How many ancestors do you have? You have two parents. Four grandparents. Eight great-grandparents. Sixteen great-great grandparents. At 10 generations you should have 1,024 ancestors. Using this logic, however, at 40 generations you’d have over 1 Trillion ancestors! That is 10x more than the number of humans who have ever lived! Not possible! Sad!
What’s the deal? The answer is pedigree collapse which describes how reproduction between two individuals who knowingly or unknowingly share an ancestor causes the family tree of their offspring to be smaller than it would otherwise be. For example, if two cousins had a child, that child would only have six great-grandparents, not eight. Don’t think this happened in your family tree? Well, it probably did: according to a Rutgers University professor, 80% of all marriages in history have been between second cousins or closer! This genealogical shrinkage makes you cousins with just about everyone. Some geneticists believe that everybody on Earth is at least 50th cousin to everybody else. This diagram shows what your family tree likely looks like:
Go back far enough and you would find that you and all your ancestors are descended from the first human tribe. Genealogists have concluded that the maximum number of unique ancestors in 1200 AD is about 2 million people. That’s it. That number is so small partly due to the Black Death. Pedigree collapse also explains how so many people can claim to be related to royalty – because they are if you go back far enough in generations.