|Diamond Theory of Ancestors|
Basically the Diamond Theory explains that we can't keep going back through the generations and doubling the number of ancestors in each generation. (That's the inverted pyramid theory). Why not? Because eventually the world's population will not be large enough to support the numbers!
In the Diamond Theory we see that as you keep going back through the generations you will eventually find cousins marrying cousins which narrows the number of unique ancestors and results in a diamond shape rather than a pyramid.
One probability model created by a demographer for genealogists, is that a child born in 1947 in England tracing back to 1492 would have 60,000 ancestors. Going back further to 1215, this child would find that 80% of the entire population of England at that time would be on his or her family tree! So anyone living in present-day England who traces his/her lineage back through English history would theoretically be related. This is why genealogists find so many people searching for the same families in the 1600s and earlier, and why we find so many "cousins" out there in our search.
Here's a different take on how many ancestors we have.
In Your Family Past, Present and Future the author, Tim Urban of Wait But Why has some very informative and interesting ideas on this topic. At the end he comes up with four conclusions. I love this one
Writing this post has really hammered home the point that humans are mainly a temporary container for their genes. In 150 years, all 7,100,000,000 people alive today will be dead, but all of our genes will be doing just fine, living in other people.Tim's article has illustrations and sources and is well worth the read.