Why Don't Snakes Have Legs?

Some snakes still carry the genes necessary to grow legs -- so why don't they? Learn how snakes may have lost their legs, evolutionarily speaking, in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:02):
Welcome to brain stuff from how stuff works, Hey, brain
stuff lorn Voga bom here. Snakes and lizards are both
card carrying members of the order Squamata. But while lizards
skinner and dash on fully formed legs, snakes opt for
a legless slither. Just why and how these animals took
such different evolutionary paths is a matter of scientific debate. However,

(00:25):
one thing is for sure. Some snakes still carry the
genetic coating that would allow them to develop legs and feet.
So why don't these snakes sprout limbs and start strutting
through the bush. The genetic machinations behind this process are
essentially switched to the off position thanks to multiple genetic
mutations that occurred more than a hundred million years ago.

(00:47):
Those mutations affected a particular gene, the sonic hedgehog gene,
which is responsible for the development of limbs. And before
you ask, yes, that is actually the genes official name
so called because of its spiky appear it's The sonic
hedgehog gene is not only vital for limbs, it also
provides guides for organs and their placement within growing animals.

(01:08):
In lizards, the sonic hedgehog gene is active and results
in legs and feet. In snakes, it's deactivated. A study
published in Nature Communications in sixteen showed how changes in
the reptile genome ultimately resulted in obvious transformations in the
creatures phenotype, which are the physical traits that come from
expressions of certain genes. In short, genetic mutations caused some

(01:31):
lizards to do away with their appendages, becoming steaks, while
still retaining the echoes of those leggy genes in their DNA.
In order to pinpoint changes in genetic sequences, the researchers
compare genomes from steaks with those of other reptiles and
vertebrates that do have complete limbs. They relied particularly on
the genome of the tagou lizard. Scientists have completely sequenced

(01:52):
its genome. Lizards happened to have DNA regulatory sequencers called
enhancers that can flip certain genes and off during a
creature's development. The sonic hedgehog gene needs those enhancers for
limbs to fully develop, which is how lizards wind up
with four legs, but in snakes, this enhancer was removed
from DNA sequences during evolution, and it's now virtually but

(02:15):
not completely eliminated, even from primitive snakes. In some snakes,
traces of the enhancer are still there, along with another
gene called h o x D, which is responsible for
guiding the development of feet. Some python embryos even develop
partial leg bones and footplates in the egg, but these
features disintegrate long before the justating creature hatches. Still there

(02:37):
are leftover signs that not all snakes are ready to
be done with the idea of sprinting along. Think of
all the tiny athletic shoes. If you peer closely at
the underside of these snakes, you'll see little nubs on
their bellies, vestigial remnants of legs that could have been
All of this means that with just a few genetic mutations,
some types of primitive snakes might be able to develop

(02:59):
limbs that essentially be activating features that are already tucked
away in their DNA, just waiting for the right variables
to trigger their development. But bonus fact of the episode,
snakes don't necessarily need feet to catch you. The black
mamba is one of the world's deadliest snakes due to
its neuro and cardio toxic venom. It can reach up
to fourteen feet long that's over four meters, and can

(03:20):
slither about twelve miles per hour. That's nineteen kilometers per hour,
which is faster than most people can run. But don't
worry too much. They're largely shy and are more likely
to flee instead of fight. Though, if you accidentally corner
a large gray snake that rears back and hisses showing
a black inner mouth, go ahead and back off. Today's

(03:44):
episode was written by Dathan Chandler and produced by Tyler Clay.
Brain Stuff is a production of i Heeart Media's How
Stuff Works. For more on this and lots of other
well evolved topics, visit our home planet, how stuff Works
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