Dr. Yan Wong explains why everyone alive in the Holy Land at the time of Jesus would have been able to claim David for an ancestor. He provides a simple mathematical explanation (exponential growth) and makes a couple of assumptions (that any two people in any one country probably won’t need to go back many generations before finding a common ancestor due to inbreeding), and then he extrapolates to the future:
What about the wider ramifications? A single immigrant who breeds into a population has roughly 80% chance of becoming a common ancestor. A single interbreeding event in the distant past will probably, therefore, graft the immigrant’s family tree onto that of the native population. That makes it very likely that King David is the direct ancestor of the populations of many other countries too.
How far do we have to go back to find the most recent common ancestor of all humans alive today? Again, estimates are remarkably short. Even taking account of distant isolation and local inbreeding, the quoted figures are 100 or so generations in the past: a mere 3,000 years ago.
And one can, of course, project this model into the future, too. The maths tells us that in 3,000 years someone alive today will be the common ancestor of all humanity.
A few thousand years after that, 80% of us (those who leave children who in turn leave children, and so on) will be ancestors of all humanity. What an inheritance!
Have you ever traced your family genealogy?