I've thought about this and read a bit about it, and I've noticed one thing that's never taken into account: intergenerational romance. Suppose I marry someone

I've thought about this and read a bit about it, and I've noticed one thing that's never taken into account: intergenerational romance. Suppose I marry someone 17 years my junior, who's dad is 90 and her mom is 30 years his junior, and suppose I already had a son with a different woman who's mother had her young and her mother also had her young. I could have a kid who's grandfather is the same age as his sibling's great grandfather. I would guess these sorts of complications magnify over time, and they need not even be that complex. When you only consider your ancestors from nth generation to be from the same generation of people you get a good, clean linear look at the geometric growth of ancestors going back, but when you include disparate ages into the mix you get a much messier picture which counterintuitively decreases the validity of the idea that we are all descended from everyone in the world at around the year 1200. You start to see how ethnicities form and merge and continue for a long time or not, you get a sense that it is cultural norms that are either conducive or destructive to their continuation, you can see how our habits and cultural norms breed us.