Actually, I have no choice.
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.All together now: "It's small world after all..."
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! [More]
(Yes, I know you'll be humming it all afternoon.)