Answer by Dr Richard Dawkins (evolutionary biologist)
Yes, we are all related. You are a (probably distant) cousin of the Queen, and of the president of the United States, and of me. You are I are cousins of each other. You can prove it to yourself.
Everybody has two parents. That means, since each parent had two parents of their own that we all have four grandparents. Then, since each grandparent has to have two parents, everyone has eight great-grandparents, and 16 great-great-grandparents and 32 great-great-greatgrandparents and so on.
You can go back any number of generations and work out the number of ancestors you must have had that same number of generations ago. All you have to do is multiply two by itself that number of times.
Suppose we go back ten centuries, that is to Anglo-Saxon times in England, just before the
Norman Conquest, and work out how many ancestors you must have had alive at that time.
If we allow four generations per century, that’s about 40 generations ago.
Two multiplied by itself 40 times comes to more than a thousand trillion. Yet the total population of the world at that time was only around three hundred million. Even today the population is seven billion, yet we have just worked out that a thousand years ago your ancestors alone were more than 150 times as numerous. And we’ve so far dealt only with your ancestors.
What about my ancestors, and the Queen’s and the president’s? What about the ancestors of every one of those seven billion people alive today? Does each one of those seven billion people have their own thousand trillion ancestors?
To make matters worse, we’ve so far only gone back ten centuries. Suppose we go back to the time of Julius Caesar: that’s about 80,000 generations. Two multiplied by itself 80 times come to more than a thousand trillion trillion. That’s more than a billion people packed into every square yard of the Earth’s land area. They’d be standing on top of each other, hundreds of millions deep!
Obviously we must have done our sums wrong. Were we wrong to say that everybody has two parents? No, that is definitely right. So, does it follow that everyone has four grandparents? Well, sort of yes, but not four separate grandparents. And that is exactly the point. First cousins sometimes marry. Their children have four grandparents, but instead of eight great-grandparents they only have six (because two great-grandparents are shared).
Cousin marriage cuts down the number of ancestors in our calculation. First-cousin marriages are not particularly common. But the same idea of cutting down the number of ancestors applies to marriages between distant cousins. And that is the answer to the riddle of the very big numbers that we calculated: we are all cousins. The real population of the world at the time of Julius Caesar was only a few million, and all of us, all seven billion of us, are descended from them. We are indeed all related. Every marriage is between more or less distant cousins, who already share lots and lots of ancestors before they have children of their own.