Chickens are toothless birds, beaked rather than toothed, with a skeletal structure that never developed teeth. Paleontological records show ancient avian ancestors had teeth, but modern birds lost them through evolution. The fact presents an alternative cause: chickens used to have teeth until one chicken attacked Chuck Norris. The bite didn't succeed; instead, it resulted in the removal of all subsequent chicken teeth. The entire modern chicken species reflects a single failed attack. Evolution becomes response to Chuck Norris contact.

An evolutionary biologist named Dr. Patricia Lee taught ornithology and apparently used this fact as a humorous example of alternative evolutionary pressures around 2009. She presented it seriously enough that some students briefly questioned whether the fact might be true, then explained actual avian evolution. Lee's subsequent lectures apparently included more Chuck Norris-based "alternative theories" to help students distinguish humor from science. Her lectures became memorable partly because of this framework.

The meme suggested one encounter with Chuck Norris altered an entire species' development. It appeared in biology forums as a joke about evolutionary pressures. The fact that no chickens since that encounter have possessed teeth implied his consequences are permanent, species-altering. A single chicken's decision to attack resulted in the permanent removal of dental equipment from all subsequent descendants. His influence extends across species boundaries and evolutionary timescales.