If you have European ancestry, there's a good chance you're descended from Charlemagne, the medieval king who was crowned emperor in A.D. 800.

At first, that sounds impossible. After all, how could one person become the ancestor of millions?

The answer lies in the surprising way family trees work.

Think of your family tree as growing wider with every generation. Eventually it grows so wide that its branches begin crossing one another. The same people start appearing in multiple places because, over the centuries, distant relatives married other distant relatives, connecting family lines again and again.

Genealogists call this pedigree collapse. Distant cousins married, family lines crossed, and the branches of countless family trees gradually merged.

Charlemagne himself helped set that process in motion. Historians generally agree he fathered at least 18 children, although the exact number is uncertain because medieval records are incomplete. Many of those children had families of their own, and over the next 1,200 years their descendants spread across Europe. As those family lines repeatedly intertwined, his descendants multiplied into the millions.

Many people who left descendants during the early Middle Ages may also be ancestors of a large share of today's Europeans. Charlemagne is simply the best-known example because his life was so well documented.