I was thinking about this today, and I did some calculations. Here they are:
Consider a problem of genealogy. How many people in the past are you related to?
Unless you are a product of cloning (not something we need to worry about in the past), you have two biological parents.
Each of your parents had two biological parents. So whilst you have two parents, you have four grandparents.
Each of your grandparents also had two parents, so you have eight great-grandparents. And so it goes. The number of people you are directly descended from doubles with each generation you go back.
2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048… and so on.
If we consider a generation to be the time it takes for a person to be born, grow up, and produce children of their own, then nowadays it’s probably around 20 - 25 years on average. If you study history at all, you’ll find that 150 years ago, particularly amongst labouring people, that figure was nearer 16 years - but we will err on the side of caution, and call a generation 25 years.
Doubling every 25 years, by the time we get back 30 generations (only 750 years), each of us is directly descended from 1,000,000,000 different people. Now that’s a bit of a problem, because the best estimates available suggest that the population of the whole world at that time did not exceed 450 million.
750 years is not such a long time. 750 years ago, the French were busy founding the Sorbonne, Oxford university had been teaching for 100 years, and Cambridge for 50. The crusades were still ongoing, and Genghis Khan’s mongol army were causing trouble in the east.
Now reverse the process. Imagine a young married couple in the year 1250. Let them have 2 children (in fact, 10 or 12 children were not uncommon in those days - yes, many died, but many did not), we’re being cautious. Two children per couple, and 25 years per generation.
Each couple has two children, but each child goes on to have two children… and on, and on… doubling each generation.
We get the same figure. Every person alive in 1250 A.D. now has 1,000,000,000 living descendants.
Bear in mind, the figures I have given account only for direct descendants. They take no account of other blood relatives - brothers, sisters, cousins, aunts, uncles etc. If you include them, you can probably multiply the figures by 10 per generation.
My conclusion? Genealogy is nonsense. You don’t have to go back very far at all, and you reach a point where everybody is related to everybody else. We’re all family. Really.