The mathematical study of genealogy indicates that everyone in the world is descended from Nefertiti and Confucius, and everyone of European ancestry is descended from Muhammad and Charlemagne
A few years ago the Genealogical Office in Dublin moved from a back room of the Heraldic Museum up the street to the National Library. The old office wasn't big enough for all the people stopping by to track down their Irish ancestors, and even the new, much larger office is often crowded. Because of its history of oppression and Catholic fecundity, Ireland has been a remarkably productive exporter of people. The population of the island has never exceeded 10 million, but more than 70 million people worldwide claim Irish ancestry. On warm summer days, as tourists throng nearby Trinity College and Dublin Castle, the line of visitors waiting to consult one of the office's professional genealogists can stretch out the door.
I suspect that many people have had a fling with genealogy somewhat like mine. In my office I have a file containing the scattered lines of Olsons and Taylors, Richmans and Sigginses (my Irish ancestors), that I gathered several years ago in a paroxysm of family-mindedness. For the most part my ancestors were a steady stream of farmers, ministers, and malcontents. Yet a few of the Old World lines hint at something grander—they include a couple of knights, and even a baron. I've never taken the trouble to find out, but I bet with a little work I could achieve that nirvana of genealogical research, demonstrated descent from a royal family.
Earlier this year I went to Dublin to learn more about the Irish side of my family and to talk about genealogy with Mark Humphrys, a young computer scientist at Dublin City University. Humphrys has dark hair, deep-blue eyes, heavily freckled arms, and a pasty complexion. He became interested in genealogy as a teenager, after hearing romantic stories about his ancestors' roles in rebellions against the English. But when he tried to trace his family further into the past, the trail ran cold. The Penal Laws imposed by England in the early eighteenth century forbade Irish Catholics from buying land or joining professions, which meant that very few permanent records of their existence were generated. "Irish people of Catholic descent are almost completely cut off from the past," Humphrys told me, as we sat in his office overlooking a busy construction site. (Dublin City University, which specializes in information technology and the life sciences, is growing as rapidly as the northern Dublin suburb in which it is located.) "The great irony about Ireland is that even though we have this long, rich history, almost no person of Irish-Catholic descent can directly connect to that history."
While a graduate student at Cambridge University, Humphrys fell in love with and married an Englishwoman, and investigating her genealogy proved more fruitful. Her family knew that they were descended from an illegitimate son of the tenth Earl of Pembroke. After just a couple of hours in the Cambridge library, Humphrys showed that the Earl of Pembroke was a direct descendant of Edward III, making Humphrys's wife the King's great-granddaughter twenty generations removed. Humphrys began to gather other genealogical tidbits related to English royalty. Many of the famous Irish rebels he'd learned about in school turned out to have ancestors who had married into prominent Protestant families, which meant they were descended from English royalty. The majority of American presidents were also of royal descent, as were many of the well-known families of Europe.
Humphrys began to notice something odd. Whenever a reliable family tree was available, almost anyone of European ancestry turned out to be descended from English royalty—even such unlikely people as Hermann Göring and Daniel Boone. Humphrys began to think that such descent was the rule rather than the exception in the Western world, even if relatively few people had the documents to demonstrate it.
Humphrys compiled his family genealogies first on paper and then using computers. He did much of his work on royal genealogies in the mid-1990s, when the World Wide Web was just coming into general use. He began to put his findings on Web pages, with hyperlinks connecting various lines of descent. Suddenly dense networks of ancestry jumped out at him. "I'd known these descents were interconnected, but I'd never known how much," he told me. "You can't see the connections reading the printed genealogies, because it's so hard to jump from tree to tree. The problem is that genealogies aren't two-dimensional, so any attempt to put them on paper is more or less doomed from the start. They aren't three-dimensional, either, or you could make a structure. They have hundreds of dimensions."
Much of Humphrys's genealogical research now appears on his Web page Royal Descents of Famous People. Sitting in his office, I asked him to show me how it works. He clicked on the name Walt Disney. Up popped a genealogy done by Brigitte Gastel Lloyd (Humphrys links to the work of others whenever possible) showing the twenty-two generations separating Disney from Edward I. Humphrys pointed at the screen. "Here we have a sir, so this woman is the daughter of a knight. Maybe this woman will marry nobility, but there's a limited pool of nobility, so eventually someone here is going to marry someone who's just wealthy. Then one of their children could marry someone who doesn't have that much money. In ten generations you can easily get from princess to peasant."
