Arthur M. Schlesinger, Jr., one the United States' great historians, is less than two lifetimes removed from a world where the United States did not exist. Through Mr. Schlesinger, you're no more than three away yourself. That's how short the history of our nation really is.
It goes on to put other events in human history in persepective, and is worth reading. It doesn't, however, mention my favorite representation of the scale of human impact, which is this quote_
Perhaps an even more effective way of grasping our extreme recentness as part of this 4.5 billion-year-old picture is to stretch your arms to their fullest extent and imagine that width as the entire history of the Earth. On this scaleâ€¦ the distance from the fingertips of one hand to the wrist of the other is Precambrian. All of complex life is in one hand, â€œand in a single stroke with a medium-grained nail file you could eradicate human history.â€
-- Bill Bryson, (quoting John McPhee) in his book "A Short History of Nearly Everything"