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

Mathematically, I think you are right - in fact, I know you are.  We each 
have two parents, Dolly and a bunch of mice being the exception.  However, 
where you are wrong is the notion that all those people were different.  In 
fact, there must be considerable overlap in the ancestor set of all people.  

In earlier times, travel was more difficult.  Logically, this suggests that 
the marriage partners for a given person were probably found within 5-10 
miles of them at the time of mating; 16-30.  If true, this suggests that 
after a period of time all the people living in a given area were a product 
fo all the forebears of those same people.  In islands this is certainly 
true.  Following this logic, the current flexibility in marriage partners 
is certainly unprecedented.  

So, I don't really have an answer, just the basic feeling that 2^40/2 is 
the number of matings, but not the number of ancestors.