It's hard to grasp the passage of time. When I was younger, I thought the longest time imaginable was how long it took to use a whole tube of tooth paste. It just seemed to never end!
Things that happened before I was around are very difficult to grasp. The years just don't make sense sometimes. I often wonder, did my father know about this or that? What about my grandmother or great grandmother? What is a generation? A generation is roughly 20-25 years on average. Let's be conservative and say 25 years. Of course, some specific generations will be longer, others shorter, but it's a good round average to work with. So there are on average, 4 generations every century. For the average person in 2012, their grand parents may have seen The Beetles hit American soil in 1964. That was only 2 generations ago.
Women got the right to vote in America, the Nineteenth Amendment in 1920. That was only 4 generations ago.
Lincoln died April 15, 1865...only 6 generations ago.
What about some of those crazy dates in human history?
We have approximately 6000 years of recorded history, since our distant ancestors learned to make marks on tablets. That's only 240 generations. Only 240 begets in recorded history.
Our very distant ancestors migrated out of Africa 125,000 years ago. Only 5,080 generations have come and gone since that early migration. The oldest homo sapiens show up in fossil records around 195,000 years ago, or only 7,880 generations ago.
Still not an easy concept to grasp, but it's humbling to realize we haven't been making footprints on this planet all that long. That puts some of those petty day-to-day things in perspective.