Important note: The ginormous family tree image at the bottom is a limited abstraction of Queen Elizabeth II’s pedigree. Since writing this article I have found two linkages between our family and the European nobility, and I will find many many more – just like nearly everyone on the planet.
Mathematically the odds of any two people sharing a distant ancestor and both having inherited the same unique #genetic trait from the ancestor is astronomically unlikely. There are just far, far too many variables which contrive to reduce the chances.
And yet it happens all the time; a person born in Norway shares genetic material with a person born in Turkey. Or even more divergently. That it happens at all between, say, an Inuit in Canada and a Maori in New Zealand is mindboggling.
Until you begin to look at the numbers.
The number of ancestors you have grows very rapidly as you go back through the generations, as we all know. Every generation back raises the number of possible ancestors by the power of 2, 2^2 = four grandparents, 2^3 = 8 great-grandparents, and so on. Assuming a conservative average of 33 years between generations (what generation really waits that long to begin having babies on average?), that comes to 30 generations in 1 000 years, 1 073 741 824 possible ancestors, which is almost certainly more than the entire population of Europe in the year 1017.
Which means two things: many more of your ancestors married your other ancestors than you probably know about, and many more of your ancestors traveled further than you probably thought possible way back then.
To give an example of the latter first, a specific genetic heritage for red hair is mapped to Ireland about 4500 years ago. But this same genetic trait was found in a burial site in the Tibetan highlands from 6000 years ago, in a location known for its use as a trade route. Either the genetic mapping is presumptuous in guessing the trait started in Ireland, or people with the trait were traveling all the way to China along trade routes nearly 3700 years before Alexander the Great.
But looking at the numbers you realize just how much endogamy must have been taking place, probably for the most part unknowingly. The estimated population of Europe at the beginning of the High Middle Ages (1000-1250) may have been 30 million, and would rise to 100 million. Any person of European genetic extraction must be descended from nearly every one of these with many many crosses to come up with the more than a billion possible ancestors from this generation. We can roughly estimate there must be 60% shared ancestors throughout the person’s ancestry, if we accept the premise that Europeans held firm to only breed with other Europeans.
But they didn’t. If we look at early Roman art we see they looked far more like the North African people of Carthage prior to the sweeping invasions from Asia. In the east, bronze-age Scythian and Cimmeran cultures show clear genetic blending from Mongolia, Siberia, and China with predominatey Eastern European, while recent studies of iron-age samples show a more insular Iranic-speaking people of south-central Asia. [Explaining this a bit more in 2019: the population in the Iron Age nearly completely displaced the earlier Bronze Age cultures; those people moved to somewhere else. The point being that a location’s genetic heritage changes, sometimes dramatically.]
The numbers dominate, they over-rule prejudice and persuasion and family myths. Our longest known chain of ancestry is 18 generations. If they never intermarried and I looked only from that most recent generation through just the ancestors there could be 262 144 people at the furthest generation. And I know two of them.
The family tree currently includes more than [4 000 individuals in 2019], almost all of them in the most recent 3 generations – aunts and uncles, cousins, and so on. I will almost certainly never fill in the hundreds of thousands of people in those 18 generations of ancestors, and all their inter-relations. But I will try to fill in some of the blanks.
And since the task of numbers is impossible, I will focus instead on accuracy.