anyone care to have a go debunking this?
https://www.youtube.com/watch?v=T_FCOOJxh4E
basically saying that all presidents apart from 1 have ancestry roots traced back to an English king in the 12th century
If you consider the time involved (over 800 years), it's quite plausible. But it's also quite likely that you and I also are descended from the same King.
Consider, you have two parents, two grandparents, four great grandparents, eight, sixteen, thirty two. For each generation you go back, the number of ancestors doubles.
So you got back 10 generations, you have 1024 ancestors.
Go back 20, you have about 1,000,000
Go back 30, you hav about 1,000,000,000 which is more people than there were in the world at that time. Obviously if you back far enough your ancestors are cousins of one form or another.
How far back is 30 generations? Well, a conservative estimate for a generation is 20 years, so 5 generations is 100 years. 30 generations is 600 years. And we are talking over 800 years, so 40+ generations. It's quite likely that anyone with english roots could (if records were perfect) trace them back to pretty much anyone who has any living descendants.