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We may never know when our ancestors walked out of the water into dry land. But it's possible they may have been walking in water for millions of years!

Hosted by: Hank Green

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Sources:

https://www.cell.com/cell/fulltext/S0092-8674(18)30050-3
https://bioone.org/journals/copeia/volume-2003/issue-3/CG-02-153R1/Punting--An-Unusual-Mode-of-Locomotion-in-the-Little/10.1643/CG-02-153R1.short
https://royalsocietypublishing.org/doi/abs/10.1098/rstb.1993.0007
https://royalsocietypublishing.org/doi/full/10.1098/rsif.2020.0701
https://www.seas.harvard.edu/news/2021/03/ancient-vertebrates-had-everything-they-needed-walk-underwater-millions-years
https://www.sciencedirect.com/science/article/abs/pii/S0944200617302052
https://onlinelibrary.wiley.com/doi/abs/10.1002/jmor.10865

Images

https://www.storyblocks.com/video/stock/fish-chub-swimming-in-the-aquarium-hrik9al3pkik77cft
https://www.storyblocks.com/video/stock/young-female-with-retriever-dog-walking-leisurely-on-the-path-at-sunset-r_qfvxo98ka3yan53
https://www.storyblocks.com/video/stock/multicoloured-koi-fish-swimming-graceful-in-a-water-of-an-garden-colorful-koi-fish-in-the-pond-with-dark-coloured-ground-moving-around-saz2atdjwj2aldr3u
https://www.storyblocks.com/video/stock/flat-fish-lying-on-the-bottom-and-trying-to-merge-with-the-mud-b3t53aeork17ww9iy
https://www.storyblocks.com/video/stock/4k-dog-running-up-driveway-gimbal-shot-sej0rxo9xizy9a7pu
https://www.shutterstock.com/image-vector/structure-motor-neuron-impulses-transmitted-through-423991636
https://www.istockphoto.com/photo/skate-fish-gm185275955-19891926
https://commons.wikimedia.org/wiki/File:Heliobatis_radians_Green_River_Formation_(cropped).jpg
https://www.istockphoto.com/photo/modern-hospital-physical-therapy-patient-with-injury-walks-on-treadmill-wearing-gm1335020216-416920392
https://commons.wikimedia.org/wiki/File:Valkyrie-robot-3.jpg
[♪ INTRO].

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 bat fish, 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 thanks as always to our patrons  for helping to make it possible.

If you’d like to join a great community,  and get to see some cool stuff like our newsletter and our monthly bloopers, you  can get started at patreon.com/scishow. [ OUTRO ].