In the absence of evidence, it can be difficult to fathom that modern birds — from sparrows to hummingbirds — evolved from dinosaurs.

The theory becomes easier to accept once you know that some dinosaurs had feathers, yet evolutionary holes remain. For instance, all modern birds have beaks instead of teeth. How did that happen?

Now for the first time, a team of scientists led by Shuo Wang of Capital Normal University of Beijing have conducted a comprehensive fossil study that paints a clearer picture of how bird beaks evolved from the toothy mouths of dinosaurs, reports Phys.org.

Wang's team found that some dinosaurs tended to lose their front teeth as they got older. To compensate, many of these dinosaurs adapted by developing a small frontal beak in old age. Researchers were able to trace through the fossil record how this process began happening earlier and earlier in a dinosaur's life, until eventually these creatures were simply born with beaks. Once the beaks grew in, teeth became obsolete and eventually disappeared entirely.

Why beaks are better than teeth

There are a number of reasons why beaks are better than teeth. For instance, beaks are made of keratin, a flexible protein that also makes hair, feathers, fingernails and even cow horns. The primary advantage of keratin structures is that they can be replenished. Teeth are bones and can be lost, but keratin regrows quickly.

Interestingly, beak development in birds seems to mimic the pattern seen in the evolution of bird-like dinosaurs. When birds are still in their eggs, beak keratin first forms at the tip of the animal's snout before eventually growing back to cover the rest of the mouth.

Scientists have also discovered that the bone gene BMP4 controls aspects of both beak growth and tooth suppression, and is probably the evolutionary vehicle that powered this transition.

All in all, this research goes to show just how powerful the evolutionary process can be. For dinosaurs to transform into birds, they had to shrink in size, sprout wings, adapt feathers, improve senses, shorten their tails, lose teeth, and of course, grow beaks.

The research can be found in the Proceedings of the National Academy of Sciences.