Descent from William the Conqueror
It is estimated that 25% of the population of Britain is descended from William the Conqueror. Consider that you need two parents, four grandparents, etc. Assuming an average of about 25 years per generation, you only need go back to 1200, quite within historical times, to need more separate ancestors than the population of the world. Therefor, we all must descend from cousin marriages, many times over, even within the last few hundred years. Davenport claimed "no people of English descent are more distantly related than 30th cousins."
Taken from Sr. Tanya Johnson's Johnson-Harris Family Genealogy, references: "Royal Descents of Famous People, by Mark Humphreys, and "In the Blood: God, Genes & Destiny," by Steve Jones, 1996.