Your Family Network- More Complicated Than You Thought
Many of us think of our ancestry as a cone extending back in time with each generation (looking backwards) having twice as many members as the prior. We each have 2 parents, 4 grandparents, 8 great-grandparents, 16 great great-grandparents, and so on. This logic makes perfect sense for several generations, but the number of direct relatives we have adds up quickly as we go back a few more generations. For example, if we go twelve generations back, we would have 4,096 great^10 grandparents. These people would probably have lived in the late 1600s. This gives rise to some interesting thoughts. Firstly, your existence required over four thousand people to meet and reproduce in a particular oder, and that’s only for the last three hundred years or so. Secondly, it is easy to see that the theory that each parental figure in our ancestry has two unique parents won’t add up.
In fact, if we kept up the pace we were at, we would have 68 billion ancestors by 1100 A.D. This brings us to the unsettling concept of pedigree collapse, which happens when people mate with relatives. This is the only explanation that keeps our ancestries in accordance with world population at any given time. The example given in the article says that if two cousins had a child together, that child would have only six unique grandparents (one set would essentially “count twice”) instead of the expected eight. This is not a pleasant concept to ponder, but Rutgers anthropology professor Robin Fox, 80% of all marriages have been between second cousins or closer. This is because earlier in human history, people spent their entire lives within a small radius where most of the other people were bound to be a relative, and because marrying close relatives is still accepted in some cultures. The pedigree collapse means that our cone of ancestry grows until about 1200 A.D., but starts to shrink again as we continue to go back in time.
It’s easy to think of our family network to have edges that connect spouses and their children with no edges ever crossing, but in reality, our family members throughout history had more connections to other family members (ew) than we would expect.
source: http://waitbutwhy.com/2014/01/your-family-past-present-and-future.html