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The very first vertebrates to walk on our planet could have done so in the depths of the ocean, millions of years before their later parents made the transition to earth.
In 2018, scientists were shocked to find the tiny rayfish (Leucoraja erinacea) and some basal sharks were able to walk along the ocean floor using many of the same neural circuits we use to walk today.
In general, it is believed that vertebrates only learned to walk when they began to avoid the sea for shore, around 380 million years ago. But other models based on the tiny rayfish – one of the most primitive animals with a backbone – suggest a much deeper origin, perhaps over 400 million years ago.
Using published video data on the scuttling dynamics of this benthic creature, mathematicians developed a model to study how early leg-like movements might have evolved in the deep sea.
The simple model they created predicts the most efficient, controlled, and balanced type of walk in a neutral buoyancy environment: the best result requires an alternating left-right-foot pattern very similar to that of the small skate.
In addition, this type of trolling does not require any additional energy costs and could be strengthened over time with the help of a simple learning scheme.
“In the context of our model, these results suggest that, despite the vast solution space of the gaits, a left-right alternating bipedal control strategy can and will be discovered and is the optimal solution for energy efficient locomotion,” the authors write. authors of the study.
Finding a real-life example of this ancient organism is like discovering a “needle in a haystack,” the team admits, but they say only rudimentary legs would be needed to achieve this pattern of foot placement. After the evolution of these foot-like fins, then the ancient creature would only need minimal neural control over its new and improved limbs.
After four episodes of learning in the model, a one-legged locomotion strategy began to emerge. After 200 episodes, a two-legged walking model took over. In the 600th episode, the modeled creature began to alternate between left and right steps.
Running roughly 50 instances of learning for 5,000 episodes, including various learning settings and rewards, the authors found that the best solution was the little skate gait 70% of the time.
This simple control strategy suggests that deep sea walking is a robust and efficient behavior similar to passive walking, like the tight-fitting toy that “walks” down a slope without the need for complex control, just gravity.
The little skate, of course, is not a completely passive plowman. Its brain cells still control six muscles for movement, but the authors say this system operates on the same principles as a passive system: “Sustained locomotion under a constant source of energy without feedback control.”
The authors are unsure why the small stingray developed a slow walk on the seabed, but they suggest that it is more efficient and cost effective than swimming at a similar pace. Further metabolic studies on the deep-sea creature will need to verify this idea.
Sometimes in nature, the little skate will use both legs at the same time to “throw” forward and quickly restart its left-right walk. This type of motion was not found in the model, but the authors believe it might be favored when faster acceleration is needed and fuel efficiency is not as important. This unusual punt requires a little more work.
“The combination of a reliable low-gravity environment and a leggy body morphology may have helped pave the way for bipedal gaits before our aquatic ancestors made it onto land,” says the diligent mathematician Lakshminarayanan Mahadevan of Harvard University.
“As our ancient ancestors migrated to the land, the strategy of control probably became more complex. But in reliable and homogeneous environments, such as the seabed, a simple strategy may be sufficient.
To complement this theoretical model, the researchers even built a simple bipedal robot based on similar deep sea conditions. In the end, the behavior of this robot showed striking similarities with the ideal walker of their model. Its regular step pattern requires no additional energy and waves on both sides of the body for added stability.
The robot, however, tends to walk slightly faster than what is seen in the small skate.
The authors admit that they may never know exactly how the first gait of walking came about, but their model helps refine some of the passive dynamics and neural circuits seen in living organisms.
“Understanding how the brain, body, and environment worked together in heterogeneous aquatic and terrestrial environments likely required proprioceptive feedback,” suggest the authors.
“But in reliable and homogeneous environments, it may just be the simple quantified strategy here that it all started.”
The study was published in the Journal of the Royal Society Interface.
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