Mindblown

From here:

The way your family trees severely collapses when you look back a reasonable (<50) number of generations. I hope this explanation is clear:

You have two parents, four grandparents, eight great-grandparents, and so on. In general, if you look n generations back, there are 2^(n+2) people who are your (n-great)-grandparents.

Let's generously say a generation is 30 years. If you look back 40 generations, to around the year 800 C.E., you have 2^42 (40-great)-grandparents. That's well over four trillion people, more people than were alive in 800 C.E. by several orders of magnitude. More people than have ever been alive.

The reason this is possible: If you looked at a list of your 2^42 40-great-grandparents, you'd see duplicates. In fact, each name would show up around 10,000 times, possibly even more. So your (mom's mom's dad's mom's ... dad) is the same as your (dad's mom's dad's dad's ... dad), and in fact your can trace your ancestry to that guy in about 10,000 different ways.

 I have nothing funny to add. Sorry.