Do Snakes Actually Benefit From No Legs?

No, modern snakes don't have legs, but their ancestors did. Snakes started losing their legs over 100 million years ago after mutations broke an enhancer that controls the Sonic hedgehog limb-building gene. Pythons and boas still keep tiny leg nubs, and python embryos briefly grow leg buds before development stalls.

We all feel for snakes. As cool as they are, with their venom-filled fangs or their rib-crushingly powerful constriction, most of us still can’t get over the fact that they have no legs. Yet, for an animal that has colonized every continent aside from Antarctica, snakes might be grateful for their leg-less existence.

Snakes are some of the most common animals in the world. From the tropics to the temperates, they do pretty well for themselves in most parts of the globe. Given their impressive range, they’re obviously highly successful in most habitats. Be it water, on the ground, deep within the earth, or even high up in the canopies, snakes are everywhere! So how did they achieve this magnificent expansion of range and habitat?

Have Snakes Always Been Legless?

Ancient snakes did have legs. The oldest known snake fossils, including Eophis underwoodi from Middle Jurassic England, push the snake lineage back to roughly 167 million years ago, and several of those early forms still bore small limbs. Today, by contrast, most snakes have no legs at all (think cobras and vipers), while pythons and boas only have tiny, redundant vestiges, the little black "spurs" tucked near the base of the tail.

So, what happened between then and today?

Researchers at the University of Florida zeroed in on Sonic hedgehog (Shh), the gene famously named after the Sega character back in 1993, which controls limb development in all four-limbed vertebrates. In a 2016 paper in Current Biology, Francisca Leal and Martin Cohn showed that in snakes, the genetic switch that turns Shh on in the limb bud is broken in some very specific ways.

All limbed vertebrates carry Sonic hedgehog, but in snakes, the version of the limb enhancer that powers it is largely silent. Stranger still, in pythons it briefly flickers on during early embryonic development, just long enough to start a leg, but then shuts down before that leg can finish forming.

What Does This Mean?

We can draw quite a few conclusions from this strange on-and-off behavior of the Shh gene.

The main one is that the brief flash of Shh activity in python and boa embryos is what produces their vestigial leg buds. The gene fires, a tiny femur and the cellular outline of an entire leg skeleton get sketched out, and then the signal cuts off long before any of that turns into a working limb.

The pattern also points to two evolutionary paths taken by snakes. "Ancient" snakes like boas and pythons still show this brief activation of Shh in the embryo, whereas more "recent" snakes like cobras and vipers don't activate it in the limb bud at all.

In both "ancient" and "recent" snakes, the reduced or non-existent activity of Shh in the limb bud comes down to mutations in a stretch of DNA called the ZRS (a limb-specific enhancer that tells Shh when and where to fire). Leal and Cohn found three deletions in the python ZRS that knock out key transcription-factor binding sites; in more advanced snakes like cobras and vipers, the ZRS has decayed so far that it's barely detectable at all.

Does this mean that "ancient" snakes have an edge over more "recent" ones?

Not at all. The legs of a python are about as useful as a wheel on a ship. They're just small black nubs with claws (often called spurs). Males do put them to one use during mating, gently rubbing the female with them as part of courtship, but beyond that, the spurs mostly just offer a window into the evolutionary history and genetic plumbing that define snakes.

Did “Ancient” Snakes Evolve Before “Recent” Snakes?

"Ancient" here just refers to a branch of the snake family tree, not a snake's literal age. An "ancient" lineage isn't necessarily older in geological terms than a "recent" one. Take, for example, two fossil snakes from the Late Cretaceous, Dinilysia patagonica and Najash rionegrina. Both lived in what is now Patagonia roughly 90 to 95 million years ago. Dinilysia patagonica was a terrestrial snake that lacked external limbs, while Najash rionegrina still carried well-developed hind legs and a pelvic girdle (small, but bony and articulated, rather than the fully load-bearing legs of a lizard). New three-dimensional skulls of Najash described in a 2019 Science Advances paper showed that early snakes hung on to those hind limbs for tens of millions of years, so leg loss wasn't a one-step event so much as a long, drawn-out fade.

Based on other fossils of extinct snakes from that time, what we can confidently say is that these leg-bearing snakes didn't actually use their hind legs for any meaningful locomotion (no walking, no swimming, no climbing). So, as early as the Late Cretaceous, snakes were already well into the process of trading a legged body plan for a leg-less one. A 2021 reanalysis even reclassified Tetrapodophis amplectus, a celebrated "four-legged snake" from 2015, as a long-bodied marine lizard (a dolichosaur) rather than a snake at all, which trimmed back the gallery of confirmed legged snakes but didn't change the broader story.

Why Did Snakes Go Leg-less?

Snakes are evolutionary rogues. For a long time, the leading hypothesis was that snakes lost their legs after their ancestors switched to a burrowing, underground lifestyle. Living in tight spaces, limbs are a liability. Think of it this way: how much easier would it be to fit into a tiny nook, crevice, or hole without four radiating structures (legs and arms) poking out from the sides of your body?

That subterranean story is still on the table, but it now has competition. The 2019 Najash work argued that the ancestors of modern snakes weren't tiny, blind burrowers at all, but larger, big-mouthed surface-dwellers whose hind legs were already shrinking. Other researchers have proposed an aquatic origin instead. What everyone agrees on is that once snakes had the long, tapered, leg-less body plan, they ran with it. The shape complemented other modes of locomotion, like swimming, and even let them use their belly scales to grip and climb vertical surfaces.

In an article published on the Florida State Parks website, Keith Morin, a park biologist at the Florida Department of Environmental Protection, said it best:

“From the way they move, to the places they can go and some of the methods of subduing prey, like constriction, having legs would simply get in the way. Over millions of years they gradually lost legs, and they’ve even lost shoulders and hips. Evidence of older species with limbs can be found in the fossil record and in the boas (Boidae), a more primitive family that still has remnants of limbs.” 

So yes, snakes do benefit from a lack of legs. However, as with most things in evolution, the answer isn’t always a clear one-liner. A legless form isn’t the only reason or evolutionary advantage that snakes picked up along the way; other morphological changes like tapering and elongation of their bodies also helped. After all, remember that snakes are true evolutionary rogues!