The idea that virtually anyone with a European ancestor descends from English royalty seems bizarre, but it accords perfectly with some recent research done by Joseph Chang, a statistician at Yale University. The mathematics of our ancestry is exceedingly complex, because the number of our ancestors increases exponentially, not linearly. These numbers are manageable in the first few generations—two parents, four grandparents, eight great-grandparents, sixteen great-great-grandparents—but they quickly spiral out of control. Go back forty generations, or about a thousand years, and each of us theoretically has more than a trillion direct ancestors—a figure that far exceeds the total number of human beings who have ever lived.
In a 1999 paper titled "Recent Common Ancestors of All Present-Day Individuals," Chang showed how to reconcile the potentially huge number of our ancestors with the quantities of people who actually lived in the past. His model is a mathematical proof that relies on such abstractions as Poisson distributions and Markov chains, but it can readily be applied to the real world. Under the conditions laid out in his paper, the most recent common ancestor of every European today (except for recent immigrants to the Continent) was someone who lived in Europe in the surprisingly recent past—only about 600 years ago. In other words, all Europeans alive today have among their ancestors the same man or woman who lived around 1400. Before that date, according to Chang's model, the number of ancestors common to all Europeans today increased, until, about a thousand years ago, a peculiar situation prevailed: 20 percent of the adult Europeans alive in 1000 would turn out to be the ancestors of no one living today (that is, they had no children or all their descendants eventually died childless); each of the remaining 80 percent would turn out to be a direct ancestor of every European living today.
Chang's model incorporates one crucial assumption: random mating in the part of the world under consideration. For example, every person in Europe would have to have an equal chance of marrying every other European of the opposite sex. As Chang acknowledges in his paper, random mating clearly does not occur in reality; an Englishman is much likelier to marry a woman from England than a woman from Italy, and a princess is much likelier to marry a prince than a pauper. These departures from randomness must push back somewhat the date of Europeans' most recent common ancestor.
But Humphrys's Web page suggests that over many generations mating patterns may be much more random than expected. Social mobility accounts for part of the mixing—what Voltaire called the slippered feet going down the stairs as the hobnailed boots ascend them. At the same time, revolutions overturn established orders, countries invade and colonize other countries, and people sometimes choose mates from far away rather than from next door. Even the world's most isolated peoples—Pacific islanders, for example—continually exchange potential mates with neighboring groups.
This constant churning of people makes it possible to apply Chang's analysis to the world as a whole. For example, almost everyone in the New World must be descended from English royalty—even people of predominantly African or Native American ancestry, because of the long history of intermarriage in the Americas. Similarly, everyone of European ancestry must descend from Muhammad. The line of descent for which records exist is through the daughter of the Emir of Seville, who is reported to have converted from Islam to Catholicism in about 1200. But many other, unrecorded descents must also exist.
Chang's model has even more dramatic implications. Because people are always migrating from continent to continent, networks of descent quickly interconnect. This means that the most recent common ancestor of all six billion people on earth today probably lived just a couple of thousand years ago. And not long before that the majority of the people on the planet were the direct ancestors of everyone alive today. Confucius, Nefertiti, and just about any other ancient historical figure who was even moderately prolific must today be counted among everyone's ancestors.
Toward the end of our conversation Humphrys pointed out something I hadn't considered. The same process works going forward in time; in essence every one of us who has children and whose line does not go extinct is suspended at the center of an immense genetic hourglass. Just as we are descended from most of the people alive on the planet a few thousand years ago, several thousand years hence each of us will be an ancestor of the entire human race—or of no one at all.
The dense interconnectedness of the human family might seem to take some of the thrill out of genealogical research. Sure, I was able to show in the Genealogical Office that my Siggins ancestors are descended from the fourteenth-century Syggens of County Wexford; but I'm also descended from most of the other people who lived in Ireland in the fourteenth century. Humphrys took issue with my disillusionment. It's true that everyone's roots go back to the same family tree, he said. But each path to our common past is different, and reconstructing that path, using whatever records are available, is its own reward. "You can ask whether everyone in the Western world is descended from Charlemagne, and the answer is yes, we're all descended from Charlemagne. But can you prove it? That's the game of genealogy."