Say you have an heirloom poem that each member of your family has to transcribe. It's in Latin, so you have no idea what it says.

Even with all the checking, we know that every new generation makes minor mistakes transcribing it, which build up over time. The same poem has somehow spread all over the world because your family is all over.

How do you find out when it was written?

Well, you compare the last few generations' worth of poems and realize it's on average 1.75 mistakes each time it's transcribed. Now you compare your family's to another one elsewhere on earth, and there are 500 differences - how many generations ago did they branch off?