Not all fish swim. A few prefer to walk, including the little skate—a sandy-colored, wok-size pancake with a tail—that scissors its stubby hind fins back and forth to run along the northwestern Atlantic Ocean’s sea floor in search of food. Now, a new study shows these ancient fish use the same neural wires to walk as we do. The finding means the neural circuitry needed for ambulation was around before animals walked on land.
“This is very exciting stuff,” says Brooke Flammang, a comparative biomechanist at the New Jersey Institute of Technology in Newark, who was not involved in the work. “It really shows that the circuitry needed for walking existed all along.”
To go from fins to limbs, animals need to use different muscles in different ways. Fish use their trunk muscles to swim, wiggling their spine in the process. Land animals, in contrast, use their leg muscles to walk, and keep their spines immobile. Scientists long thought the transition from swimming to walking happened when fish transitioned from sea to land, as nerve cells gradually activated more and more specific muscles, taking the trunk muscles and wiggling spine out of the equation. But some fish such as the little skate propel themselves through the water without moving their spines. Instead, the skate uses its pectoral muscles to extend and contract its lobe-shaped front fins at the same time, sort of like a bird flapping its wings. On the sea floor, it uses its hind fins to walk.
To find out how a fish walks like a land animal, Jeremy Dasen, a neuroscientist at the New York University School of Medicine in New York City, started with snakes. He had shown in previous work that a certain gene turns off limb development in snakes—making snakes essentially limbless four-legged animals—and he wanted to compare that finding in other species. So he went in search of a new model organism. He found what he was looking for at the Marine Biological Laboratory in Woods Hole, Massachusetts. The lab collects and studies skates, so they happily sent Dasen 20 of the fish’s eggs at a time. This enabled Dasen and colleagues to watch how the little skate’s muscles and motor circuits grew during development.
Early on, before their fins begin to grow, the skate embryos move by wiggling the length of their body. By late gestation, only their tails undulate this way. The part of their spines near their fins stop moving and instead their front fins move up and down. Upon hatching, their hind limbs move rhythmically back and forth, one at time, without the assistance of their spine, just as land animals do.
While Dasen and his colleagues watched the skates take their first steps, they also examined the circuits that controlled the fish’s movement. They found that skates use the same neurons to walk as land animals, the team reports today in Cell. Because skates are an evolutionarily ancient animal, that means the neurons essential for walking originated in species that separated from other four-legged vertebrates, or tetrapods, about 420 million years ago.
“Generally, people think of evolution as moving from simple to more complicated,” Dasen says. “But in fact, this relatively more simple and primitive species actually uses the same sort of complicated network to generate that type of behavior.”
When the researchers probed the blueprints underlying the shared circuitry, they found the genes that direct this development are also the same. The same genes that control the skate’s front fin versus back fin are the same type of genes that discriminate arm neurons from leg ones in people.
Even Dasen is a little stunned. The find is “quite remarkable,” he says. “It’s not like tetrapods had to invent an entirely new system. They basically had a lot of the main elements in place, and that enabled skates and tetrapods to evolve the walking behavior.”
Flammang agrees, saying what’s most exciting about this is that it really underlines the idea that there is a vertebrate body plan that allows for changes that result in different kinds of animals, adding that the work “really unlocks a way of understanding the differences between organisms.”