Photograph courtesy James Brown
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Tetrapodophis catching a lizard.
Photograph courtesy James Brown

Why Snakes Don't Have Legs (For Now)

Snakes used to have legs. Now they have evolved, but the gene to grow limbs still exists.

Imagine a snake that has legs but can still slither. That’s how snakes used to be, and there’s evidence that legs have reemerged in some snakes. Why?

Trying to discover whether snakes evolved to slither and reevolved to produce legs, or whether there’s another explanation for what appears to be double evolution, two scientists recently discovered the “Sonic hedgehog” gene in pythons.

Sonic hedgehog is a gene that is necessary for limb development, one of several genes named after the hedgehog-like spines that grow from embryos when limb development begins.

The scientist who helped carry out the snake study shows his findings in compelling imagery.

“It’s one of the strangest body plans in vertebrates,” University of Florida biologist Martin J. Cohn said when describing the findings.

An enhancer controls the activation of the Sonic hedgehog gene, causing it to flicker on and off. In snakes, a mutation in the enhancer keeps the gene from staying in the “on” position, allowing for limbs to grow.

“In lizards, Sonic hedgehog stays on and acts as the motor, driving limb development all the way down into the fingers and the toes,” explained Cohn.

In the recent study Cohn and Francesca Leal published in Current Biology, researchers discovered Sonic hedgehog flickered to “on” at certain stages of python development. That’s why pythons sometimes have small claws protruding from their skin: They are beginning to develop limbs.

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A four legged snake eats a mouse

What that means is it’s possible for the pythons of today to grow legs—and it doesn’t require an entire evolutionary cycle for them to do so. They just need the right genetic mutation to help along the limbs they already have.

Cohn sees future implications for evolutionary study.

“I think it could provide a foundation for doing further comparative studies to look at how organs and structures are lost over evolutionary time.”

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