Amphibious Robot Offers Evolution Clue

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March 8, 2007 — Salamanders swim with the slippery ease of an eel. But as soon as their toes touch bottom, they switch rapidly to the four-legged gait typical of lizards.

Because the critters haven't changed much in about 150 million years, most researchers think that salamanders may resemble the first land-based vertebrates. But how so-called tetrapods evolved from swimmers to walkers remains a secret locked away in the complex circuitry of their spinal cords.

Now researchers have replicated that circuitry with a computer model and a salamander-like robot — Salamandra Robotica — that may help tease out some answers.

"We hypothesize that the evolutionary transition was made by reusing and extending a previous circuit designed for swimming, rather than developing a completely new locomotion circuit," said Auke Jan Ijspeert, a professor at the Swiss Federal Institute of Technology in Lausanne.

Ijspeert and his team reported their results in this week's issue of Science.

The researchers started with a computer model for the spinal cord of a lamprey eel-like animal and then added functions for limbs. They used the model along with a laptop and a wireless signal to control the walking and swimming of Salamandra Robotica.

They found that although the spinal circuitry that controls locomotion is fairly complex and sophisticated, the stimulations needed to change the robot's gait were simple. A variation in just two simple signals sent from the brain stem did the trick. Applying asymmetrical signals to neurons along the spine caused the robot to turn left or right, whether swimming or walking.

"This exactly replicates how gait transitions can be induced by electrical stimulation in a real salamander," said Ijspeert.

While the model suggests an explanation for how the spinal cord and neurons work together to change the animal's velocity, direction, and type of gait, it leaves out information that is created as a result of the motion, said Keith Sillar, Professor of Neuroscience in the School of Biology at the University of St Andrews, Fife, Scotland.

"What has yet to be built into the system is sensory feedback — information that is created in sensory neurons as a result of the movements themselves," he said. "But this is an important first step and such details can be added as more is understood," he said.

Adding those details to a later model could allow scientists to explore the evolution of tetrapods directly.

"One of the things we know from fossils of the Devonian tetrapodomorphs is that limbs evolved while vertebrates were still primarily aquatic fish," said John Long, a professor of biology at Vassar College in Poughkeepsie, N.Y., whose team developed a four-flippered aquatic tetrapod to test ideas about how living and fossil vertebrates swim.

"What were these critters doing? Were they walking in the water, using their limbs to punt like a crab?" he asked.

A computer model could answer such questions of how the aquatic use of limbs was possibly co-opted for actual terrestrial movement, he said.