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A bird’s beak is a remarkable thing. It can be dull or spectacularly colored, pointed or blunt, long or squat, thin or comically large. It can be used to tear flesh, probe flowers, or crack the toughest nuts. Birds aren’t the only animals with beaks, but they’re the only major group of animals in which a beak is the exclusive option. No modern birds have teeth. But why?

A new study, appearing in the current issue of Science, examines the evolution of the avian beak by going all the way back to modern birds’ ancestors: dinosaurs. All birds have a gene that deactivates the formation of teeth (yep, birds can grow teeth, we’ll get to that in a minute). The researchers, from the University of California, Riverside, found that this gene can be traced back to a common ancestor of all modern birds, which lived some 100 million years ago. 

To solve this puzzle, the researchers used a that catalogues the genetic history of nearly all living bird orders—48 species in total. They were looking for two specific types of genes: one responsible for dentin, the substance that (mostly) makes up teeth, and another for the enamel that protects them. Upon finding these genes, researchers then located the mutations that deactivate them, and combed the fossil record to figure out when those mutations developed. 

They concluded that the loss of teeth and the development of the beak was a two-stage process, though the steps basically happened simultaneously. The paper states: “In the first stage, tooth loss and partial beak development began on the anterior portion of both the upper and lower jaws. The second stage involved concurrent progression of tooth loss and beak development from the anterior portion of both jaws to the back of the rostrum.” (Rostrum is another word for beak, and anterior basically means toward the front.) 

The development of the bird’s beak and the loss of the bird’s teeth appear, say the researchers, to have taken place at around the same time; there are early birds in the fossil record, like Ichthyornis, that have a partial beak in the front of the mouth and teeth in the back, an in-between development. Mark Springer of the University of California, Riverside says the researchers weren’t able to pinpoint the loss of teeth, but that the presence of certain mutations “indicate that dentin (and teeth) were lost no later than ~101 million years ago.” The loss of the enamel, probably the first step in the process of eliminating teeth, can be more precisely dated to around 116 million years ago.

The new research goes a long way in describing how the birds swapped teeth for beaks, but why is still a mystery. Interestingly, the mutations responsible for the miraculous transformation can be overcome. Back in 2006, scientists at the University of Manchester and the University of Wisconsin so that it actually grew teeth. 

“People used to think that birds lost their teeth in order to lighten their skeleton so they could fly better,” explained Stephen Brusatte, a paleontologist currently at the University of Edinburgh who has studied and written extensively on the overlap between dinosaurs and birds.. “Yeah, maybe, but definitely the loss of teeth did not coincide with the evolution of flight, because there were a lot of birds that could fly which had teeth.” He’s speaking specifically about , widely considered the “first bird.”. Archaeopteryx flew, and sported plumes and chompers. “Why would an entire major group of animals lose their teeth?” Brusatte says. “It’s been a really open question.” The answer, in other words, isn’t known. 

But Brusatte is firm that the idea that birds lost their teeth to save weight only makes sense from a narrative point of view, not a scientific one. Flying mammals like bats have the ability to fly without forgoing teeth for a beak.  Beaks aren’t really a sacrifice, after all. “Beaks can be used to eat all kinds of different things,” he says. “Maybe beaks are even more versatile than teeth in some ways.”

Brusatte points out that beaks are excellent for cutting, especially (but not exclusively) for plant matter; there are many non-avian animals that sport beaks for that very purpose. There’s the parrotfish, which boasts a beak to chip away at hard coral on reefs; the , a scaled mammal which lacks teeth and crushes the insects it eats on its palate and in its belly; and turtles, which vary almost as much as birds and can use their differently shaped beaks to eat plant matter or other animals. 

We still don’t know exactly why birds lost their teeth. But that very uncertainty is what keeps Brusatte going. “When there's big questions out there, that's what makes science pretty exciting,” he says.