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People who have traced their genealogy extensively often reach three conclusions….

  • First, they conclude that they are descended from some famous person of the distant past (i.e. Emperor Charlemagne).
  • Secondly, they conclude that they are related to some famous modern people.
  • Thirdly, they conclude that they are their own cousins, which is to say that some pair of their ancestors were cousins.

This sounds improbable to most of us, but the math of ancestry says that it’s almost inevitable.

Let’s take a look

The number of ancestors doubles each generation back: Let “n” be the number of generations back, the number of ancestors in that generation is 2n. (a person has one set of parents, 4 grandparents, 8 great-grandparents, 16 great-great grandparents, and so on).

Follow the math and you’ll notice a problem

Go back to someone’s claim to be descended from Charlemagne. Charlemagne lived in 800AD, which is about 40 generations back. Now, 240 is a little over one trillion. This means that, if our claimant filled out an ancestry chart back forty generations, he or she would have to list a trillion names. But there were only around 300 million in the world’s population at that time.  The present population of the world is about seven billion.

Implications of the shortfall

This shortfall of potential ancestors implies these two things:

  • Persons descended from one geographic area have to have many ancestors in common.
  • Among any one person’s ancestors, intermarriage of cousins was almost inevitable.

So what does this mean? We are all much more inter-related than we might think.

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