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."