“How Many Ancestors do We Have?” by Dr. Terrence M. Punch, CM, FIGRS
Until very recent times, it took two to tango. Barring miracles or divine intervention every human being was the result of sexual activity between a man and a woman. Dolly the sheep and other clones were science fiction notions, futuristic fantasies, or they were before this generation. I suppose a hair-splitter who considers that il faut faire des précisions would argue that the first humanoids resulted from the mating of some sort of ape-like creatures. But for most of us ordinary folk, the traditional method of procreation served thousands of years of ancestors well enough to get or beget us here.
With that in mind, anyone tracing his or her ancestry will start with two parents, four grandparents, generally eight great-grandparents. We would double the number of our ancestors with each generation, so we’d go 2-4-8-16-32-64-128-256-512, etc., until twenty generations back you’d count 1,048,576 forebears. If you allow an average of 30 years per generation, then 600 years ago, you would theoretically be able to claim that many people born circa 1400 as being your ancestors. If you took a generation to be 25 years rather than 30, you’d get that number a century sooner, about 1492, the time Columbus reached the Americas. Need I add that, if you followed that track mathematically, you should have had 65,108,864 ancestors born in the generation of 1300.
What’s wrong with this picture? Let’s assume for the sake of argument that historical demographers are correct in their estimates of early populations. Colin McEvedy & Richard Jones, in their Atlas of World Population History (1978), calculate that there were 350,000,000 people alive in the entire world in the year 1400. The number collapses further when you realize that figure includes everyone then alive, regardless of which generation they belonged to. Since there would be parents and children and some grandchildren, then even the most generous estimate would have to cut that number by about sixty percent, let’s say that the generation of 1400, worldwide, computes out to somewhere between 125 and 135 million souls. And here you are with 65,000,000 ancestors born in that generation. Quite a bit wrong with that picture, I’d say.
Perhaps your family all came from Ireland, with its estimated one million living souls in the year 1500, meaning that about 330-340,000 people were born in that generation in Erin. But hold on, you’ve got three times that many ancestors belonging to that group. It can’t be done, can it? Perhaps your family wasn’t all Irish after all, or perhaps people were marrying relatives?
Here we come to the concept of implex or, if you prefer, pedigree collapse. As we work backwards in time through several generations in all our family lines, we will, sooner or later, discover that the same person or couple appears twice or even more often. Each time that happens, the number of different ancestors we have is diminished. This is not a sign of incest, and when such a discovery is made, you needn’t fear that your children will be driveling idiots.
In some cultures, cousins were obliged or at least encouraged to marry in order to keep wealth and property within the family or social class. A lord might have a child by a commoner, but he was expected to marry his social peers and produce legitimate heirs. In Europe until recently, the practice was for members of royal families to marry inside the ranks of families considered suitable matches. Failure to do so resulted in becoming disqualified from succeeding to the throne. The outstanding example of that was the exclusion of the three children of Archduke Franz Ferdinand from the succession in Austria-Hungary before World War One. Their mother was a countess, but she wasn’t royalty, so the children of the marriage were nobility but not imperial and royal highnesses.
Genealogists will learn that they have examples of implex, or pedigree collapse, in their own family trees. It is normal, and mathematically guaranteed to have occurred not once, but many times, over the past several centuries, and as a rule is nothing to be concerned about. If one case of intermarriage occurred six generations back, that is, about 200 years ago, then all the ancestors of that repeated couple will be the same, reducing the number of possible ancestors you have accordingly. One demographer has calculated that would give you 123,554 ancestors in 1500 instead of 1,048,576. But it is more than likely that in the five centuries between then and now, implex will have taken place several times in various parts of your family tree. Once you factor in social class, religion, geographic location and so forth, you will discover that you have considerably fewer than the theoretical million or so great-greats than you may have expected. It would surprise most students of population history to find anyone with more than several thousand ancestors born in the generation of 1500. A number between four and six thousand would be far more realistic, assuming that your ancestors all came from the same country.
Again, consider how difficult travel was before the age of steam power, and you can see that most of our ancestors had to marry from within a fairly circumscribed geographical area. Add in the fact that, apart from those engaged in mercantile trade or who were nobility, most people were born, lived, and died inside a very limited area. Engagement in war took some men far afield, and the devastation of war caused refugees to leave familiar places. Yet, given the constraints of class and economics, the selection of possible mates was usually quite limited. Villagers tended to marry within the village or inside the bounds of two or three neighboring parishes. The chances of marrying a second, third or fourth cousin were not just likely, but probable. Clergy of all denominations kept an eye on the degree of consanguinity (relationship by blood) between prospective married couples, and, all in all, the resultant progeny were normal and healthy.
The point of discussing implex, or pedigree collapse, is to point out that we may indeed be descended from kings and peasants, but not from as many different ones as simple binary calculation would lead us to believe. If you find that two of your great-great-great-great-grandparents turn up twice in the family tree, don’t fret; it’s to be expected and is perfectly all right.