How snakes lost their limbs

The body evolution of snakes has been among the most dramatic in animal history, morphing from two-legged burrowing animals to the slithery, legless carnivores we know and love (well, some of us).

It’s been 100 million years since snakes lost their limbs. More primitive snake species, such as boas and pythons, still have hind legs in the form of tiny claws that retract into their muscles.

In advanced species, such as cobras, limbs have completely disappeared.

Now, researchers writing in the journal Current Biology claim a string of DNA involved in coding limb-growth is mutated in today’s snakes, which goes some way to explaining their lack of appendages.

“This is one of many components of the DNA instructions needed for making limbs in humans and, essentially, all other legged vertebrates. In snakes, it’s broken,” explains Axel Visel, a geneticist at the Lawrence Berkeley National Laboratory in the US and lead author on the paper.

“It’s probably one of several evolutionary steps that occurred in snakes, which, unlike most mammals and reptiles, can no longer form limbs.”

The fossil record is sparse for snakes and their ancestors, partially because their skeletons are delicate, making them less likely to remain intact.

Instead, Visel and his team analysed the recorded genomes of six modern-day snake species, which vary in morphology, marking important points in the evolutionary journey of snakes.

The team homed in on a stretch of DNA called the Zone of Polarising Activity Regulatory Sequence, or ZRS, in the Sonic hedgehog gene.

They collected the ZRS sequence from a bunch of animals – including advanced snake species – and used genome editing to replace that strand in mice.

Mice with ZRS lifted from cobras developed severely shortened, malformed limbs. Those with ZRS from other animals, including humans and even fish, developed limbs as normal.

“Using these new genomic tools, we can begin to explore how different evolutionary versions of the same enhancer affect limb development and actually see what happens,” explains Visel.

“We used to be mostly staring at sequences and speculating about molecular evolution, but now, we can really take these studies to the next level.”

By comparing ZRS in each different snake species, the researchers were then able to create a new iteration of python ZRS in which the missing strand had been replaced. This new hybrid strand resulted in mice developing limbs as per usual.

Despite essentially de-evolving the most advanced snake species, Visel emphasises that the evolutionary path from legged to legless isn’t as straightforward as one DNA strand.

“Loss of limbs has occurred multiple times independently during animal evolution, and it’s safe to assume that mutations affecting other genes were involved.”

But he’s confident that the finding brings experts one step closer to understanding the mysteries of body morphology.

“It’s a complex problem, but with the introduction of genome-editing tools, we can finally start tying specific DNA changes to alterations in body shape more systematically.”