Friend: "Grammy, the president's been shot!"
Grandmother: "Another one?!"
It was then that I realized her grandmother had lived (and was old enough to remember) every presidential shooting from Lincoln to Kennedy. It blew my mind.
It's amazing how much can happen in a lifetime. This point was driven home in a recent post on Reddit. As of March 2011, our 10th president John Tyler (born 1790) had two living grand children! This got me thinking: If you passed a baton from person to person, what's the least number of hands it could touch and reach the first even human?
According to Wikipedia, modern humans showed up about 200,000 years ago. If you passed the baton from a centenarian to a new born that would live to be a centenarian, it would take only 2000 people.1 This is roughly the number of people attending my high school. If you wanted to go back to the time of Christ, it would only take 20 people.
 Some people will no doubt quibble that the average life span was in the 30s for much of our history. True, but this neglects two points. First, we're doing an order of magnitude estimate and this fact won't change the answer by an order of magnitude. More importantly, "life expectancy" is the average life span. Life expectancy may have been low, but this was largely due to high infant mortality rates. Back in the day, if you were lucky enough to make it to adulthood, you stood a good chance of making it to old age. Since we're only looking for the oldest person at any one time, there's a good chance there was centenarian somewhere in the world.