[This idea was born as a comment here, but I decided to create a separate post out of it]. What if your mother stood right behind you, and your mother's mother stood right behind her? Then your great grandma and then your great great grandma. Imagine them all lined up, one foot apart, stretching out into the distance. If a generation is deemed to be 25 years, a line of your ancestors as long as a football field (300 feet) would stretch backwards 7,500 years. The woman at the end of that 300 foot line would have lived during the time when agriculture just began in ancient Egypt. You'd still recognize each of your ancestors in that 300 foot line to be fully modern humans, biologically speaking. Isn't it amazing to think that you could run along side that entire 300 foot line of your ancestors in only 15 seconds (I'm assuming you’re not an Olympic caliber sprinter) to end up standing next to one of your own ancestors who was alive 7,500 years ago? Now think even further back. In An Ancestor’s Tale, Richard Dawkins calculated that 20,000,000 great-grandparents ago, our relatives were small shrew-like animals living at the end of the Cretaceous period. What if you spaced out your relatives one foot apart to extend all the way back to these shrew-like creatures? That line would be 3,787 miles long. That's about the length of highway running from my hometown of St. Louis, Missouri to Anchorage Alaska. Imagine speeding alongside that line of your relatives at 60 mph, seeing the generations of your relatives whizzing by, more than 5,000 of them every minute. It wouldn’t take long to reach the last of your relatives who looks like you. In fact, your trip would have barely begun. Biologically modern humans (those whose bodies are the functional equivalent of our own bodies) came onto the scene between 100,000 and 200,000 years ago. Driving at highway speed parallel to that line of your own relatives, you'd run out of your biologically modern human relatives less than one-minute after beginning your trip. That's only 4,000 generations. [More . . . ]