Brooke Shields comes from royalty; so do you

Actress Brooke Shields has a pretty impressive pedigree; hanging on her family tree are Catherine de Medici and Lucrezia Borgia, Charlemagne and El Cid, William the Conqueror and King Harold, vanquished by William at the Battle of Hastings.

Shields also descends from five popes, a whole mess of early New England settlers and the royal houses of virtually every European country. She counts Renaissance pundit Niccolò Machiavelli and conquistador Hernando Cortés as ancestors.

What is it about Brooke? Well, nothing, at least genealogically.

Even without a documented connection to a notable forebear, experts say the odds are virtually 100 percent that every person on Earth is descended from one royal personage or another.

"Millions of people have provable descents from medieval monarchs," said Mark Humphrys, a genealogy enthusiast and professor of computer science at Dublin City University in Ireland.

By the same token, for every king in a person's family tree there are thousands and thousands of nobodies.

It works the other way, too. Anybody who had children more than a few hundred years ago is likely to have millions of descendants today, and quite a few famous ones.

Take King Edward III, who ruled England during the 14th century and had nine children who survived to adulthood. Among his documented descendants are presidents (George Washington, Thomas Jefferson, John Quincy Adams, Zachary Taylor, both Roosevelts), authors (Jane Austen, Lord Byron, Alfred Lord Tennyson, Elizabeth Barrett Browning), generals (Robert E. Lee), scientists (Charles Darwin) and actors (Humphrey Bogart, Audrey Hepburn, Brooke Shields). Some experts estimate that 80 percent of England's present population descends from Edward III.

The longer ago somebody lived, the more descendants a person is likely to have today. Humphrys estimates that Muhammad, the founder of Islam, appears on the family tree of every person in the Western world.

Some have tried to establish a line between Muhammad, who was born in the sixth century, and the medieval English monarchs, and thus to most if not all people of European descent. Nobody has succeeded, but one proposed lineage comes close.

The proposed genealogy runs through Muhammad's daughter Fatima. Her husband, Ali, also a cousin of Muhammad, is considered by Shiite Muslims the legitimate heir to leadership of Islam.

Ali and Fatima had a son, al-Hasan, who died in 670. About 300 years later, his ninth great-grandson, Ismail, carried the line to Europe when he became Imam of Seville.

Many genealogists dispute the connection between al-Hasan and Ismail, claiming it includes characters invented by medieval genealogists trying to link the Abbadid dynasty, founded by Ismail's son, to Muhammad.

The last emir in that dynasty was supposed to have had a daughter named Zaida, who is said to have changed her name to Isabel upon converting to Christianity and marrying King Alfonso VI.

Yet there is no good evidence demonstrating that Isabel, who bore one son by Alfonso VI, is the same person as Zaida. So the line between Muhammad and the English monarchs probably breaks at this point. But if you give the Zaida/Isabel story the benefit of the doubt, too, the line eventually leads to Isabel's fifth great-granddaughter, Maria de Padilla.

Maria married another king, Peter the Cruel. Their great-great-granddaughter was Queen Isabel. Her daughter Juana married a Hapsburg, and eventually gave rise to a Medici, a Bourbon and long line of Italian princes and dukes, spreading the Muhammadan line all over Europe.

Finally, 43 generations from Muhammad, there's an Italian princess, Marina Torlonia, whose granddaughter is Brooke Shields.