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Our ancient fishy ancestors first transitioned from life in the sea to walking on land
millions of years ago, and it’s usually understood to have been a gradual change.
That makes sense. I mean, gravity would be a real downer if
your little fish fins are used to life in water.
But when along that timeline our vertebrate ancestors
developed the ability to walk is still a mystery.
We may never fully know the answer, but thanks to one
little fish that loves to stroll along the seafloor, we are getting a little closer.
And it turns out, ancient fish might have been ready for
walking millions of years before they took to the land.
Controlling the way we move about the world is a vital
function of our nervous system.
There are just two basic types of locomotion we see in vertebrates.
For us and many other land animals, we prefer to get around by directly controlling
our limbs, called ambulatory locomotion.
That applies both to four-legged gaits, like most
land vertebrates, and two-legged gaits like ours.
So what is the other main type of vertebrate movement?
Have you figured it out?
Well it depends on a back-and-forth movement using muscles along the spine.
It’s called undulatory locomotion, the way that most fish get around.
However, there are some fishy exceptions, like skates and rays.
These close relatives of sharks actually have a pretty immobile spine.
So instead of the typical side-to-side wiggle, they have large
pectoral fins to generate forward propulsion.
But even with the ability to glide along on these wing-like fins,
some species of skate opt to stroll along a sandy bottom instead.
One particular species, the little skate, uses unique movements of their
small pelvic fins to create a distinct walk when they’re cruising along.
Adorably referred to as ‘punting’, it’s actually the preferred
mode of transportation for this fish.
Now you might be wondering, why bother walking when you can swim?
Well it’s actually an important part of their feeding strategy.
Little skates use electroreceptors in their head and throughout their
pectoral fins to help locate their prey.
By keeping their big fins still while they punt along the seafloor,
they might be better able to pick up on sensitive changes in their
environment that could reveal their next meal.
Now, walking along on fins is not unique to the skates and rays.
There are some other incredible fish species, like flatfish and
red-lipped batfish, that also prefer to stroll along on limb-like fins.
But these walking fish still use undulatory locomotion,
the side to side swim you see in most fish families.
Whereas the little skate also has a very specific left-right
saunter more characteristic of ambulatory locomotion.
And while we’re not totally sure yet, this could tell us
something about how ambulatory locomotion evolved.
It is widely thought that four-limbed land animals evolved their
ability to walk starting from that fishy undulatory movement,
with species shifting to more direct limb control over time.
See, walking is not just about having limbs.
Specialized cells along the spine called motor neurons tell those limbs what to do.
And neurons don’t keep well in the fossil record.
But looking at the nervous system of the little skate is helping
us better understand how and when walking may have evolved.
If the little skate’s genes are any clue, fish may have
had some of the necessary tools long before they ever left the water.
Our motor neurons are guided by certain proteins called transcription
factors that determine which genes get used by the cell, and to what extent.
Essentially, they guide what the neurons do.
A study published in the journal Cell in 2018 looked at skate embryos and found
that they had a lot of the same transcription factors and other molecules also
present in the motor neurons of land animals.
This includes factors that control that left-right walking motion of limbs,
as well as the bending and straightening of limbs.
Not only that, but the way the neurons were connected
to muscles was also very similar.
The researchers were able to show that the skates have the genetic
blueprint for controlling movement, as well as bending and straightening limbs.
And this potentially shifts the evolution of walking way back.
It suggests that the common ancestor of skates and modern land vertebrates
may have already had the neural tools required for complex walking behavior
50 million years before land animals first began strutting their stuff.
To build on this research of skate neural pathways, researchers at Harvard used a
theoretical model to show how underwater walking may have evolved in that
ancient common ancestor, even building a bipedal robot to show their work.
Using a mathematical framework, they considered the environmental and physical
constraints that an ancient sea-floor dwelling fish would have experienced.
They showed how the left-foot, right-foot type gait that little skates share with land
vertebrates can evolve with the help of the neutral buoyancy of water.
Essentially, their framework showed it wouldn’t take much
specialization for fish to begin strolling around fairly efficiently.
The constraints of moving onto land wouldn’t be a hard requirement.
This provides further evidence that long before our fishy ancestors made their way
into land, they had already worked out the kinks of a steady walking motion.
Now while we still might never know exactly how the first land walkers came to be,
these studies can help us better understand vertebrate evolutionary history.
They can help us in other ways too, like in building better walking robots.
The little skate can also now serve as a model organism to help us study
limb control, which could help us improve recovery from spinal injuries.
So thank you to the little skate. Your adorable stride is teaching us so much.
And from here on out I’m not swimming at the beach, I am punting.
Thank you for watching this episode of SciShow, and
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