The finding is reported in one of eight studies published today in Science by the Avian Phylogenomics Consortium, an international collaboration examining the evolution of living birds.
In total 23 papers are being simultaneously published this morning, revealing new insights into questions such as what makes a bird a bird; how colourful feathers and vocal learning developed; and the evolutionary tree of all avians.
The tooth study answers a question that has long baffled evolutionary biologists: did tooth loss in birds happen convergently across a number of bird lineages or through a common ancestor?
It uses data from whole-genome sequencing of 48 bird species that represent nearly all living bird orders, as well as the American alligator, a representative of Crocodylia (the closest living relatives of birds).
Co-author Professor Mark Springer, of the University of California Riverside, says it has been known that modern birds descended from a toothed ancestor since the discovery of Archaeopteryx in 1861.
Archaeopteryx lived around 150 million years ago and is considered a transitional species between dinosaurs and birds. Although it had wings, it also had jaws with sharp teeth and stood on two legs.
Later fossil finds also revealed animals with partial beaks, but in the back of the jaw they still had teeth.
Springer says this makes evolutionary sense.
"You can't expect that an ancestor would have gone through a stage when all of the teeth would have been lost, but there was no beak," Springer, of the Department of Biology, says.
"That would have made it too hard to do all the things they needed to do like feeding themselves or their young.
"Once you get that partial beak, then you can continue to lose the teeth more posterially until they become completely replaced by the beak."
However, he says the history of final tooth loss in the ancestry of modern birds has remained elusive for more than 150 years.
Mutant tooth genes the key
For the study the team looked for shared mutations in the six genes that are essential for the formation of dentin and enamel, the building blocks of teeth.
"Tooth formation is very complex in terms of the hundreds of genes involved … most of these genes are not only involved in tooth formation they are involved in other developmental processes as well."
The six genes they isolated were the "best candidates for being specific to teeth".
"If teeth are gone then natural selection should not maintain these genes," says Springer.
If the genes were found to be non-functioning due to mutations in all 48 bird species they studied, this would indicate loss through a common ancestor.
"We can then use this record of mutations to estimate when the teeth were in fact lost," he says.
The team, which also includes Professor Thomas Gilbert, a visiting academic at Curtin University in Western Australia, found enamel-related genes were disabled in the common ancestor of modern birds about 116 million years ago.
The researchers also examined the genomes of a number of toothless vertebrates including three turtles and four mammals (pangolin, aardvark, sloth, and armadillo) for these same mutations.
They found these vertebrates also had mutations in the dentin and enamel genes, making them non-functional.
By comparison all six genes were functional in the American alligator.
"Dead genes" are an important tool, says Springer. They are "chock full of information, it's like finding a fossil in the rocks".
"They are still in our genome, but full of these inactivating mutations so they don't function anymore.
"This DNA from the crypt is a powerful tool for unlocking secrets of evolutionary history."
Read more at Discovery News
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