If we double the number of ancestors in each generation, 2 parents, 4 grandparents, and so on, we can see that by the time we are back 10 generations, we have the potential for 1024 ancestors. If we were to go back to the time of Charlemagne, we would find we had the potential for 281 trillion ancestors all living at that one moment in history. But this is statistically impossible! The world’s population at that time does not support these numbers. So where did our ancestors go?
It is estimated that 80% of the marriages in history were between second cousins. Why? Because the population base was smaller, people lived in small communities and migrated within those same small communities. The theory in genealogical research is that our family trees are actually shaped like a diamond, not an inverted pyramid. Tracing back a few generations gives a wider shape. Keep going and you find the shape narrowing, eventually, the theory holds, converging to only a few ancestors.
This may sound mind-boggling but I've seen the truth of it. I am back a total of 14 generations which takes me to the last half of the 1500s. I've found that in two cases so far, I am descended from more than one child of one specific couple. Need an example? Pieter Uziele and his wife Cornelia Damen were my 8th great grandparents. I descend from two of their children: Sophia Uziele and her sister Maria Uziele. Remember, they are my 7th great-grandmothers and are in my 10th generation. I also descend from two children of Jochem Lambertse Van Valkenburg and his wife Eva Hendrickse Vrooman, who were my 8th great-grandparents. Their son Isaac and his sister Jannetie are my 7th great-grandparents and are in my 10th generation. So we see the gene pool narrowing in my 11th generation!
How is this possible? In the pyramid theory of doubling ancestors each generation, these four 7th great-grandparents would give me eight distinct individuals as ancestors for my 8th great-grandparents - but they don't. Because they are sets of siblings, I have only four new distinct individuals as ancestors for my 8th great-grandparents - half the number I should have if the doubling theory held true.
Assuming I have double sets of siblings at least three times on that 10th generation, I've lost six individuals from my 11th generation. That carries over to my 12th generation, but doubles the number I lose for a total of 12 ancestors. If I had three more double sets of siblings in my 11th generation, I've lost another six individuals in my 12th - for a total of 18 fewer individuals. Keep doing this for a few more generations and you'll see the shape your ancestral tree is taking. It’s not an inverted pyramid, it’s a diamond.
Luckily for the human race, this tendency to marry cousins reversed itself in more recent years, due to larger population bases and easier access to possible mates. Otherwise, our search for the missing link might prove to be just that !
One very interesting probability model created by a demographer for genealogists, is that a child born in 1947 in Englad 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 or 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. I've found hundreds of cousins in the last year while searching via the Internet.
Genealogy is fascinating, and becomes even more so when we make those human contacts in present-day times with folks as far away as Norway who are descended from the same immigrant ancestor of 1624. I've become almost blasé about new cousins - I expect to find them, and I do!
Inverted Pyramid Theory of Doubling Ancestors
Inverted Pyramid Theory
In this theory the number of ancestors double each generation. I can't represent the rest of the generations on this page, so following is the number of theoretical ancestors in each generation, starting at Generation 12 where the figure above leaves off.
Gen. 12: 2048
Gen. 13: 4096
Gen. 14: 8192
Gen. 15: 16384
Gen. 16: 32768
Diamond Theory of Ancestors
In this theory the pyramid begins to narrow beyond the 10th generation. I can't represent this with numbers as they would be unknown, so I am representing the basic shape with x representing the number of individuals in each generation. I will, however make some assumptions about the number of parents and grandparents back to the 10th generation.
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.
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 including this, my favorite 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 Urban. https://waitbutwhy.com/2014/01/your-family-past-present-and-future.html)
Tim's article has illustrations and sources and is well worth the read.
Lorine McGinnis Schulze is a Canadian genealogist who has been involved with genealogy and history for more than thirty years. In 1996 Lorine created the Olive Tree Genealogy website and its companion blog. Lorine is the author of many published genealogical and historical articles and books.