Homer's Evolution is the tweith couch gag of season 18 and the second longest couch gag ever used on the show. (The first being the one from "Homerazzi"). It starts with Homer as a single-celled organism and going through time to become the modern day Homer.
A single-celled organism, resembling Homer, separates into a lot of cells (all of them yelling "D'oh!") and eventually turns into a jellyfish and then into a fish-like creature and swims onto land, avoiding an octopus Mr. Burns who tries to catch him. It grows legs and turns into an amphibian that eats a dragonfly-like Lenny. He walks along and keeps evolving from a dimetrodon to a primitive mammal as he sees dinosaurs and a pteranodon resembling Agnes Skinner; a tyrannosaurus rex resembling Bart tries to eat him, and a stegosaurus resembling Lisa prevents Bart from eating Homer by swiping at him with her tail. After surviving the meteor that wiped out the dinosaurs, Homer continues jumping from tree to tree to become an ape . He becomes a caveman walking in a snow storm, while another caveman Moe, devolves into a small, shrew-like animal as he walks the other way. Caveman Homer then evolves into conquistador, then a pilgrim, goes through Victorian times and eventually becomes the modern Homer. He gets to his house and sits on the couch, when Marge asks him what took him so long. Homer then sighs in exhaustion.
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. In another, instead of Marge asking him what took him so long, she asks him "Did you bring the milk?".