http://www.straightdope.com/classics/a2_083b.html
“Pedigree collapse” and “diamond-shaped arrays” seems like a very convoluted answer to this question. The uncomplicated answer is that couples, on average, have more than two children. So, your lineage of parents, grandparents, great-grandparents, etc., is not solely your own, but shared my multiple people (you and your siblings).
Isn’t this a much simpler explanation, or am I missing something?
It’s certainly simpler, but what it misses is that it doesn’t really answer the question. Read it again. Your siblings are inconsequential to the point being made. If you draw a family tree rooted at you and moving up through ancestors, it will double in size with each generation. Eventually there will be more leaves than there could have been people. Cecil’s point is that some of those leaves will be duplicates, until eventually all the leaves are Adam or Eve. (Actually, Noah and his wife would work also.)
OK, Cecil explained that much more clearly than I just did, so I’m not likely to clear up your confusion. Oh, well. I tried!
I think faced with a bottleneck like this, a genealogist would distinguish clients by their kids; Shem, Ham, and Curly.
Does this not make sense to anyone else? Won’t you always have more real ancestors the farther you go back? You are always adding people even if the rate of people/generation you add goes down.
My reading of the column, flight, is that they’d have 2 million ancestors alive in 1200, not 2 million between then and now.
No, you’re always adding slots for people. More and more of those slots get filled by the same people. Past a certain spot, only slots are added - no new people at all.
To chime in with the other voices trying to clarify this thought experiment… you have X many people at generation t. When you move back to generation t + 1, you’re not keeping all of those X people… you’re ONLY looking at the people who are the parents of generation X people. (I suppose there’s a fair chance that someone in generation X is also the parent of someone else in generation X, so you’re moving to someone in generation Y who was also in generation X. But this would not be the case for the majority of people in generation X.)
That just seems to muddy it up more … as if you are stating that if you go far enough back, people were their own parents. In a sense it’s true though. The same people may appear in multiple generations in your family tree. Say, for example, a young boy named Jesus is born to a couple, call them Joseph and Mary. Jesus wants to trace his lineage to a past celebrity named David. This is very important to him. He finds that he can trace the lineage through 13 (or so) generations on his father’s side and 10 (or so) on his mother’s side. That means David appears in his family tree going back 10 generations, but when he traces back 13 generations, he fills in one of the new slots with David again, who was already used at generation 10. The 8,192 slots at generation 13 would probably also contain many duplicates that first appear at that generation.
Gah, this is why nobody pays me to write a syndicated column. It makes sense in my head, but I just can’t seem to translate it into pixels very well!
Think of it this way: instead of adding a new generation all the way across the bottom/top of the tree, add in only those people who have not been previously used, and add them only once if they appear in the same generation multiple times. At some point, the number of new people added this way is less than the number of new people added the next (in time) generation, and grows ever smaller the earlier in time you get. Finally, at some point, you don’t have anyone to add, because Eve and Adam already got added (or whichever “people” are considered the starting members of the race; possibly not just two people, but certainly some relatively small number).