The following is a Plus Edition article written by and copyright by Dick Eastman.

How many ancestors do you have? 

It is a simple question and would appear to have a simple answer: Any of us can count the number of our ancestors by performing a very obvious mathematical progression: two parents, four grandparents, eight great-grandparents, and so forth. In the past twenty generations, you have a mathematical chance of having more than one million ancestors. Thirty generations produces more than a billion ancestors, and forty generations results in more than one trillion.

The calculation is correct from a mathematical viewpoint but it ignores one obvious fact: there have never been that many humans on the face of the earth since the dawn of evolution! The reality is that it is impossible to have one trillion unique ancestors, regardless of the mathematics involved. 

In reality, we all have multiple lines of descent from single individuals. That is, if you could fill out a pedigree chart of all the names of all your ancestors going back forty generations, you would see the same individuals appearing time and time again in various places in the chart. For example, your mother’s great-great-grandfather might be the same person as your father’s great-great-great-grandfather, and so on and so forth, repeated thousands of times with different individuals at different places in your family tree. The further back you go in time, the more often certain individuals will appear in multiple locations in your ancestry.

The correct term for this is “inbreeding.” That’s not a very polite word in the English language, but it is a fact: if you go back enough generations, every person on the face of the earth has hundreds or even thousands of cases of inbreeding in our family trees. When hearing the word “inbreeding,” some people will picture the residents of remote mountain hamlets or rural farming communities. In fact, inbreeding is much more common at the opposite end of the social spectrum: the royal families of England and Europe have far more inbreeding than the rest of us. 

Queen Elizabeth II and her husband Prince Philip were second cousins once removed, both being descended from Christian IX of Denmark. They were also third cousins as great-great-grandchildren of Queen Victoria. That results in obvious inbreeding when looking at Prince Charles’ pedigree chart. 

Genealogists often prefer to use the phrase “pedigree collapse,” a term first coined by the late Robert C. Gunderson, the Senior Royalty Research Specialist at the Family History Library in Salt Lake City. Gunderson correctly calculated that, if you kept multiplying by two the progenitors of a person born today, doubling his parents, their parents, etc., the person would have something like two hundred and eighty-one trillion forebears alive at the time of Charlemagne. Obviously, there were nowhere near that many people around when Charlemagne was King of France in 800 A.D., or at any other time. 

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