Subject: What happened in human history to make O blood dominant in our society?

The answer posted at 
http://www.madsci.org/posts/archives/feb98/886034359.Gb.r.html
does not answer my question. That answer has to do with why O is common, not 
*how* it became common.

O-type blood is most common because there is a larger % of that allele.  Did 
this happen as the result of a mutation in human history, or perhaps O-type 
blood was at one point more resistant to a disease that wiped out a large 
population of humans?  If A & B type blood are dominant, shouldn't A & B type 
blood have eventually, through time, taken their places as most common?

Re: What happened in human history to make O blood dominant in our society?