The egg. The chicken is the product of countless genetic mutations in a sequence leading from reptiles all the way to avian. Reptiles had been laying eggs long before avian, though slightly different, had evolved. Over time, Avian has developed several characteristics. Such characteristics led to a proto-chicken who was very similar to, but not quite yet, a chicken. Once upon a time, there was a creature that was almost unbelievably similar to, yet not quite entirely, a chicken. This proto-chicken gave birth to an egg which, after fertilization, had the final mutations in the genetic form required to eventually become a chicken.