The little skate has been both swimming and walking for hundreds of millions of years. Jeff Rotman/The Image Bank/Getty Images
The late-Devonian vertebrate land invasion, roughly 382 million years ago, was a big deal in Earth's history. Previously confined to the ocean, our tetrapod forefathers took to the surface world and, over the course of millions of years, traded fins and gills for limbs and lungs. (Tetrapod just refers to vertebrates with two pairs of limbs.)
Here's a quick video refresher, courtesy of the journal Cell and associate professor and co-author Jeremy S. Dasen and his colleagues:
The remarkable thing, says the team of researchers, is that the neural circuits involved in ambulatory limb control were already established millions of years before the first tetrapod strutted its stuff. In other words, much of the software was in place well before the walk-about hardware.
The researchers studied the neural circuitry of the little skate (Leucoraja erinacea). This cartilaginous fish might not be much to look at, but it's considered one of the most primitive vertebrates alive today. Travel back roughly 420 million years and you'll find a common ancestor of both skates and tetrapods.
The little skate is also interesting because it's one of several ambulatory fish that "walk" across the seafloor. The skate uses its large pectoral fins to swim and smaller pelvic fins to walk with alternating, left-right motions, much like the gait of a land animal. This similarity impressed the researchers, but the similarities would go beyond movement.
Dasen and his colleagues employed RNA sequencing to study the expressed genes in the skate's motor neurons. Many of these genes pop up in mammals as well — and that includes neural subtypes involved in the muscle control of bending and straightening limbs. This, according to the study findings, constitutes a conserved genetic program for walking.
Dasen says that neither "swimming" nor "walking" accurately describe the skate's movements, but perhaps this isn't too surprising given the human-centric nature of language.
"The skate/ray mode I would call 'ambulatory swimming' whereas the axial/tail-based is more like 'spinal swimming,'" Dasen notes via email. "The ambulatory swimming mode is really the one which made walking possible in both skates and tetrapods."
The study sheds light on the underwater history of walking, but the researchers hope that it will lead to an improved understanding of motor neurons and even the treatment of human neurological disorders.
Dasen stresses that while the neural complexity of higher organisms hinders our studies of animals such as mice, the little skate's archaic simplicity makes it a perfect starting point.
"I think one of the advantages of studying neural circuits in skates is that they can accomplish this behavior using a relatively simple set of connections between neurons and muscle," Dasen says. "We hope we can exploit this simplicity to understand the basic architecture of the circuits controlling walking."
The exact wiring of these circuits is still not fully understood in humans or other tetrapods, but such knowledge could one day aid in the treatment and repair of human spinal cord injuries and motor neuron diseases such as amyotrophic lateral sclerosis (ALS).
But as the saying goes, you have to crawl before you can walk – or should we say swim?
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