Let my people go home: our common ancestry

They say there are only seven, more or less, basic story plots. Springing from them are millions of stories. Similarly, there's a basic fact of genealogy, we all have two parents, four grandparents, and so on until in a relatively few generations further back there are so many that any two people must have a common ancestor. The variations and spinoffs of that basic genealogical fact continue to fascinate, especially on television where people from all walks of life are shown to have ancestry involved with significant historical events.

Earlier this month Joshua S. Weitz from the Georgia Institute of Technology published a paper (preprint) Let my people go (home) to Spain: a genealogical model of Jewish identities since 1492 (pdf).
In it he demonstrates that the Spanish government's recently announced fast-track path to citizenship for any individual who is Jewish and whose ancestors were expelled from Spain during the inquisition-related dislocation of Spanish Jews in 1492 means that most Jews qualify.
"The basis for this conclusion is that not having a link to an ancestral
group must be a property of all of an individual's ancestors, the probability of which declines (nearly) superexponentially with each successive generation. These findings highlight unexpected incongruities induced by genealogical dynamics between present-day and ancestral identities."

Although the details vary a parallel argument can be made for royal ancestry, and for descent from elite, and much less than elite groups in history.