Subject: How many ancestors does the average person have after 40 generations?

Starting with parents, grandparents, great grand parents, it 
seems 2^40 is the answer. But that's over a trillion humans! 
More people than have ever lived. Impossible! After 41 
generations there's twice as many, 2 trillion. Incest and 
intermarrages could cut the numbers somewhat. Also if humanity 
began with 2 "parents", there should be a method to began 
reducing the population as we count ancestors backwards. 
Obviously, something is wrong here. My reasoning is pathetic. 
Please explain your answer, so I can understand what is 
happening. Words, math, graphs,and references would be deeply 
appreciated. Many thanks, Bob

Re: How many ancestors does the average person have after 40 generations?