Researchers believe that snakes evolved from lizards separately perhaps thirty times. Each time, the evolution was aided by a common DNA mutation in the Zone of Polarizing Activity Regulatory Sequence, a regulatory region of the “Sonic Hedgehog Gene” (I kid you not) which is required for limb development.
Since limblessness evolved so often, it’s clear that there are some situations in which there is great value to an animal having no limbs.
What could those situations be?
At this point, you could stop and write down what you think the advantages could be from having no limbs.
Or, if that sounds too much like work, you can proceed with this article and I will tell you.
First, let’s note some common features of snakes apart from an absence of limbs:
- Most snakes swallow their prey whole. This is usually facilitated by their jaw bones being joined by movable cartilage that allows them to open their mouths wider than their body width. The major advantage of this method of ingestion is that the snake doesn’t have to engage in the messy and inefficient practice of picking apart the carcass of its prey, particularly if the carcass is in a cramped environment. The prey is, instead, handled by the snake’s powerful digestive system.
- Snakes do not have outer ears. This much is obvious; what might not be obvious is that shakes are also missing the three inner ear bones that resonate with vibrations from the air. Instead, because the jaw bones move independently of each other, a snake resting his jaw on the ground has stereo hearing that helps to locate its prey. The underside of its body can also detect vibrations from a long way away.
- Snakes have a forked tongue which they constantly flick in and out. This allows them to sample chemicals in the environment and convey those chemicals to a nasal organ in its mouth, providing a directional taste/smell perception. Snakes have no need for open nostrils or mouths to take in smells and tastes; the forked tongue is much more sensitive to direction than nostrils.
- Snakes have transparent eyelids. One would be forgiven for thinking that snakes had no eyelids; in fact, their eyelids just remain permanently closed but allow the snake to see through them. In this way, snakes do not need to close their eyes to protect themselves from the environment.
- Most snakes are covered in scales, including specialized, surface-gripping belly scales that assist in movement. Thus, snakes are much less prone to scratches and punctures from the passing environment, while remaining more stable on the ground.
- Snakes have very elongated bodies, with their organs spread out in a line. This allows the snake to go through a relatively small hole while at the same time be much larger than a normal animal passing through such a hole.
- Snakes can move by undulating their bodies, without having to use legs.
For land snakes, the most common method is called lateral undulation (see https://www.youtube.com/watch?v=e-TmYs1Eq8E), where the body alternatively flexes to the left and right. This creates a backward moving wave, in which the body pushes forward from resistance points on the ground at exactly the same speed as the wave. The snake’s body passes along a standing wave, allowing its body to easily pass even through a body-diameter hole.
Water snakes use the same lateral undulation to swim through the water, but the backward moving wave moves faster than the body moving forward.
Other types of motion are:
- (i) sidewinding, where there are no resistance points to push against, such as on a sand dune, so one side of the snake remains in contact with the “ground”, while the other is lifted up and over, and then the ground side is brought up and over;
- (ii) concertina, in environments where lateral motion is restricted, meaning the forward end is braced against, say, the walls of a burrow, the back portion is brought forward, then the back portion braced, and the front portion pushed forward; and
- (iii) rectilinear where the snake’s body remains straight but waves pass through the belly scales, lifting, pulling forward, and setting down successive sections of the underside of the body.
Okay, now do you want to guess why snakes lost their legs?
In that case, let’s move on to the answer.
The leading evolutionary theory about snakes is that they evolved from burrowing predator lizards. In this environment, all the snake characteristics above were a major plus:
- swallow the prey whole
- no ears to catch the side of the burrow or ear holes to gather dirt
- a transparent scale to protect their eyes from dirt
- scales to protect against the walls of the burrow and specialized belly scales to grab the ground
- a forked smelling/tasting tongue to take air samples safely back in the mostly closed mouth
- an elongated body allowing the combination of small diameter and largeness
and of course:
- a way of wiggling through the burrow
In a burrow environment, limbs would be not just useless, but a positive disadvantage. They would catch on the wall of the burrow; they’d need to lift the body up in a confined space to walk; and, when the snake swallowed a large prey, they would struggle to hold up the increased weight of the elongated body and walk at the same time.
Result: the first time the Sonic The Hedgehog gene mutated, that was it for the limbs.
A second evolutionary theory posits that snakes evolved from swimming lizards. The above-listed snake characteristics would all fit, although now in the context of the underwater environment. For instance, the transparent eyelids are to protect the cornea from osmosis, and an undulating movement is to move through water. Again, the legs would be equally as useless as they would be in a burrow and in some situations a positive disadvantage. For example, the legs would make the undulating body less streamlined, and could catch on the sides of underwater burrows.
Perhaps both evolutionary theories are correct; perhaps some snakes evolved one way and others, the other.
Either way, it’s clear why limbs became vestigial. They no longer cut the mustard.
So, now you know why the snake lost its legs.