The simple math is that you had about 68 billion ancestors around 1100 AD. Pluck anyone of them and you would not exist today. Wilder still is that the world population goes was far less. This is because of a concept called pedigree collapse, which is what happens when people end up with a mate who is somewhat or very closely related to them. This post really hammers home the point that humans are mainly a temporary container for their genes.