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The frog that re-evolved to reclaim its missing teeth

For as many as 200 million years, frogs lived without lower teeth. But one frog species has managed to “re-evolve,” restoring those missing teeth.

As far back as 230 million years ago, frogs took a new evolutionary leap, moving on without the teeth in their lower jaw. But strangely, within the past 20 million years, one frog species – just one that we know of! – evolved to grow back those missing teeth.

It’s clear evidence, according to John Wiens of Stony Brook University in New York, that complex traits long lost in an animal’s evolutionary past can occasionally make a surprising comeback.

The frog that “re-evolved” to reclaim its lower set of teeth, Gastrotheca guentheri, lives in the jungles of Colombia and Ecuador. It is one of 58 frog species known as “marsupial frogs,” so nicknamed because, like kangaroos, they carry their young in pouches. Female marsupial frogs carry their fertilized eggs in pouches on their backs. In some species, the eggs develop into tadpoles; in others, they hatch out as tiny froglets.

Gastrotheca guentheri. Photo by William E. Duellman, courtesy of the Biodiversity Institute, University of Kansas

There’s a concept in evolutionary biology called “Dollo’s law.” It states, according to Dr. Wiens, that once a complex trait is lost during evolution, it will not re-evolve again. We see it in snakes that descended from reptiles with legs. The earliest turtles and birds had teeth, but lost them as they evolved into their present-day descendants. The tails of our primate ancestors disappeared somewhere along the way as we became modern humans.

But Dollo’s law has recently been fraught with controversy. Scientists have been finding tantalizing signs of exceptions to that rule, and proving an exception to a rule is never easy!

So how did Professor Wiens make his case about the re-evolved lower jaw teeth in Gastrotheca guentheri? He explained to BBC News,

I combined data from fossils and DNA sequences with new statistical methods and showed that frogs lost their teeth on the lower jaw more than 230 million years ago, but that they re-appeared in Gastrotheca guentheri within the past 20 million years. That means that teeth were absent on the lower jaw for more than 200 million years before re-evolving in Gastrotheca guentheri.

The loss of mandibular [lower jaw] teeth in the ancestor of modern frogs and their re-appearance in Gastrotheca guentheri provides very strong evidence for the controversial idea that complex anatomical traits that are evolutionarily lost can re-evolve, even after being absent for hundreds of millions of years.

How was it possible for this one frog species, Gastrotheca guentheri, to re-evolve its lower jaw teeth?

This study also suggests a mechanism for how this re-evolution could happen. Even though teeth were lost on the lower jaw more than 200 million years ago, they are maintained on the upper jaw in most frogs . . . . [This] implies that the mechanisms for developing teeth on the lower jaw were present all along . . . what Gastrotheca guentheri did was to put teeth back on the lower jaw, rather than having to re-evolve all the mechanisms for making teeth “from scratch.”

Gastrotheca guentheri. Photo by William E. Duellman, courtesy of the Biodiversity Institute, University of Kansas

Nature is full of surprises! Around 230 million years ago, frogs lost their lower teeth, evolving toothless lower jaws. Then, out of thousands of frog species in the world, one species, Gastrotheca guentheri, was able to reclaim those missing lower jaw teeth!

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