Birds are related to reptiles and their ancestry can be traced all the way back to the dinosaurs 150+ million years ago. In the fossil record we find transitional species with feathers, teeth, plus older reptilian features..
We can look back genetically and find the “instruction manual” that the previous bird ancestors used. We glanced into the genetic past of birds and we found that they retain the instruction manual their ancestors used to make teeth. So why don’t we see birds with teeth outside the window if they have the genes?
30 years ago: Scientists took a layer of cells from the mouth of a mouse embryo and placed it in the beak region of a chick embryo and the cells combined, causing the chick to grow rudimentary teeth in its beak! The bird embryo had the “instruction manual” for teeth already and just needed the “go” signal, which they received from the mouse cells. The mouse and chick cells combined and they used chemical “language” to talk to each other. The genes for the “go” signal chemically turned on the genes for instructions for teeth, and together they developed teeth on the chick’s beak!
2006: We discovered a small mutation in the DNA of a chick embryo that affected its development and caused it to develop rudimentary teeth. This mutation is important because it changed the arrangement of tissue layers in the beak. In modern birds their beak tissues develop too far apart to exchange their “chemical language” and thus, never talk about making teeth. This small mutation caused the layers of tissue to move closer and they began communicating again. We then looked at these rudimentary teeth in the chick’s beak to look at how they were developing and we then saw that the teeth were being built just like reptiles and not mammals.
Why still “rudimentary” teeth?
If you visualize genes lined up along chromosomes like close neighbors living next door in a neighborhood, we find that in birds the houses/genes for making enamel are in the same “neighborhoods” in reptiles and mammals. We looked for the houses/enamel making genes in the chick DNA and the genes are still in their “neighborhood” but they are but badly mutated...
Visualize visiting an old neighborhood where several houses are broken down and have been abandoned for 80 million years! Who cares if you throw a brick through a window of an broken down abandoned house? Well natural selection doesn't care what you do with abandoned genes either! If there is a mutation in a gene that is not needed, it will be passed on because it doesn't harm anything. Reproduction will continue to pass on old genes as they accumulate mutation, like an old house falling apart through time. These old genes will be useless just like the old house unless it can become renewed…which for a house could be a new tenant and a paint job, but for genes its mutation. Mutation will continue in the genes and sometimes (quite often actually) old useless genes gain a new function because the right mutation.
This is the story of birds and their teeth. Birds have broken down genes for their “go” signal that “talks” to the instructional genes for teeth. This allowed mutations in the neighborhood of enamel making genes to accumulate unnoticed and eventually cause the weak “rudimentary teeth” in the chick embryo. Perhaps these old one day will mutate and become the signal for something brand new.
So if birds did not evolve from the reptilian-like ancestors we find in the fossil record, why then do birds have the ancient instruction for their teeth?