Why did snakes evolve their legs, and is it really more advantageous to walk on their stomachs?

Snakes are one of the coldest killers in nature. They are natural hunting machines. They use spitting letters to analyze chemicals in the air, and they use the ground to sense the vibrations brought by prey and predators. Some snakes even have infrared sensing capabilities. The body temperature of the prey can also be sensed in the absence of light.

Not surprisingly, these advanced "equipments" allow snakes to adapt to a variety of habitats, from deserts to oceans, from tropical rain forests to alpine grasslands, and snakes are found in most corners of the earth.

Adapting to these different environments means snakes need to be able to dig sand, swim, climb trees, and sometimes even glide through the air.

What's interesting is that while snakes do all these vastly different jobs, they all share strikingly similar body structures, most notably being slender and limbless.

So the question is, why did snakes evolve their legs, and is it really more advantageous to walk with their stomachs?

snakes used to have legs

It is now generally believed that the earliest four-legged vertebrates began to appear 397 million years ago, and the limb structure likely came from the fins of fish.

Birds, mammals, amphibians and reptiles all belong to the category of quadrupeds, and snakes are classified as reptiles, so it should have legs.

In fact, there are many studies (including the fossil record) that can now prove that snakes did have legs, but at some stage they gave up all limbs.

Snake bones are very light, so few well-preserved fossils are found. The oldest animal fossils that can be called snakes date back to between 143 and 167 million years ago.

But true serpents — fossilized snakes with slender bodies and no legs that appeared between 113 million and 94 million years ago — sometimes have regressed hindlimbs.

For example, a 95-million-year-old snake fossil found in 2011 had a pair of 2-centimeter-long hind limbs, but no forelimbs.

As for why snakes should slowly and completely delete their legs, no one can actually answer this question, because the fossils of snakes are too limited.

But to be sure, the removal of all limbs is very common in reptile evolution, and in the order Squamata, snake-like body structures have evolved independently at least 26 times.

Two Hypotheses for Snakes Losing Legs

From all these reptiles that have completely deleted their legs, scientists have now come up with two hypotheses to explain why snakes lost their legs during evolution.

The first hypothesis is that the four-legged ancestors of snakes entered the ocean at some point, and the limbs would bring resistance to swimming, so they slowly evolved the limbs.

Over time, the snakes, which had completely lost their limbs, returned to land, but they have no plans to add limbs.

Before 2015, most of the ancient snake fossils found lived in the ocean, which provides some evidence to support this hypothesis.

However, in 2015, the discovery of the fossil of a snake that apparently did not live in the ocean, but was also losing limbs, provided some evidence for another hypothesis.

The second hypothesis is that the four-legged ancestors of snakes burrowed on land, just like today's earthworms. In this way of life, the legs are also an obstacle.

So, as they adapted to living underground, their legs gradually shortened and lost—first the forelimbs, then the hindlimbs.

Both hypotheses have a large number of supporters, and it cannot be ruled out that both are correct. Different ancestors of snakes may have evolved almost identical physiological structures in different environments in response to similar environmental stress.

This is also very common in nature, and it is called convergent evolution.

Personally, I'm leaning towards the second hypothesis, because I think it's hard to understand losing legs after entering the ocean, why doesn't it turn into tails and "fins" like cetaceans do now?

And to return from the ocean to land I find it more difficult to understand, what pressure or temptation can make them give up the whole ocean and compete for a small beach?

In fact, the second hypothesis also has many loopholes, the most obvious of which is that I should use my limbs as a "shovel" to dig holes better, why should I discard it?

The explanation given by some paleontologists is that the four-legged ancestors of snakes may have rarely burrowed their own burrows at first, but occupied other people's burrows.

Well, let's just say it's very difficult to figure out why snakes don't have legs, and that requires more fossil evidence.

While there is much debate about the evolutionary impetus for snakes to lose limbs, the good news is that scientists have now figured out why snakes lack legs at the genetic level.

Finally: Genetic Tuning in Snakes

In fact, the fact that snakes once had legs is not only reflected in the fossil evidence. Scientists have discovered that snakes and lizards have similar genomes a long time ago, and discovered some genes that may be involved in the development of reptile limbs, and snakes also have these genes.

In addition, some snakes still have degenerate "hind legs", such as pythons, which have leg remnants on either side of their cloaca.

As technology has advanced, scientists have identified genes known as sonic hedgehog (SHH) that play an important role in the formation and size of reptile limbs.

After the gene editing technology became more and more mature, scientists tracked the embryonic activity of pythons and found the reason why the pythons have hind legs but not fully developed.

The results of the study found that three of the "switches" that control the activity of the SHH gene in the python were missing, which ultimately prevented the python's legs from continuing to develop.

These "switches" are now genetically known as enhancers, and some mutations cause the snake's enhancer to prevent the activation of its genes that control limb development, completely changing the snake's appearance.

at last

The diversity of snakes is thought to have exploded after the extinction of the dinosaurs, and they began to challenge and adapt to various environments, but they don't seem to have limbs anymore.

Modern snakes seem to be in line with a philosophical truth - "remain unchanged", the body structure does not need to change much in different environments, but it can still live well.