Time
Here is a really decent attempt to give the proper impression for the amount of time that has taken place since the earliest found prokaryotes appeared in the fossil record. The website is 135 feet wide. Make sure you scroll down first to get the whole dark area into it as that is where the events are. They said it works best with Firefox but I don't have it. I was still able to get it though. Needless to say pretty cool. Hit the scroll to the right when you have time and it gives you a good impression for how much time passed. The cambrium explosion is in there too, all 70 million years of it.