Fossil records are too spotty for scientists to pinpoint when the shift occurred. Until now, so was genetic information.
But because of the newly reported sequencing of 48 different bird genomes (representing at least one species from each major lineage), we can now trace beaks back to their origin.
Led by Mark Springer, a professor of biology at University of California at Riverside, researchers used this new data to track down the genetic changes that indicate a switch from teeth to beaks.
Tooth formation requires a lot of genes, with six genes absolutely essential to the formation of dentin and enamel -- the building blocks of teeth. Springer and his colleagues went looking for mutations that would inactivate these genes, tracking them throughout the 48 bird genomes that had been sequenced.
"Such mutations would provide evidence that teeth were lost once in the common ancestor of all living birds," Springer said.
They found those mutations in every single genome. Based on the mutations they found, Springer and his colleagues believe that a common ancestor of all birds lost the ability to produce enamel-covered teeth around 116 million years ago.
The shift from teeth to beaks didn't happen overnight: Fossil evidence suggests that some birds had beaks at the front of their mouths and teeth in the back, with beaks slowly taking over the back of the mouth as well.
The full beak was an advantageous choice. Birds developed muscular gizzards for grinding up their food, and adapted to use their beaks to build nests, feed their young and defend themselves. Having a set of teeth in the back just wasn't useful anymore.
"It is very satisfying to finally have answers to questions that have not been crystal clear from the fossil record," Springer said. "Our results highlight the power of DNA from the crypt to unravel the evolutionary history of vertebrates. Together, genomic fossils and fossils from rocks provide a powerful combination for reconstructing the past."