A researcher by the name of Matthew Harris noticed bumps on the beak of a mutant chicken embryo. The bumps resembled the baby teeth of an alligator, the animal we now recognize as the closest living relative to birds.
That fact that chickens are still capable of growing teeth today reveals that the genetic blueprint for growing teeth never disappeared. Rather, it has been lying dormant all this time.
Many organisms possess genes or traits that they do not express. Breeders have long known that genes that affect the amount and quality of cow's milk are carried by bulls, even though they're not expressed. Breeders select these genes by observing which bulls' sisters produce the best milk. These are the bulls that get to mate.
Similarly, although adult blind cave fish lack eyes, they carry the genetic blueprint for creating eyes, the same as their sighted relatives do.
What put this mutant chicken embryo's tooth-growing mechanism into action again? Scientists aren't sure exactly, but the mutation that creates teeth in chickens is caused by a recessive trait called talpid. It seems that the molecular signals of talpid bring together two embryonic mouth tissues, the two from which teeth develop in mammals. These two tissues are normally separate in chickens.
Scientists have been able to replicate this phenomenon in normal chickens by creating a virus that mimics talpid's molecular signals. The results are teeth that develop for a short period of time and then get reabsorbed into the beak.
Like fossils, primitive genes that linger around today reveal the genetic ties between different organisms. They're further evidence of evolution at play.