When I was a kid my mum made a family tree.
2 parents, 4 grandparents, 8 great grandparents and so on. Very quickly it gets very big. Going back only 6 generations to around the year 1800 and you already have 64 great-great-great-great grand parents.
But then I thought. Just how many great.. grandparents did I have say, 1,000 years ago when King Harold had to pull an arrow out of his eye at the battle of Hastings?
Well, 1000 years ago is - roughly speaking - about 40 generations. And I’m sure you’ve noticed the series above expands exponentially as you go back in time.
Okay then, so we have 2^40, which is…
or 1099 billion great grandparents!!!!!!!!!!!!!
WTF - there are only 6 billion people living on the planet at the moment!
I asked my mother for an explanation, but she didn’t have one. And as I couldn’t find the answer anywhere else I didn’t think about it again until many years later on Sunday night, when I read the answer in Bill Bryson’s “A Short History of Nearly Everything”:
“The reason, of course, is incest. If you look around you on a bus or in a park or café or any crowded place, most of the people you see are very probably relatives.”
Ah of course: your line is not pure and there will have been many occasions when a relative from your mother’s side has procreated with one on your father’s side! And Bryson goes on to say:
“Indeed, if you are in a partnership now with someone from your own race and country, the chances are excellent that you are at some level related.”
Isn’t that incest? Yucks! No wonder foreigners find Indonesian women attractive!