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The evolution of walking may have happened earlier than thought -- and underwater

"We found that the interneurons, nearly a dozen types, are also highly conserved between skates and land mammals," researcher Jeremy Dasen said.

By
Brooks Hays
Researchers think the relatives of the little skate, a primitive skate species, were some of the first animals to develop ambulatory locomotion. Photo by Heekyung Jung, et al./Cell
Researchers think the relatives of the little skate, a primitive skate species, were some of the first animals to develop ambulatory locomotion. Photo by Heekyung Jung, et al./Cell

Feb. 8 (UPI) -- The first animals to walk may have evolved locomotion underwater.

Often, the development of limbed locomotion and the ability to walk is traditionally associated with the transition from life in the sea to life on land.

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But new research, published this week in the journal Cell, suggests the first adulatory species may have opted to stay in the sea. The findings also suggest the first walkers developed limbs much earlier than previously thought.

"We were surprised to learn that certain species of fish also can walk," Jeremy Dasen, a developmental neurobiologist at the New York University School of Medicine, said in a news release. "In addition, they use a neural and genetic developmental program that is almost identical to the one used by higher vertebrates, including humans."

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To better understand the neural processes linked with adulatory motion, Dasen and his colleagues analyzed the walking abilities of the little skate, Leucoraja erinacea. The cartilaginous fish is closely related to sharks and rays and is considered one of the most primitive creatures in the sea. The little skate has barely evolved since it first emerged and looks very much likes its predecessors that lived several million years ago.

While the little skate's larger pectoral fins are used for swimming, the species is able to walk along the ocean floor using its bottom pair of fins. The skate uses a left-right motion similar to the pattern used by walkers on land.

When scientists analyzed the genes expressed by the skate's motor control neurons, they found many of the same genes expressed by mammals' motor control neurons. The also found overlap among the neuronal genes in mammals and the little skate used to control the muscles that bend and unbend limbs.

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"These findings suggest [that] the genetic program that determines the ability of the nerves in the spinal cord to articulate muscles actually originated millions of years earlier than we have assumed they appeared," Dasen said. "This fin-based movement and walking movements use the same developmental program."

Researchers also analyzed the skate's interneurons, which form the circuitry linking motor control neurons with the central nervous system. The circuitry features central pattern generators, which sequence the order in which neurons fire and trigger muscle activation.

"We found that the interneurons, nearly a dozen types, are also highly conserved between skates and land mammals," Dasen said.

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Unlike mammal models like mice and chickens, which feature more complex neural networks and a larger variety of muscles, the little skate is relatively simple -- an ideal model in which to study locomotion. In future studies, Dasen hopes to better understand how the skate's locomotion neurons are integrated with other neurons, as well as how neuronal genes are regulated.

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