Walking is a lot more complicated than putting one foot in front of the other. For that to happen, motor neurons in the brain and spinal cord must instantaneously coordinate the muscles you need to move forward, then manage the limbs, lungs, and brain to work in harmony to get you where you need to go. The origins of this elaborate organizational strategy are murky: Until recently, the most accepted theory is the one you’ve seen drawn out on high school biology posters, showing that the ability to walk evolved as vertebrates transitioned from sea to land.
But new research, released Thursday, revises that theory in a counterintuitive way. In the journal Cell, an international team of scientists report that the ability of spinal cord nerves to articulate muscles for walking emerged millions of years ago in the sea.
“We have learned that some of the things we generally think evolved in more ‘advanced’ animal species, such as the nerve cells controlling walking, are actually much more ancient than previously thought,” co-author and New York University neuroscientist Jeremy Dasen, Ph.D., tells Inverse.
This means that the first creatures that developed the ability to walk — the common ancestor linking fish and humans — stayed underwater. Some of their descendants eventually became walking invertebrates on land, while others remain on the ocean floor today, still walking.
One of these seafloor dwellers, the little skate, was the focus of this new study. Skates, which look similar to rays, are cartilaginous fish that haven’t changed much in the hundreds of millions of years that they’ve existed. And they “walk,” but you probably couldn’t tell by looking. Previous research showed that they wave their smaller pelvic fins in alternating left-right motions to creep along the ocean floor — which would hardly be noticeable to a scuba diver floating over them in the western Atlantic Ocean.
“One of the most surprising findings was how similar the movement of the skates’ pelvic fins are to the way we use our legs during walking,” says Dasen. “We could only appreciate this from taking videos from underneath skates while they are walking. This showed that many of the basic elements of walking, like the alternation between left and right legs, leg extensions and flexion, were present in skates.”
Dasen and his team began studying a group of skates as they developed in their egg cases. In a skate embryo, the tail is the strongest thing that pushes its locomotion, but after it hatches, the tail eventually regresses — presumably because locomotion through pelvic fins is poised to dominate.
A follow-up experiment on the skates used RNA sequencing to assess what genes were expressed in the skate’s motor neurons and compared them to genes linked to mammal locomotion. This showed that skates and mammals actually have a lot in common, including molecules expressed in the motor neurons of land vertebrates, molecular switches that control muscles, and interneurons that control locomotion.
“Many of the genes we studied in skates were known to be very important for the function of motor neurons that control walking in mammals,” says Dasen. “Some of these genes produce proteins that are known to function as ‘genetic switches’, which turn genes on or off. Our study shows that these same switches are used in both skates and mammals to help wire the nerve circuits essential for walking.”
Taken together, the observations indicate that the circuits involved in limb control began with a vertebrate ancestor millions of years before anything walked on land. By the time our ancestors wiggled onto the sand with their primordial limbs, the processes that generated their movement had long been established. With this in mind, Dasen and his team will continue to study the little skates to understand how exactly their motor neurons connect, with the hope that one day this knowledge can help people with serious spinal injuries.
“We actually know very little about how the nerve cells in the brain and spinal cord communicate with the motor neurons that control walking,” says Dasen.
“We hope we can take advantage of the relative simplicity of the skate fin to figure out some of the important nerve connections that make walking possible, and eventually test whether these same connections are important for mammals.”
If you liked this article, check out this video explaining the research created by the study’s authors: