There's an interesting little factoid about myself I've neglected to mention before, so I thought I'd drop it in now: I'm a descendant of Charlemagne, the first Holy Roman Emperor.
"Why, that's amazing, Jeff!" you say, yawning. And it is. But you want to know something really amazing? If you have any European ancestry, you're a descendant of Charlemagne, too.
See? Pretty interesting now that it's about you, isn't it?
"But Jeff," you object, "How can both you and I be descended from Charlemagne?" It's simple, really. It just involves complex math.
Don't worry -- I'm not going to make you do the math. Why would I, when others have already done it? At any rate, it's all based on the theory of Most Recent Common Ancestor.
As you know, you have two parents, four grandparents, eight great-grandparents, sixteen great-great-grandparents, 32 great-great-great...well, you see my point. In this you are not unique; everyone alive can say the same thing, at least from a genetic standpoint.
But of course, this leads to fun with math. It only takes 15 generations -- about 300 years' time -- to get you to 32,768 ancestors in that generation. 30 generations -- 600 years' time -- and you have 230 ancestors, or 1,073,741,824. Go back to 1 C.E. and you've got 2100 ancestors in that generation, or roughly 1.2 nonillion ancestors.
Of course, that's roughly 500 billion billion times the number of people who have ever lived, give or take a billion or so. And that's kinda, sorta impossible. But math doesn't lie. So how can this be?
The answer, of course, is inbreeding. Oh, yes, there's plenty of your and my ancestors who married their first cousins and whatnot, but I'm not talking exclusively about that. There are also plenty of your and my ancestors who unwittingly married their sixth cousins twice removed. Plenty of your and my ancestors who married someone who shared a few ancestors a dozen generations back. Indeed, if you're dating someone, you're related to them, though it may be a long way back and a lot of tracing before you get to the point you figure out exactly how.
Now, here's the fun part: obviously, all of us share common ancestors. And not just from the very first hominids to evolve into modern humans. Using mitochondrial DNA, scientists have shown that all of us share a common maternal ancestor who lived on the order of 100,000 years ago. And when I mean all of us, I mean "all of us" -- you, me, Osama bin Laden, Kanye West, Hillary Clinton, Hu Jintao, Ann Althouse -- every human alive today shares that common maternal ancestor. And every human shares a common paternal ancestor who lived on the order of 60,000 years ago.
But wait! There's more! The threads that make up your and my ancestry cross and recross and recross again, to the point where we start to discover more and more people who are ancestors to everyone. And the most recent common ancestor -- the most recent person to be an ancestor to every single human alive -- lived only about 3,000 years ago.
That person, of course, is an ancestor to all humans alive. And because he lived 3,000 years ago -- 500 years after the establishment of the Egyptian royal dynasty -- it's very likely that nearly everyone alive today could theoretically trace their ancestry to the leaders of Egypt. After all, we know that there are some people alive today who can; that means, most likely, that everyone can.
And if you look at narrower populations, as one might expect, things get more fine-grained. Almost everyone in the world -- and almost certainly everyone reading this -- is descended from the Chinese philosopher Confucius, who lived around 500 B.C.E. That's because we know there are people descended from him living today, and we also know that population migrations should have spread his descendants throughout Asia, Europe, and Africa, and eventually into Australia and the Americas.
Meanwhile, the west -- loosely defined as Europe and the Middle East -- likely has an MRCA of about 1000 C.E. -- or about a millenium ago. And this is why, if you have European ancestry, you probably are descended from Charlemagne. Like Confucius and the Egyptian royals, we know that there are descendants of Charlemagne alive today, and using statistical models, that means you are probably descended from him.
There's another person you're probably descended from. And I find this beautiful, in that it should drive the wingnuts crazy: Almost every westerner is descended from the Prophet Mohammed. Statistical models show that virtually all Muslims can trace their ancestry back to Mohammed, and we know there was a reasonable amount of interbreeding in Muslim-occupied Spain. It's a virtual certainty that you and I, like the Queen of England and George W. Bush and, not for nothing, almost all Jews, can trace your ancestry back to the founder of Islam.
If you go back about 10,000 years, you hit a point at which all of us share all ancestors. Beyond that point, each of us has the same heritage, all the way back to the very first single-celled organism to replecate itself. And this is what I find fascinating about all this. Humans are very closely related to each other. The most distantly related humans are more closely related than are two random chimpanzees plucked from different troops. We are, all of us, very similar, and all of us are related by blood. Which is why I cannot believe, in the end, that racial differences of any meaningful nature exist: we are all cousins, after all, and all of us the grandchildren of the same grandmothers and grandfathers. We are too alike to be too different.
(Tangental thoughts begun thanks to Amanda)