Why King Cobra and other snakes don’t have legs

Snakes are one of the most dangerous yet fascinating reptiles. With their stunning patterns, slithering noises and poisonous tongues, these reptiles are one of the most fabled members of the reptile family. For those unaware, snakes and lizards form the highest order of the reptile family of Squamata. Yet, apart from some lizards, these are the only reptiles that don’t have legs. Is this a natural miracle or a genetic condition? Find out here!
Surprisingly, used to have legs. They still carry the genetic machinations required for them to develop legs. Then why do they not possess these limbs? Well, the reason lies in multiple genetic mutations that occurred more than 100 million years ago. These mutations affected a gene named the Sonic Hedgehog gene (SHH) – which is responsible for the development of limbs. It is not only vital for limbs but also provides guidance for the organs and their placement in growing animals. In lizards and other reptiles, this gene is active and thus leads to them having legs and feet. However, in snakes, this gene is deactivated.

The “Phenotype loss is associated with widespread divergence of the gene regulatory landscape in evolution” published by Juliana Gusson Roscito and collaborators in Nature Communications in 2016, showed how changes in the reptile genome resulted in visible transformations in the creatures' phenotype or physical traits that come from expressions of certain genes. To pinpoint the changes in genetic sequences, the researchers compared genomes from snakes with those of other reptiles and vertebrates that do have complete limbs. Lizards have DNA regulatory sequences called enhancers that can flip certain genes on and off during the early years of growth. The Sonic Hedgehog gene requires these enhancers for limbs to fully develop. “In lizards, Sonic Hedgehog stays on and acts as the motor, driving limb development down into the fingers and the toes,” explained University of Florida biologist Martin J. Cohn.
In lizards the completion of this process led to them having four legs, however, in snakes, this enhancer was removed from DNA sequences during evolution. This has led to the gene almost disappearing from some snakes, but not all. Certain snakes have traces of enhancers along with another gene called HOXD, which leads to the development of the feet. Some python embryos develop leg bones and footplates but these traits break down long before the gestating animal is born. In a study by Cohn and Francisca Leal in Current Biology, they discovered that the SHH flickered to “on” in certain stages of python development which is why some of them have tiny claws. They are beginning to develop limbs.
While snakes have adapted perfectly to their limbless existence, their genetic history suggests that evolution is never truly set in stone. The presence of dormant limb-related genes raises intriguing possibilities—could future mutations or scientific intervention reignite limb development in snakes? While this remains speculative, it serves as a fascinating reminder that evolution is an ongoing process, constantly shaping life in unexpected ways.
What does this mean? Well, snakes are just some genetic mutations away from being able to crawl and not slither towards anything under the sun.

Snakes once had legs, but evolution turned off the genes responsible for limb growth. However, traces of these genes remain, meaning snakes could potentially develop legs again with the right genetic changes.