"Everybody comes from 2 parents, 4 grandparents, 8 great-grandparents, 16 great-great grandparents, etc. Every generation back, we have twice as many lineal ancestors.
If there are, say 25 years to a generation, then 64 generations is 64 X 25 = 1600 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 direct-line ancestors (more than the national debt :) ) -- or so it seems -- to say nothing about collateral relatives!
But this is far more than the population of the Earth, then or now. It is far more than the number of human beings who ever lived.
Something is wrong with our calculation. But what??
We have assumed all those lineal ancestors to be different people. But this is not the case. The same distant ancestor is related to us by many different routes. We are repeatedly connected with our relatives. Every marriage brings cousins (distant or close) together.
If we go back far enough, any two people anywhere on Earth have a common ancestor. Whenever a new American President is elected, someone -- generally in England -- discovers that the new President is a distant relative of the Queen of England. This announcement is intended, perhaps, to soothe any residual American longing for a king of our own.
Of course, when two people derive from the same small corner of the planet, and their genealogies are well-recorded, the last common ancestor can be discovered.
But whether it can be discovered or not, a relationship is clear: We are all cousins -- all members of a vast world-girdling family.
It's time, I think, for a reunion!" ~~Carl Sagan, 1989