Primitive fish's sea-floor shuffle illuminates the origins of walking

The little skate walks using the same nerves and genes as mammals.

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 Cell1, 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 ocean2.

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 floor3. 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.”

https://www.nature.com/articles/d41586-018-01700-3