BioEd Online

The genes and nerve cells that allow people and other mammals to walk around can also be found in a primitive fish known as a skate, according to a study. The findings suggest that the nerve cells essential for walking evolved millions of years earlier than previously thought.

By studying the little skate (Leucoraja erinacea), a close relative of sharks and rays that can walk on the sea floor, a team of neuroscientists found that the nerve networks that control this ability are the same as those in mammals. The results, published on 8 February in Cell, support the argument that the nerves that control walking first appeared in fish at least 420 million years ago, more than 20 million years before the first four-legged animals crawled out of the ocean.

The analysis provides some of the only direct evidence showing that the nerve cells for walking arose before vertebrates moved onto land. Bones, teeth and other tough body structures are well preserved in the fossil record, but softer tissues such as muscles and nerves decay quickly and are often lost to time, says Tetsuya Nakamura, an evolutionary biologist at Rutgers University in Piscataway, New Jersey. So if researchers want to study the nerves that control locomotion in an ancient animal, they often turn to modern creatures that scientists believe are good approximations of their ancestors.

“This study is the first deep foray into the origins of limb-control circuits,” says neuroscientist Joseph Fetcho of Cornell University in Ithaca, New York. There are some fish species that are more closely related to mammals that can walk on the sea floor. But it’s “really cool” that skates, which are relatively primitive, are able to move their limbs in the same way as people do, Fetcho says. “We’re not as special as we think we are.”

One small step

Skates have changed little from their ancestors that inhabited the oceans hundreds of millions of years ago. They use a set of large fins to swim, and a separate set of smaller fins to walk on the sea floor. “When you look at videos of skates walking, these fins look a lot like legs,” says Jeremy Dasen, a neuroscientist at New York University School of Medicine in New York City and a co-author of the study. The fins move like legs, too, he says. As the fish crawls, the left and right fins alternate, bending and stretching out much like our legs do when we run.

That’s not just a coincidence. The researchers found that the nerve cells that control the muscles for bending and straightening the limbs in people are also present in skates. What’s more, when Dasen and his team looked at the genes expressed in the nerve cells that enable a skate to move, they discovered that many of them appear in both skates and mammals.

Because of these similarities, the nerve networks required for walking might already have been present in the common ancestor of skates and mammals about 420 million years ago, the researchers concluded. But how skates and humans evolved the ability to walk on two limbs is still a mystery. They might have developed it independently using a shared set of genes and nerve cells present in the common ancestor, says Heekyung Jung, a neuroscientist now at Stanford University in California, who led the research.

But some researchers urge caution when interpreting the results. “We must be very careful about looking at any living group and thinking it represents ancestral conditions,” says Michael Coates, an evolutionary biologist at the University of Chicago in Illinois. To confirm that the common ancestor of skates and humans had a set of genes and nerve cells for walking, the team should analyse a larger sample of animals, including fish that are more closely related to mammals, he says.

Dasen remains hopeful, however, that this line of research could lead to bigger things. By studying skates, which only use six muscle groups per walking fin, scientists could find out how the nerve networks that control walking are wired in more recently evolved organisms such as mice and people, which use hundreds of muscles to move, he says. Eventually, that knowledge might help physicians to repair damaged circuits in spinal-cord injuries, or treat neurological disorders that affect a person’s ability to move.