The chances that anyone on this forum is my biological brother or sister, purely from a statistical point of view, is rather low. But how many generations back would we have to go to have a fair chance that one of my ancestors married one of your ancestors (or procreated anyway)?
I was born in 1985. Less than 20 years before my birth my two parents were being conceived by a combined 4 parents of their own.
And if we go back even further, to 40 years before my birth, then I'll find an additional 8 ancestors.
Around the 60 year mark I have another 16 ancestors
80 years back gives me an additional 32 ancestors
And so on
. . .
By the time I've gone back to the year 1545 I would expect to have around 8.3 million LIVING relatives. Only 50 years after Columbus had discovered North America more people alive were grandparents to me than are currently active publishers with the Watchtower today. Just 22 generations back . . . 22 conceptions and births and surely one of my ancestors would have witnessed Martin Luther's separation from the Catholic Church. Or would have served under King Henry the VIII. Just 22 generations ago.
So, are you related to me? Well of course - we're all related. But we probably don't have to go back the thousands or tens of thousands of years we might expect to find our common ancestry. Perhaps 500 years ago one of my 8 million living relatives married one of your 8 million living relatives.
Chances are none of you here are my biological brother or sister. But there's a pretty good chance you're my cousin from the last millennium.
http://io9.gizmodo.com/5791530/why-humans-all-much-more-related-than-you-think