500 AD are only related to you by documentation, not by DNA. Our ancestors from the Middle Ages are completely unrelated to us by DNA.

OK, let's look at things a little differently. Suppose you and your mother had a DNA test, and they examined how the DNA corresponds to each other. Only half of your DNA comes from her, so it's going to be a 50% match. You and your grandmother only share 25% of your DNA, so it's a 25% match. Your great-grandmother only shares 12.5% of her DNA with you, so it's a 12.5% match. Meaning, only 1/8 of your DNA matches your great grandmother. As you go back further and further in time, you are less and less related to your ancestors.

Now, you might say that the other half, coming from your father, would match your mother if your father and mother were related, and this is true. So let's ask this question: how closely do you match with your siblings and cousins? Siblings (sharing parents) only match about 50% of the time. Cousins (sharing grandparents)? 25% of the time. Second cousins (sharing a great grandparent)? 12.5% of the time. And so on.

The point is this: your relationship with a person, the further up the family tree you go, means less and less DNA will match. In fact, at some point, you'll get stronger matches with all but complete strangers who just randomly happened to share sequences with you through some fluke of nature.

In reality, people aren't marrying perfect strangers. Cousins marry cousins all too often in history. Certainly second or third cousins end up marrying just by virtue of the fact that people didn't typically travel very far before automobiles. So in reality, you'd have communities that were more or less closed with second and third cousins marrying each other, sometimes unwittingly. This creates a sort of pattern in those communities. You're borrowing DNA from your mother or father, but if your mother and father share DNA, you're guaranteed to get that shared DNA. Because of these more or less closed communities, it is possible to identify as a member of one of these groups.

But nowadays, this is all but impossible, since the human race has been mixing up for many generations now. Starting with the age of exploration in the 1500s and rapidly accelerating from there, various cultures that were once isolated are now mixed. For instance, in my family tree, I am literally related to Pocahontas. Someone white married her sister or cousin or something, and after so many generations, my family line ended up being connected with theirs. (I can't tell you how -- I just know that it is.) And this is true for virtually everyone.

Had we remained in isolation from each other, then this mixing would have been happening at a much slower rate.

The net result of this is that despite the fact that I can trace my fathers' and mother's lineage through to Anglo-Saxon times, there has been enough mixing that I'm also related to various Indian tribes and others too. If there is a community I belong to, it's possibly "American" which is possibly a large subset of "European". I've probably got tons of German and French ancestors, along with Italians or Irish as well. Just because my fathers fathers and mothers fathers came from England doesn't mean all of my ancestors did.