Actress Brooke Shields has a pretty impressive pedigree — hanging from her family tree are Catherine de Medici and Lucrezia Borgia, Charlemagne and El Cid, William the Conquerer 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 Niccolo Machiavelli and conquistador Hernando Cortes 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. "The number of people with unprovable descents must be massive."

By the same token, for every king in a person's family tree there are thousands and thousands of nobodies whose births, deaths and lives went completely unrecorded by history. We'll never know about them, because until recently vital records were a rarity for all but the noble classes.

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.

A slight twist of fate could have prevented the existence of all of them. In 1312 the close adviser and probable lover of Edward II, Piers Gaveston, was murdered by a group of barons frustrated with their king's ineffectual rule. The next year the beleaguered king produced the son who became Edward III.

Had Edward II been killed along with Gaveston in 1312 — a definite possibility at the time — Edward III would never have been born. He wouldn't have produced the lines of descent that ultimately branched out to include all those presidents, writers and Hollywood stars — not to mention everybody else.

Of course, the only reason we're talking about Edward III is that history remembers him. For every medieval monarch there are countless long-dead nobodies whose intrigues, peccadilloes and luck have steered the course of history simply by determining where, when and with whom they reproduced.