Here is a list, from parent to child, showing approximate birth year for each:
I am a grandson of Arthur Conrad, and thus a 24th great-grandson of King John--the King John of Magna Carta fame.
William I the Conqueror King of England 1027
Henry I "Beauclerc" King Of England 1068
Matilda (Maud) Empress Of Germany 1102
Henry II "Plantagenet" King Of England 1133
John "Lackland" King of England 1166
Joan Princess of England 1188
Margred Verch Llywelyn 1210
Maud de Clifford 1234
Eleanor Giffard 1275
Elizabeth le Strange 1308
Roger Corbet 1330
Robert Corbet 1383
Roger Corbet 1412
Anne Corbet 1438
Blanche Sturry 1472
Thomas Whitcomb 1502
William Whitcomb 1528
John Whitcomb 1558
John Whitcomb 1588
Johnathan Whitcomb 1628
Johnathan Whitcomb 1669
Johnathan Whitcomb 1690
William Whitcomb 1719
Oliver Whitcomb 1749
Oliver Whitcomb 1772
Hannah Whitcomb 1806
Lucinda Haws 1828
Mary Elizabeth Holdaway 1856
Arthur Marion Conrad 1882
This discovery is somewhat tempered by some realizations:
- The descent is through Joan, who was an illegitimate daughter (although she married and became Princess of Wales, and later got Pope Honorius III to declare her birth legitimate).
- King John must have millions and millions of descendants by now.
- Although he was an innovator in some ways (ex. the Pipe Rolls), he had many disagreeable traits, and was recently selected as the "worst Briton" of his century.
So, by the time you reach William the Conqueror, my 28th great-grandfather, there are well over two billion ancestors in my family tree. In particular, William the Conqueror is only one of nearly 550 million 28th great-grandfathers (and an equal number of 28th great-grandmothers).
I don't know how many people were living in western Europe back then, but I doubt that it was anywhere near a billion. So, it doesn't seem very unlikely that one of them in particular was a progenitor.