On Greg Laden's Blog, I've been joining the discussion a bit about race and its meaninglessness as a biological thing for humans. If you want to take that up, please join that thread. Here, I'm taking up the more genealogical side of relationship.
I've mentioned before that I've been doing some genealogy. This has included running mitochondrial and Y chromosome DNA samples. (Well, having someone else do so :-) The mitochondria pass solely from mother to child, so it gives a pointer on where my strictly matrilineal line comes from. The Y chromosomes pass strictly from father to son, so points to where the strictly patrilineal side comes from. In both cases 'points' is a generous description for something that really means 'within a few thousand miles, give or take a lot'. For the strictly patrilineal, I already 'know' the location outside the US (colonies as it turned out) -- Leonhart Krumbein of Strasbourg, who came in 1754 on the ship Brothers.
For the strictly matrilineal side, I had some hope of an interesting result. That side runs back to the early history of Maryland, and then is lost. Could have been that one of the women was a Native American. I'd have liked that. Unfortunately, both sides point to the most common haplogroups in Europe (H for the maternal side, R for the paternal). So no great surprises or novelty there. Oh well. It also established (not that this was a question) that I'm a mutant. (So are you, don't worry.) My mitochondria differ in 7 places (out of about 1500 checked) from the reference sequence. A little looking around shows one of the mutations to be fairly uncommon, even though the rest are very common.
By the time we're looking at 10 generations back (which is where I've tracked, loosely, the matrilineal side to), we've got 1024 ancestors. These tests only speak to 2 of those 1024. Quite a lot of variety could exist in the remaining 1022, but it won't show up in these two parts of our genome. At 20 generations back (about 1350), we've got about a million ancestors. That starts to get interesting, as it gets to be comparable to the population of significant areas (all of England at the time, for instance). Go to 30 generations (about 1050) and there are a billion ancestors -- greater than the population of the world at the time. Make it 40 generations, about 750, back to grandpa Chuck, and we've got a trillion ancestors -- more than 2000 times the world population of the time.
Grandpa Chuck is Charlemagne, who shows up in my tree. Given world population at the time, he probably shows up at least 2000 times in my tree if the whole thing were to be discovered. Back in college a friend (hi Derek) mentioned his descent from Charlemagne. This was before I knew about pedigree collapse so I was skeptical. Now that I do, it's more the converse -- it'd take evidence to show that you (for any of you, anywhere in the world) do not share him as an ancestor. Pedigree collapse is this business that as you go back in time, world population drops, but your number of ancestors keeps increasing. You hit a point where the same person must show up multiple times in your ancestry.
Related is that with a trillion slots to fill in your ancestry, even some connections that may seem intuitively unlikely are certain to happen. Intuition isn't a very good guide when numbers get large (among other times).
So I already know my real list of relatives -- everybody, everywhere. The only question would be how closely related we are. Even before starting the genealogy, I'd realized this. Even after greatly extending my knowledge of who came from where, 60% of the 'where' is still unknown. I take the rest of the world in that 60%.