Sixty-five million years ago, an asteroid is believed to have crashed into Earth. The impact wiped out huge numbers of species, including almost all of the dinosaurs.
One group of dinosaurs managed to survive the disaster. Today, we know them as birds.
The idea that birds evolved from dinosaurs has been around since the 19th century, when scientists discovered the fossil of an early bird called Archaeopteryx. It had wings and feathers, but it also looked a lot like a dinosaur. More recent fossils look similar.
But these early birds didn't look the same as modern ones. In particular, they didn't have beaks: they had snouts, like those of their dinosaur ancestors.
To understand how one changed into another, a team has been tampering with the molecular processes that make up a beak in chickens.
By doing so, they have managed to create a chicken embryo with a dinosaur-like snout and palate, similar to that of small feathered dinosaurs like Velociraptor. The results are published in the journal Evolution.
The team's aim was to understand how the bird beak evolved, because the beak is such a vital part of bird anatomy. It has been crucial for their success. The 10,000 or more bird species occupy a wide range of habitats, and many have specialised beaks to help them survive.
But they did not set out to create a "dino-chicken", say lead authors Bhart-Anjan Bhullar of Yale University in New Haven and Arkhat Abzhanov of Harvard University in Cambridge, US.
"Whenever you examine an important evolutionary transformation, you want to learn the underlying mechanism," says Bhullar.
The beak is also the part of the avian skeleton that has "diversified most extensively and most radically", says Bhullar.
Despite this diversity – ranging from flamingos to pelicans - very little work has been done to figure out "what the heck a beak actually is", he adds.
"I wanted to know what the beak was skeletally, functionally and when this major transformation occurred from a normal vertebrate snout to the very unique structures used in birds."
To begin to understand this, the team trawled though changes in the ways genes are expressed in the embryos of chickens and several other animals. They looked at the embryos of mice, emus, alligators, lizards and turtles, representing many of the major animal groups.
They found that birds have a unique cluster of genes related to facial development, which the non-beaked creatures lacked.
When they silenced these genes, the beak structure reverted back to its ancestral state. So too did the palatal bone in the roof of the mouth.
To make this genetic tweak, Bhullar and his colleagues isolated the proteins that would have gone on to develop beaks. Then they suppressed them using tiny beads coated with an inhibiting substance.
When their skeletons started to develop inside the eggs, these animals had short, rounded bones instead of elongated, fused beaks that bird skeletons have.
"By affecting this early protein you are actually altering gene expression," added Bhullar.
The work highlights that beaks develop very differently from snouts, using a different set of genes, says Michael Benton of Bristol University in the UK. "That's what proves the beak is a real adaptation or 'thing', not just a slightly different nose shape."
The shift from snouts to beaks happened well into the evolution of birds, 40-50 million years after Archaeopteryx, says Benton.
For now Bhullar has no plans, or ethical approval, to hatch the snouted chickens. But he believes they would have been able to survive "just fine".
"These weren't drastic modifications," says Bhullar. "They are far less weird than many breeds of chicken developed by chicken hobbyists and breeders."
"The rest of the animal looked OK, but one needs to think about this carefully from an ethical point of view."
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