All humans can trace their family tree back to a surprisingly small group of common ancestors. It's a question of basic mathematics - there simply aren't enough ancestors to go around.
Let's say you were born in 1975, your parents were both born in 1950, your four grandparents were born in 1925, your eight great-grandparents in 1900, and so on. In other words, your number of ancestors doubles every 25 years the further back in time you go. If you take this back just 1,000 years, you'll find that you have well over 500 billion ancestors in a single generation.The Presurfer
Considering there's fewer than seven billion people on this planet there's something seriously wrong here. The solution, of course, is that you don't have 500 billion distinct ancestors, but rather a much, much smaller number of ancestors reappear over and over and over again in your family tree.