Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

The Joke

Two fish are talking underwater. One asks, "Hey, Frank! Wanna evolve to walk on land?" and Frank replies, "Sure!" In the next panel, they have evolved into primitive land animals and are walking on shore, but Frank complains: "Blech. It is dry and smelly up here, and there are spiders."

In the third panel, the two creatures decide to stay on land only long enough to "evolve internal fertilization" -- and then they will go back to the water. Frank says "Perfect." The final panel shows a parent explaining to a child: "And that's where dolphins come from." This presents a humorous (and wildly inaccurate) origin story for dolphins: that they were sea creatures who briefly evolved onto land just to pick up internal fertilization, then went back to the ocean.

The Humor

The comic anthropomorphizes evolution as a conscious, deliberate choice that animals make, as if species can decide to evolve specific traits like picking items off a menu. The idea that creatures would go through the enormous trouble of evolving to live on land -- only to immediately want to go back because land is unpleasant -- is inherently funny. The punchline about dolphins ties it all together with a kernel of real science delivered in the most absurd way possible.

References

There is genuine evolutionary science behind this joke. Dolphins and other cetaceans (whales, porpoises) did indeed evolve from land-dwelling mammals. Their ancestors were terrestrial creatures that returned to the sea roughly 50 million years ago. Internal fertilization -- which is standard in mammals but not in most fish -- is something cetaceans retained from their land-dwelling ancestors. So while the comic's version of events is comedically oversimplified and teleological, the broad strokes (sea to land, evolve mammalian traits, then back to sea) are loosely based on real evolutionary history.