Homer's Evolution couch gag

Homer's Evolution is a couch gag that starts with Homer as a single-celled organism and going through time to become the modern day Homer.

Plot
A single-celled organism, resembling Homer, separates into a lot of cells and eventually turns into a fish-like creature and swims onto land. As it grows legs and walks along, and keeps evolving as he sees dinosaurs, one resembling Bart, and becomes a caveman, while a caveman Moe, devolves into a small animal. Caveman Homer keeps evolving going through Victorian times and eventually becomes the normal Homer and sits on the couch when Marge asks him what took him so long.

Behind the Scenes
There are two versions of this that are no different until the end. When Marge asks Homer what took him so long, in one, he sighs and in another he says "traffic".