For example, it is estimated that there were about 700,000 people living in England at the time of the Conquest (1066 AD). From this one can infer that a person of English ancestry is descended from everyone living in England at the time of the Conquest (who had descendants). My family has traced its roots to William the Conquerer--but so can every other person of English Ancestry. In other terms William the Conquerer is the father of all the English. But so, too, is the poorest farmer's wife at that time is also the mother of all the English.
Similar arguments can be made about any ethnic group. All the Japanese today are all descended from a group of several hundred thousand Japanese who lived on those Island a 1,000 or 2,000 years ago. The same can be said for the French, Russians, Zulus, or Laotians.
By extension, one might plausibly believe that every person in the world can trace at least one line back to Abraham, Isaac, and Jacob, given the amount of migration of peoples there has been throughout history.