What does a mouse have in common with a cartilaginous fish known as a little skate?
At first glance, you might think not much. One’s fluffy, with big ears and whiskers; the other breathes with gills and ripples its way around the ocean. One is a lab animal or household pest; the other is most likely to be seen in the wild, or the bottom of a shallow pool at an aquarium. But it turns out these two vertebrates have something crucial in common: the ability to walk. And the reason why could change the way we think about the evolution of walking in land animals—including humans.
A new genetic study from scientists at New York University reveals something surprising: Like mice, little skates possess the genetic blueprint that allows for the right-left alternation pattern of locomotion that four-legged land animals use. Those genes were passed down from a common ancestor that lived 420 million years ago, long before the first vertebrates ever crawled from sea to shore.
In other words, some animals may have had the neural pathways necessary for walking even before they lived on land.
Published today in the journal Cell, the new research began with a basic question: how did different motor behaviors evolve or change in various species over time? Author Jeremy Dasen, an associate professor at the NYU Neuroscience Institute, had previously worked on the movement of snakes. He was inspired to look into skates after reading Neil Shubin’s book, Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body, but didn’t really know where to start.
“I had no idea what a skate looked like,” Dasen says. “I’d eaten it in a restaurant before. So I did what everyone does, I went onto Google to find videos of skates.” One of the first things he found was a Youtube video of a clearnose skate engaging in walking behavior. “I was like, wow, that’s really cool! How does it do that?” he says.
Using skates collected by the Marine Biological Laboratory at Woods Hole, Dasen and others endeavored to find out. First, the basics: Little skates are bottom-dwellers who live all along the East Coast in the Atlantic Ocean. They don’t actually have legs, and their walking doesn’t look like a human going for a stroll. What they use are anterior pelvic fins called “crus,” located under the much bigger diamond-shaped sail-like fin that undulates when they swim.
When they’re feeding, or need to move more slowly, they engage their crus in a left-right alternating movement along the ocean floor. From the bottom, it almost looks like little feet propelling the skate forward.
But Dasen and his team weren’t just interested in the biomechanics; they wanted to identify the genes that controlled the motor neural pathways for skate walking.
When looking at the layout of a vertebrate, geneticists often begin with Hox genes, which play a crucial role in determining an organism’s body plan. If the genes are knocked out or misordered, it can spell disaster for the animal (as in the experiment in which a fly grew legs instead of antennae on its head after scientists intentionally knocked out certain Hox genes).
Dasen and his colleagues also looked at a genetic transcription factor called Foxp1, located at the spinal cord in tetrapods. The simplified explanation is that it works by triggering motor neurons that allow for the walking movement.
“If you knock [Foxp1] out in model organisms like mice, they’ve lost all the ability to coordinate their limb muscles,” Dasen says. “They have a severe type of motor discoordination that prevents them from walking normally.” It’s not that the mice without Foxp1 don’t have the limbs or muscles necessary to walk—they just don’t have their circuitry wired correctly to do so.
That combination of genes in little skates that allows them to step their way across the seafloor in search of dinner goes all the way back to a common ancestor that lived 420 million years ago—a surprise to the researchers, since the ability to walk was thought to come after the transition from sea to land began, not before. The fact that such genetic traits stuck around for so long, and evolved in such unique ways across different species only added to Dasen’s excitement.
“There’s a lot of literature on the evolution of limbs, but it doesn’t really consider the neuronal side of things because it’s much harder to study,” Dasen says. “There’s no fossil record for neurons and nerves. There’s much better ways of studying evolution by looking at bony structures.”
Plenty of researchers have looked to the fossil record for details about the earliest land dwellers. There’s Elginerpeton pancheni, an early tetrapod that lived outside the ocean sometime around 375 million years ago. And then there’s Acanthostega, another ancient vertebrate that scientists recently analyzed to learn about its limb growth patterns and sexual maturity.
Meanwhile, other biologists have gleaned clues by looking at some of the weirdest fish alive today, many of which have ancient lineages. Some have looked at coelacanths and sarcopterygians, or lungfish (the latter use their pelvic fins to move in a motion like walking). Others have investigated bishr movement. The African fish species is equipped with lungs as well as gills, so it can survive out of water—and has a movement similar to walking when forced to live on land, as seen in the 2014 experiment conducted by University of Ottawa biologist Emily Standen and others.
Standen says she greatly admires the new research on little skates. “I would’ve expected that there would’ve been quite a bit of similarity [in the systems behind different animal’s movement], but the fact that it’s as close as it is was a lovely surprise,” she says. “It speaks to what I believe in quite strongly, that the nervous system and how it develops and functions is very flexible.”
That flexibility has clearly been key across evolutionary history. Thanks to that 420-million-year-old ancestor, we now have everything from fish who swim, to snakes that slither, to mice that walk, to skates that use a combination of movements—with the Foxp1 gene expressed or suppressed depending on the animal’s unique body plan and locomotion.
And now that we know a little more what’s controlling that movement in skates, it’s possible that knowledge could have a future use in understanding bipedalism in humans.
“The basic principal by which motor neurons connect to different circuits is not really worked out [in complex organisms], so the skate is a way to look at that in a more simplified system,” Dasen says. But he doesn’t want to get too ahead of himself in predicting what that might mean for the future. Dasen just hopes that upon seeing the research, people will simply think, “Gee whiz, that’s really neat. They can walk!”