Study of Chicken Teeth Sheds Light on Evolution

A new study shows that chickens, which don't have teeth, still have the genes that make them, and in special cases, those genes can be switched back on. Scientists now think that as animals evolve, they lose the ability to turn those genes on at the right time during development — not the genes themselves.

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It is a well-known fact of science that chickens don't have teeth. So well known that it's been incorporated into everyday speech, rare as hen's teeth, the saying goes. But facts have a way of shifting in science. Researchers at the University of Wisconsin have found a mutant strain of chicken that does appear to have teeth. And as NPR's Joe Palca reports, the finding sheds light on how evolution works.

JOE PALCA, reporting:

The notion that chickens can't make teeth has been around a good long while. But in 1821 a French zoologist named Geoffroy Saint-Hilaire tried challenging it. In an article entitled (French Spoken), On the Teeth of Birds, he described sharp brown structures beneath the beaks of parrots he was studying. Saint-Hilaire said these structures were nascent teeth. C'est impossible, asserted his contemporary Georges Leopold Chretien Frederic Dagobert, Baron Cuvier--everyone knows birds can't make teeth. Cuvier said the sharp brown structures might be sprouting feathers sprouting in the wrong place. But teeth? No. And so the dogma endured, until recently.

Enter biologist John Fallon of the University of Wisconsin and his graduate student Matthew Harris. Harris was working in the lab late one night when he saw strange structures beneath the beak of a chick embryo he was studying. Fallon says Harris showed him the structures the next day.

Mr. JOHN FALLON (Biologist, University of Wisconsin): They looked like teeth.

PALCA: And to Fallon they didn't look just like any teeth. As he reports in the latest issue of the journal Current Biology, they looked like alligator teeth. Alligator baby teeth, mind you, not the big choppers they get as adults, but alligator teeth all the same. Fallon says that may sound peculiar, but it really isn't that strange because birds descend from a class of reptiles, reptiles with teeth, known as Arcosaurs.

Mr. FALLON: The closest living relative of birds in that lineage are the crocodilians, and alligator being one.

PALCA: Now, you may be saying to yourself, wait a minute, how come no one spotted these teeth before? Good question. The reason is Fallon was working with a strain of chickens called Talpid 2 mutants. These mutants grow abnormally and don't live long enough to hatch, but they do help scientists understand the causes of birth defects. Fallon says something about the Talpid 2 mutation allows a feature lost 60 million years ago, teeth in this case, to pop out again.

And it turns out that hens' teeth aren't the only lost features that can pop back up among animals. Michael Levine is a biologist at the University of California Berkeley. He says under the right circumstances snakes can grow the legs their ancestors once had. And cavefish can grow the eyes their ancestors once had. It's as if the animals have a genetic memory of their ancient past, and Levine says in a way they do.

Mr. MICHAEL LEVINE (Biologist, University of California, Berkeley): The genes for teeth are still in the bird genomes. The genes for limbs are still in the snake genomes. And the genes for eyes are still in the cavefish genomes. So what do we make of this? The genes have not been lost, they've been retained despite the loss of the structures and the reason is simple. The genes are used for other purposes.

PALCA: In the case of birds, the same genes that once made teeth are now used to make feathers. And Levine says this realization that genes are retained has helped scientists understand how evolution works. Levine says nearly all animals have the same set of genes.

Mr. LEVINE: What's different about them is how they use these genes. Where and when these genes are turned on and off is radically different from animal to animal.

PALCA: And it's the intricate dance of genes interacting with each other that changes during the course of evolution. And there's one funny coincidence about the chicken teeth story. Remember that guy who said birds couldn't make teeth? Georges Leopold Chretien Frederic Dagobert, Baron Cuvier? Well, it turns out that he trained a scientist, who trained a scientist, who trained a scientist, who trained a scientist, who trained a scientist, who trained John Fallon, the man who showed that Baron Cuvier was wrong about chicken teeth. An example of evolution in science, perhaps. Joe Palca, NPR News, Washington.

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