If we double the number of ancestors in each generation, 2 parents, 4 grandparents, and so on, we can see that by the time we are back 10 generations, we have the potential for 1024 ancestors. If we were to go back to the time of the Norman Conquest, we would find we had the potential for 200 trillion ancestors all living at that one moment in history. Obviously this is impossible.  So what happened? 

It is estimated that 80% of the marriages in history were between second cousins.  This is because the population was smaller, people lived in small communities and migrated within those same small communities. The theory in genealogical research is that our family trees are actually shaped like a diamond. Tracing back a few generations gives a wider shape. Keep going and you find the shape narrowing, eventually, converging to only a few ancestors.

So how many ancestors do I have?  We have no way to know, perhaps not as many as you thought?  However this is one thing that I always remind people of…

 

think of anything that has happened in history, ever… an ancestor of yours was alive during that event.