Just discovered an interesting article published last month which says that, according to genealogy experts, 'the odds are virtually 100 percent that every person on Earth is descended from one royal personage or another' - i.e. we are all members of a royal family!
The actress Brooke Shields is presented as an example - her ancestors include five Popes, Charlemagne, William the Conqueror and the Prophet Mohammed. Indeed, according to Mark Humphrys of Dublin City University, the founder of Islam 'appears on the family tree of every person in the Western world.' Likewise, he estimates that 80% of English people are related to King Edward III.
Such a theory is based on our common descent from Adam and Eve and the statistical fact that '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.' It does raise some interesting possibilities - I certainly never thought of Edward III and the Prophet Mohammed as distant relations!