It's time to get an answer to the age-old question: Why don't birds have teeth? New research reveals it may have something to do with their genes. This is the closest answer the researchers could come up with. 

The researchers, from the University of California, looked at the birds tooth gene and tried to figure out when birds developed "edentulism," an absence of teeth. Live Science reported that the ancient birds have been fragmented, meaning it's difficult to find clues in determining the edentulism in over 150 years. But, it is the modern birds that hold the key in understanding what happened.

Today's birds have curved beaks and a well-functioning digestive system that helps them to grind and process food. Back in 1861, the finding of the fossil bird Archaeopteryx in Germany suggested birds descended from toothed reptiles; incidentally scientists are aware now birds did indeed come from theropod dinosaurs. Archaeopteryx is known as the "first bird."

"DNA from the crypt [the fossilized bird] is a powerful tool for unlocking secrets of evolutionary history," Mark Springer said in a statement. Springer is a professor of biology at the University of California, Riverside, and one of the study's lead researchers.

Today's modern birds might be born with the tooth gene turned off. According to the researchers, all birds have a gene or mutation that deactivates the formation of teeth, which can be traced back to the common ancestors of the modern birds, the Audubon Magazine reported.

The researchers investigated the gene that governs tooth production. In vertebrates, tooth formation involves six genes, which are crucial for the formation of enamel (the hard tissue that coats teeth) and dentin (the calcified material underneath the enamel). The researchers looked for mutations that would have deactivated these six genes in the genomes of 48 bird species.

"The closest living modern reptile relative of birds is the alligator," Springer said. "All six genes are functional in the American alligator."

A mutation in the dentin and enamel-related genes was found, and it was shared among bird species. This indicated their ancestor did indeed lose the ability to form teeth over time. And, the researchers also found all bird species have the same dentin and enamel mutations.

"The presence of several inactivating mutations that are shared by all 48 bird species suggests that the outer enamel covering of teeth was lost around 116 million years ago," Springer said.

These mutations that cause the edentulism can be overcome. In 2006, scientists at the University of Manchester and the University of Wisconsin were able to manipulate a chicken's genes so that it could grow teeth, and it worked.

While researchers give a reason for the tooth loss in birds, it has left a number of questions unanswered, such as, why do birds have beaks?  Researchers try to explain that the beak is essential for the tearing of food, but they do not say why the birds have beaks.

Stephen Brusatte, a paleontologist at the University of Edinburgh, who has studied and written extensively on the overlap between dinosaurs and birds, said scientists used to believe the reason birds lost their teeth was they wanted to lighten their skeletal load so they could fly. Brusatte is skeptical now even more with the new research.

"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," Brusatte said. "Why would an entire major group of animals lose their teeth? It's been a really open question."

Brusatte said flying mammals, such as bats, have the ability to fly without losing their teeth for a beak. Beaks, Brusatte said, are used for eating all sorts of things.  

"Maybe beaks are even more versatile than teeth in some ways," Brusatte said.