The problem of relations

Here is a curious problem. We may safely assume that you had two parents; each of your parents had two parents, so that you had four grandparents. Arguing along similar lines you must have had eight great grandparents and so on. Assuming an average of three generations per century the number of your ancestors since the Christian Era began must have been nearly 1 trillion–
1,000,000,000,000,000,000 or 10¹⁸
This is vastly more people than have ever lived on the Earth. What can we do about it?

– J. Newton Friend, Numbers: Fun & Facts, 1954

via Futility Closet