The Simplistic Jew's story yesterday brought to mind some research that deserves a wider audience.
Simple (and I didn't mean to insult him above) tells the story of family that owned a document tracing their lineage to King David. I think we all know Jewish families that make similar claims. I'm related to Rashi. I can trace my line to the Marsha. It looks great on shadchan applications, and it impresses the easily impressed, but it's all hooey.
The mathematics of our ancestry is exceedingly complex, because the number of our ancestors increases exponentially, not linearly. These numbers are manageable in the first few generations—two parents, four grandparents, eight great-grandparents, sixteen great-great-grandparents—but they quickly spiral out of control. Go back forty generations, or about a thousand years, and each of us theoretically has more than a trillion direct ancestors—a figure that far exceeds the total number of human beings who have ever lived.
In a 1999 paper titled "Recent Common Ancestors of All Present-Day Individuals," Joseph Chang, a statistician at Yale University, showed how to reconcile the potentially huge number of our ancestors with the quantities of people who actually lived in the past. He has found that 80 percent of the adult Europeans alive in 1000 are direct ancestors of every European living today.
The dense interconnectedness of the human family means that a few thousand years ago the majority of the people on the planet (ie: everyone with a line that survived) were the direct ancestors of everyone alive today.
So we're all direct descendants of King David, all os us, including non-Jews. We're all probably descendant of Rashi and the Maharsha, too. Factor in rapes and intermarriage, and it's fair to argue that we're all also direct descendants of Mohammed, and Confucius.
Sort of takes the fun out of the genealogy game, eh?
[Source: The Atlantic Monthly] If you can't read the subscription-protected article, send me an email and I'll send it to you.