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4 minutes de lecture

You are probably descended from royalty. So is everyone else

You are probably descended from royalty. So is everyone else You are probably descended from royalty. So is everyone else
Source: Depiction of Charlemagne, Cathedral in Moulins, France.
Une brève histoire de tous ceux qui ont vécu
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Une brève histoire de tous ceux qui ont vécu
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We are all special, which also means that none of us is. This is merely a numbers game. You have two parents, four grandparents, eight great-grandparents, and so on. Each generation back the number of ancestors you have doubles. But this ancestral expansion is not borne back ceaselessly into the past. If it were, your family tree when Charlemagne was Le Grand Fromage would harbor around 137,438,953,472 individuals on it—more people than were alive then, now, or in total. What this means is that pedigrees begin to fold in on themselves a few generations back, and become less arboreal, and more a mesh or weblike. You can be, and in fact are, descended from the same individual many times over. Your great-great-great-great-great-grandmother might hold that position in your family tree twice, or many times, as her lines of descent branch out from her, but collapse onto you. The further back through time we go, the more these lines will coalesce on fewer individuals. Pedigree is a word derived from the middle French phrase pied de grue—the crane's foot—as the digits and hallux spread from a single joint at the bottom of the tibia, roughly equivalent to our ankle. This branching describes one or a few generations of a family tree, but it's wholly inaccurate as we climb upward into the past. Rather, each person can act as a node into whom the genetic past flows, and from whom the future spills out, if indeed they left descendants at all.

This I find relatively easy to digest. The simple logic is that there are more living people on Earth now than at any single moment in the past, which means that many fewer people act as multiple ancestors of people alive today. But how can we say with utter confidence that any individual European is, like Christopher Lee, directly descended from the great European conciliator?

The answer came before high-powered DNA sequencing and ancient genetic analysis. Instead it comes from mathematics. Joseph Chang is a statistician from Yale University and wished to analyze our ancestry not with genetics or family trees, but just with numbers. By asking how recently the people of Europe would have a common ancestor, he constructed a mathematical model that incorporated the number of ancestors an individual is presumed to have had (each with two parents), and given the current population size, the point at which all those possible lines of ascent up the family trees would cross. The answer was merely 600 years ago. Sometime at the end of the thirteenth century lived a man or woman from whom all Europeans could trace ancestry, if records permitted (which they don’t). If this sounds unlikely or weird, remember that this individual is one of thousands of lines of descent that you and everyone else has at this moment in time, and whoever this unknown individual was, they represent a tiny proportion of your total familial webbed pedigree. But if we could document the total family tree of everyone alive back through 600 years, among the impenetrable mess, everyone European alive would be able to select a line that would cross everyone else’s around the time of Richard II.

Chang’s calculations get even weirder if you go back a few more centuries. A thousand years in the past, the numbers say something very clear, and a bit disorienting. One fifth of people alive a millennium ago in Europe are the ancestors of no one alive today. Their lines of descent petered out at some point, when they or one of their progeny did not leave any of their own. Conversely, the remaining 80 percent are the ancestor of everyone living today. All lines of ancestry coalesce on every individual in the tenth century.

One way to think of it is to accept that everyone of European descent should have billions of ancestors at a time in the tenth century, but there weren’t billions of people around then, so try to cram them into the number of people that actually were. The math that falls out of that apparent impasse is that all of the billions of lines of ancestry have coalesced into not just a small number of people, but effectively literally everyone who was alive at that time. So, by inference, if Charlemagne was alive in the ninth century, which we know he was, and he left descendants who are alive today, which we also know is true, then he is the ancestor of everyone of European descent alive in Europe today.

It’s not even relevant that he had eighteen children, a decent brood for any era. If he’d had one child who lived and whose family propagated through the ages until now, the story would be the same. The fact that he had eighteen increases the chances of his being in the 80 percent rather than the 20 percent who left no twenty-first century descendants, but most of his contemporaries, to whom you are all also directly related, will have had fewer than eighteen kids, and some only one, and yet they are all also in your family tree, unequivocally, definitely and assuredly.

(…)

You are of royal descent, because everyone is. You are of Viking descent, because everyone is. You are of Saracen, Roman, Goth, Hun, Jewish descent, because, well, you get the idea. All Europeans are descended from exactly the same people, and not that long ago. Everyone alive in the tenth century who left descendants is the ancestor of every living European today, including Charlemagne, and his children Drogo, Pippin, and, of course, not forgetting Hugh. If you’re broadly eastern Asian, you’re almost certain to have Genghis Kahn sitting atop your tree somewhere in the same manner, as is often claimed. If you’re a human being on Earth, you almost certainly have Nefertiti, Confucius, or anyone we can actually name from ancient history in your tree, if they left children. The further back we go, the more the certainty of ancestry increases, though the knowledge of our ancestors decreases. It is simultaneously wonderful, trivial, meaningless, and fun.

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