It's hard to get my mind wrapped around this but I did develop an equation to determine the number of great grandparents you have which is Number of X Great Grandparents = 2^(X+2) An example of this is below. So what happens when the number of great grandparent = the world population?

Number of your 20th Great Grandparents = 4,194,307 and they would have possibly lived near the year 1310.