Everybody has 2 parents, 4 grandparents, 8 great-grandparents, 16 great-great-grandparents, etc. Every generation back we go, we have twice as many lineal ancestors. You can see that this is very much a Persian chessboard kind of problem. If there are, say, 25 years to a generation, then 64 generations ago is 64 X 25 = 1,600 years ago, or just before the fall of the Roman Empire. So..., every one of us alive today had in the year 400 some 18.5 quintillion ancestors - or so it seems. And this says nothing about collateral relatives. But this is far more than the population of the Earth, then or now; it is far more than the total number of human beings who have ever lived. Something is wrong with our calculation. What? Well, we have assumed all those lineal ancestors to be different people. But this, of course, is not the case. The same ancestor is related to us by many different routes. We are repeatedly, multiply connected with each of our relatives - a huge number of times for the more distant relations.
Something like this is true of the whole human population. If we go far enough back, any two people on Earth have a common ancestor. Whenever a new American President is elected, there's bound to be someone - generally in England - to discover that the new President is related to the Queen or King of England. This is thought to bind the English-speaking peoples together. When two people derive from the same nation or culture, or from the same small corner of the world, and their genealogies are well-recorded, it is likely that the last common ancestor can be discovered. But whether it can be discovered or not, the relationships are clear. We are all cousins - everyone on Earth.
—Carl Sagan, Billions and Billions, p. 23.