When Ryan Tubridy appeared on the RTE TV program Who Do You Think You Are? last year, he was startled to discover he descends from the 14th century King Edward III of England. This seemed at odds with his self-image as proud son of a Fianna Fail family of Irish nationalists. The program did not really explain the big picture to Tubridy. It did not explain that such descents from medieval European monarchs are actually quite common for ordinary people in the West. For instance, all the following provably descend from the medieval English Royal house: Barack Obama, John McCain, Sarah Palin, George W. Bush, Washington, Jefferson, both Roosevelts, Winston Churchill, Wellington, Gladstone, Thatcher, Tony Blair, David Cameron, Hermann Goering, David Hume, Bertrand Russell, Darwin, Dawkins, Bill Gates, Jane Austen, Shelley, Byron, George Orwell, Walt Disney, Uma Thurman, Brad Pitt, Hugh Grant and Paris Hilton. In fact, I have collected Royal descents for over 300 famous people, showing how interconnected the West really is. Tubridy's Royal descent means he is related to all of these people. If we want Irish heroes, note that Garret Mor Fitzgerald, Garret Og Fitzgerald and Silken Thomas descend from Edward I, as do Parnell, St. Oliver Plunkett and William Smith O'Brien. Robert Emmet descends from Edward III. Lord Edward Fitzgerald descends from Charles II. As for popular culture, working with expert Leo van de Pas, I recently discovered that Chris de Burgh and Rosanna Davison descend from Edward I. [1] Tubridy's Royal descent means he is related to all of these too. Tubridy's descent through the House of York also means he descends from Brian Boru, High King of Ireland. In fact, most people who descend from medieval English royalty can find a descent from Brian Boru. This is because Brian Boru is the ancestor of every English monarch from 1399 to the present. Almost certainly, more people in England can prove descent from Brian Boru than people in Ireland. But genealogy has recently gone beyond just collecting amazing descents. It has started to develop a theory of amazing descents. It is now clear there is a vast "Western family tree" from which millions of people across the West provably descend. You either can connect to this tree (like Tubridy) or (like me) you can't. This leads to the question of whether everybody in the West is on this tree, even if they can't prove it. The answer is probably yes. To be precise, recent mathematical models and computer simulations suggest that the "Most Recent Common Ancestor" of Ireland, or Britain, or the West, or even the world, could have lived within historical times, even AD. For example, Charlemagne, the Holy Roman Emperor who died in 814 AD, is probably the ancestor of almost everyone in the West today. The Egyptian Pharaoh Cheops (around 2500 BC) is probably the ancestor of almost every human on earth. And Brian Boru (died 1014 AD) is probably ancestor of all modern Irish (except recent immigrants). It might look something like this: 99 percent of Irish descend from Charlemagne. 95 percent descend from Brian Boru. 90 percent descend from William the Conqueror. 50 percent descend from Edward I. 30 percent descend from Edward III. 5 percent descend from Henry VII. 1 percent descend from James I. These numbers are only guesses, but most genealogists would agree with the broad picture. This view of the world's family tree is quite new. Genealogists have been collecting Royal descents since the 19th century, but the computer simulations only came to attention with a landmark paper in Nature in 2004. So everyone in Ireland may descend from Brian Boru, but can you prove it? Tubridy is ahead of the game there. Proving a Royal descent is almost commonplace in England, but is still not so common in Ireland. Most Irish people, if they are of Catholic descent, cannot find a line back through the penal times of the early 18th century. This is the case with my family. I have been working for 25 years on a link to the Anglo-Irish family of Blennerhassett, but I have not proved it. I have even launched a competition with a €2,000 prize [2] for anyone who can prove it, and link me to the western family tree. For the moment I remain, like most Irish, on the outside. But nothing stands still, and as Irish people marry with other westerners, their children get provable Royal descents. For instance, my English wife has more descents from Edward III than I can even graph. [3] So all my descendants will descend from Edward III (and Brian Boru). In the future, everyone in Ireland will be of proven Royal descent at birth. Whether everyone in Ireland will have to revise their self-image, as Tubridy may have had to, remains to be seen.