The following is an excerpt from A Brief History of Everyone Who Ever Lived by Adam Rutherford.
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.
At least that’s the theory. With the advent of easy and cheap DNA sequencing came the possibility of testing this math. DNA is the bearer of biological ancestry, and you get all of your DNA from your two parents, pretty much a 50:50 split. They in turn got all of their DNA from their parents, so one quarter of your DNA is the same as a quarter of each of your grandparents. If you have a cousin, then you share around an eighth of your DNA, as you have a pair of grandparents in common. These shared bits of DNA are not the same sections though. And it doesn’t keep halving perfectly as you meander up through your family tree. Remember that DNA gets shuffled when a sperm or egg is made, and every single shuffle is different, but it’s quite clumsy shuffling. In the newly shuffled deck, that is, your own personal genome, big chunks of it are the same as your father or mother. The more closely related two people are, the more DNA they will share in big chunks. This is why identical twins are identical (all the chunks are the same), and why siblings and parents look similar (half of their DNA is the same as each other). In genetics, we call these sections of DNA identical by descent (IBD), and they are very useful for measuring the relatedness of two individuals.
In 2013, geneticists Peter Ralph and Graham Coop showed that DNA says exactly the same thing as Chang’s mathematical ancestry: Our family trees are not trees at all, but entangled meshes. They looked for lengths of identical by descent DNA in 2,257 people from around Europe (to mitigate the influence of recent migration, all the subjects selected had four grandparents from the same region or country). By measuring the lengths of the shared DNA, they could estimate how long ago that deck got shuffled, and therefore how related any two people are. Computing and DNA have empowered this field, and this is shown in their dataset and the number crunching that follows.
Joseph Chang’s mathematical calculation didn’t account for something very obvious, which is that we don’t mate randomly. We typically marry within socioeconomic groups, within small geographical areas, within shared languages. But with Coop and Ralph’s genetic analysis, it didn’t seem to matter that much. Ancestry is such that genes can spread very quickly over generations. It might seem that a remote tribe would have been isolated from others for centuries in, for example, the Amazon. But no one is isolated indefinitely, and it only takes a very small number of people to breed out with people from beyond their direct gene pool for that DNA to rapidly descend through the generations.
Chang factored that into a further study of common ancestry beyond Europe, and concluded in 2003 that the most recent common ancestor of everyone alive today on Earth lived only around 3,400 years ago.
He used two calculations, one that simply crunched the math of ancestry, and another that incorporated a simplified model of towns and migration and ports and people. In the computer model, a port has a higher rate of immigration, and growth rates are higher. With all these and other factors input, the computer calculates when lines of ancestry cross, and the number comes out at around 1400 BCE. It places that person somewhere in Asia, too, but that is more likely to do with the geographical center point from which the migrations are calculated. If this sounds too recent, or baffling because of remote populations in South America or the islands of the South Pacific, remember that no population is known to have remained isolated over a sustained period of time, even in those remote locations. The in flux of the Spanish into South America meant their genes spread rapidly into decimated indigenous tribes, and eventually to the most remote peoples. The inhabitants of the minuscule Pingelap and Mokil atolls in the mid-Pacific have incorporated Europeans into their gene pools after they were discovered in the years of the nineteenth century. Even religiously isolated groups such as the Samaritans, who number fewer than 800 and are sequestered within Israel, have elected to outbreed in order to expand their limited gene pool.
When Chang factored in new, highly conservative variables, such as reducing the number of migrants across the Bering Straits to one person every ten generations, the age of the most recent common ancestor of everyone alive went up to 3,600 years ago.
This number may not feel right, and when I talk about it in lectures, it often results in a frown of disbelief. We’re not very good at imagining generational time. We see families as discrete units in our lifetimes, which they are. But they’re fluid and continuous over longer periods beyond our view, and our family trees sprawl in all directions. The concluding paragraph of Chang’s otherwise tricky mathematical and highly technical study is neither of those things. It’s beautiful writing, extremely unusual in an academic paper, and it deserves to be shared in full:
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.